200 research outputs found
Anisotropic drop spreading on superhydrophobic grates during drop impact
No full textWe study the influence of geometric anisotropy of micro-grate structures on the spreading dynamics of water drops after impact. It is found that the maximal spreading diameter along the parallel direction to grates becomes larger than that along the transverse direction beyond a certain Weber number, while the extent of such an asymmetric spreading increases with the structural pitch of grates and Weber number. By employing grates covered with nanostructures, we exclude the possible influences coming from the Cassie-to-Wenzel transition and the circumferential contact angle variation on the spreading diameter. Then, based on a simplified energy balance model incorporating slip length, we propose that slip length selectively enhances the spreading diameter along the parallel direction, being responsible for the asymmetric drop spreading. We believe that our work will help better understand the role of microstructures in controlling the drop dynamics during impact, which has relevance to various engineering applications
FIGURE 5 in A new species of Cephalaeschna Selys, 1883 (Odonata: Anisoptera: Aeshnidae) from Neora Valley National Park, West Bengal, India, with notes on C. acanthifrons Joshi & Kunte, 2017 and C. viridifrons (Fraser, 1922)
No full textFIGURE 5: A. Cephalaeschna acanthifrons holotype from Arunachal Pradesh, thorax [Photo by Subhajit Mazumder]; B. Cephalaeschna viridifrons from Neora Valley National Park, West Bengal, India [Photo by the author]; C. C. acanthifrons, face [Photo by Subhajit Mazumder]; D. C. viridifrons from Neora Valley National Park, face [Photo by the author]; E. C. viridifrons from Neora Valley National Park, abdomen dorsal view [Photo by the author]; F. C. acanthifrons holotype, anal appendages [Photo by Shantanu Joshi, NCBS]; G. C. viridifrons from Assam, anal appendages (reproduced from Asahina 1981a); H. C. viridifrons from Nepal, anal appendages (reproduced from Asahina 1981a); I. C. viridifrons from Neora Valley National Park, anal appendages [Photo by the author].Published as part of Dawn, Prosenjit, 2021, A new species of Cephalaeschna Selys, 1883 (Odonata: Anisoptera: Aeshnidae) from Neora Valley National Park, West Bengal, India, with notes on C. acanthifrons Joshi & Kunte, 2017 and C. viridifrons (Fraser, 1922), pp. 371-380 in Zootaxa 4949 (2) on page 378, DOI: 10.11646/zootaxa.4949.2.10, http://zenodo.org/record/463619
Antibacterial Surfaces Mechanisms, Design and Development
No full textThe spread of disease-causing microorganisms through high-touch surfaces and their increased tolerance against antimicrobials and the host immune system is responsible for several fatal diseases. By the year 2050, Antimicrobial resistance (AMR) is expected to cause 10 million deaths annually and a loss of US$100 trillion. Today, some bacterial species (e.g., Carbapenem-resistant Enterobacteriaceae group of bacteria) are immune to all major classes of available antibiotics. This has encouraged the scientific community to develop alternatives to antibiotics to fight the AMR.
Primary sources of spread and resistance acquisition among bacteria include cross-contamination of surfaces in hospitals, catheters, stethoscopes, and surgical tools. Any such abiotic surface is vulnerable to bacterial colonization that begins with a few primary colonizers attaching themselves to the surface to condition it for further attachment of arriving bacteria. After initial attachment, bacteria start to proliferate and develop into surface-bound colonies. It then forms a robust protective layer of biofilm that brings advantages to bacterial survival against environmental odds. Hence, the initial stage of attachment is a weak link in the bacterial journey to forming a protective biofilm. Exploiting this weak link, nanostructured surfaces hinder initial attachment by physically rupturing the cell without the involvement of any chemical or biocides, hence are consistently called “promising” in controlling bacterial proliferation.
Although various theories over the past few years have tried to explain the behavior of bacteria on these nanostructures, there is a lack of consensus on the precise mechanism that leads to bacterial death. To efficiently restrain bacterial colonization, it is of profound importance to understand the fundamental cause of bacterial death on these nanopillars. Only such fundamental understanding can guide us to the answer to the question: What precise nanopillars feature participate in bacterial cell-rupture and how?
In this doctoral dissertation, we investigated the mechano-response of E. Coli cells as it attaches itself to a regular array of precise dimension-controlled nanopillars. Overcoming the fabrication limitations, two sets of ordered arrays of nanopillars by varying one dimension at a time makes it possible to study the involvement of individual dimensions on the response of single bacterial cell, which is crucial in understanding the rupture mechanism. The bacterial cell extends out via thread-like projections in the direction of neighboring pillars to establish contact with them. At a particular interpillar spacing (pitch) of straight pillars, the attached nanopillars appear to bend towards the cell due to the application of force. This displacement of pillars and hence the force increases with interpillar spacing. Bactericidal efficacy was proportional to the applied force, and hence interpillar spacing. The method of calculating force applied by bacteria on nanopillars adds direct experimental evidence towards the proposed mechanism of bacterial interaction with nanopillars at the single-cell level. We have focussed on one bacterial strain E. Coli; however, this method of studying bacterial-nanopillar interaction can pinpoint the governing parameter for cell rupture for different bacterial strains.
After establishing the fundamentals of mechano-bactericidal mechanism, the subsequent work progresses to dual action antibacterial surfaces that aim towards studying alternatives to biocide coatings aiding from mechanical rupture of cells. A common non-selective way to kill bacteria without using antibiotic chemicals, and hence following the risk of developing antibacterial resistance, is to use photocatalytic materials. They produce reactive oxygen species (ROS) in the presence of light and water that cause bacterial death on the surface. The dual action surfaces benefit from nanostructures and photocatalytic antibacterial coatings over it. We establish the design principles of such “dual-action” surfaces, and answer several open questions, for example: which material should the nanostructures be made of? What is the optimum photocatalyst thickness? What geometries are most effective? In this work, TiO2 is used as the photocatalytic coating on nanostructures made of Si and SiO2. It is demonstrated that TiO2-coated “black-silica" (nanostructured SiO2), is more effective in producing the bactericidal effect. The bacterial kill rate is improved by 73% on replacing the underlying Si nanopillars with SiO2 nanopillars.
To understand the dynamics of light absorption and subsequent ROS diffusion in such systems, FDTD and FEM simulations were used for modeling. FDTD simulations show that parasitic absorption in the underlying base pillar of high extinction coefficient leads to significant loss of incident optical energy. Hence, the “total absorption” of a system can be a misleading proxy for photocatalytic activity. Only absorption in the photocatalyst (TiO2) matters, which can be enhanced by fabricating nanopillars with a more transparent material like SiO2 or PDMS, having a low extinction coefficient. Further, FDTD coupled with FEM simulations shows that taller nanopillars don’t always lead to higher bulk ROS concentration, despite more absorption. Beyond 5 µm height, ROS are unable to diffuse out of the nanopillar forest.
After articulating the design rules, the next step is to come up with a scalable process that can be deployed as practical antibacterial surfaces. In this work, we further extend the effectiveness of the TiO2-coated B-Si. By substituting TiO2 with TiO2 nanoparticles, the effective surface area for the production of ROS increases significantly. The extraction of photocarriers also improves because bulk of TiO2 is always within a few nm of a surface. The films are fabricated with three different techniques, all of which are scalable to large-areas. We establish the impact of the different techniques on the film’s topology and ability to kill bacteria.
Antibacterial photocatalytic coatings are a promising alternative; however, the band gaps of most metal oxides are too wide, requiring UV/blue illumination. To deal with this, we discovered a new antibacterial photocatalyst, Mn2V2O7 (MVO), that works in ambient light or low-intensity solar radiation. The β-phase has a bandgap of 1.7 eV, so MVO absorbs visible light up to 600 nm.7 Under visible light, MVO reduces bacterial load by four orders of magnitude. MVO can be coated into films by drop-casting, which kills 76% of bacteria.
In conclusion, work done in this thesis address the problem of spread of antimicrobial resistant bacteria via surfaces. We establish the mechanism of interaction of bacteria with nano-pillars also called as mechano-bactericidal mechanism. This formulates the understanding behind contact-kill mechanism of nanostructures. We extended efficiency of nanopillars by coating it with photocatalytic material that non-selectively degrades any organic material including bacterial cells, hence adds as a second line of defense again bacterial colonization. Using FEM and FDTD simulations, we articulated the design rules of such coated nanostructures. We developed technique to coat mesoporous photocatalyst on these nanostructures allowing larrge area deployment. At last, we overcame the UV-activated limitation of photocatalysts by enabling a visible light-activated antibacterial material suitable for large area coatings.MHRD, DS
Droplet, Jets, and Leaky Surfaces
No full textSurface structuring on a micro-nano scale, combined with a low surface energy coating, leads to anti-wetting properties. Such surfaces also exhibit other properties such as self-cleaning, antifouling, bacteriostatic, drag reduction, and anti-icing. Hierarchical structures with dual-scale roughness provide the superhydrophobic surface with lower droplet adhesion and better protection against failure (i.e., Wenzel transition). This understanding has led to the study of nanostructured sieves, as the sieve wires (having diameters ranging from 10 to 100 microns) provide the higher-level roughness required in dual-scale surfaces. Thus, for sieves, a single nano-structuring step leads to dual-scale rough surfaces. Further, the pores in sieves provide an additional structural feature for enabling other applications such as oil-water separation. Hence, nanostructured sieves are being investigated today for novel applications.
Studying the impact of droplets on sieves with different wettability is fascinating as their porosity leads to several exciting scenarios that can be explored for potential use. This thesis investigates the different outcomes of droplet impact on sieves and explores new possibilities. The first part of the study explores droplet impact at the low Weber number regime. The formation of different cavities and their collapse have been studied. The focusing of kinetic energy in the cavity collapse process and the associated singularity leads to the generation of a single droplet. This work reports a new kind of cavity formation phenomenon unique to sieve configurations. In contrast to cavities observed for droplet impact on solid surfaces, this cavity is formed during the droplet impact's recoil phase. Hence, it is called the recoil cavity. The cavity formation and collapse are explained using experimental results and theoretical modeling.
The collapse of the recoil cavity leads to the generation of a satellite-free single droplet underneath the sieve. Essentially this phenomenon of ejecting a single drop opens up avenues for novel applications. This thesis explores the drop-on-demand technique for material jetting and printing. Interestingly, we found that using superhydrophobic sieves could eliminate a long-standing problem of clogging in printing. We explored the clogging issue in-depth and showed our technique's unique capability in printing high mass loading and large particle size.
We report printing of ink with mass loading as high as 71% using our technique. Further, the use of this printing technique has been demonstrated for various applications. Electronic circuits and devices have been printed on flexible substrates. 3D printing has also been demonstrated using high mass loading ink. Printing of live cells and bacteria has also been achieved using this technique.
This thesis explores droplet impact at a high Weber number regime in the second part. We developed double sieve-based air-transparent surfaces capable of repelling rain droplets impacting at terminal velocities. Such air-transparent surfaces will find use in roofs and windows of homes and public places. Current understanding would point towards the use of nanostructured superhydrophobic sieves. However, liquid leaks through such superhydrophobic sieves when the dynamic pressure of the impacting droplet is larger than the anti-penetration Laplace pressure. When the droplet penetrates through the sieve, it comes out in the form of jets. Due to Rayleigh-Plateau instability, the ejected jets break into smaller droplets. This jetting dictates the outcome of the impact. Contrary to the common understanding, we explain our experimental results, which show that the jet velocity can be larger than the impact velocity. This increase in velocity of the ejected droplet makes it difficult to stop the ejected jet using a second superhydrophobic sieve. We use a combination of superhydrophilic and superhydrophobic sieves to repel raindrops impacting at the terminal velocity.
Overall, this thesis deals with the droplet interaction with sieves of different wettability. The present work evolves to innovate interesting applications and solve significant problems
Investigating Mechanical Properties of Suspended Ovarian Cancer Cells and Clusters
No full textThe peritoneal cavity of a patient suffering from advanced epithelial ovarian cancer is filled with disseminated multicellular aggregates, commonly known as spheroids. These spheroids colonize abdominal organs leading to metastasis. Metastasizing spheroidal cancer cells frequently become resistant to chemotherapeutic drugs, thus demanding a rigorous investigation of mechanisms underlying their formation and stability. Spheroids obtained by tapping the malignant ascites of ovarian cancer patients show heterogeneous morphologies: some exhibit a dysmorphic ‘moruloid’ (mulberry-like) phenotype, and others show smooth compacted surfaces and an internal lumen, giving them a ‘blastuloid’ appearance. Additionally, blastuloid spheroids reveal the presence of a basement membrane coat surrounding them which was also raised the interest in understanding its role in their structure, and its contribution to their localized stiffness. These morphologies could represent consequences of phenotypically heterogenous cell types, or indicate progressive stages of metastasis with moruloid phenotypes maturing into blastuloid counterpart.
There is a burgeoning body of literature on biophysical investigations of tumorigenic cellular ensembles. Of these, most studies focus on the migrational dynamics of spheroidal or tumoroidal cells within stromal-like extracellular matrix (ECM) microenvironments. In fluid microenvironments, the assembly of multicellular structures from suspended single cells likely employ distinct mechanisms. Although elegant theoretical models have been constructed recently to explain dynamical structural transitions, technical difficulties of efficiently imaging floating clusters have allowed few biophysical characterizations of spheroids. Notable experimental exceptions include efforts to mechanically analyze spheroids using microtweezers, wherein those constituted from breast cancer cells were found to be softer than from untransformed controls, and investigations using cavitational rheology to determine the cortical tension in spheroids of HEK293 cells. A pertinent study by Panwhar and coworkers recently describes a high throughput approach using virtual liquid-bound channels to show that the stiffness of multicellular spheroids is an order of magnitude lower than that of cells that constitute them. Although these investigations have not studied temporal topological transitions between multicellular morphologies, they lay the foundation for such studies within fluid microenvironments.
In this dissertation, I try to investigate the mechanics and biophysics of ovarian cancer starting at the single cell level, thereby moving to spheroids. I used AFM as an elegant tool to characterize the stiffness of single cells in suspension. The difficulty in imaging suspended cells and 3D cultures was overcome using a novel technique employing noble agar as a substratum to immobilize them. Three different cell lines representing ovarian cancer were studied for this and compared to their respective adhered states. The technique was also used for spheroids of the mentioned morphologies to sense localized stiffness changes as they mature, thereby making it versatile and size independent.
Subsequently, we combine microfluidics with high- speed time-lapse videography and imaging analysis to investigate the consequences of lumen formation and the basement membrane (BM) coat on the architecture and integrity of ovarian cancer spheroids providing architectural robustness to the transitory ovarian cancer metastatic niche within constrained flow spaces. We investigate their time of transit, relaxation dynamics, disintegration statistics, particle image velocimetry (PIV), shape evolution, etc as tools to further understand their structural and temporal integrity.
We further extended these techniques to study the effect of chemoresistant drugs and inter-cellular motion inhibitors, and their effect on the cluster mechanics and biophysics.
In future, some design iterations can be tried to trap the clusters and monitor the long-term effect of shear on them. Keeping them trapped for a longer duration can give insight into the real-time changes in a fluidic environment
Droplet Interface Oscillations using Electrowetting-on-Dielectric (EWOD) for Open-Chip Microfluidic Applications
No full textDroplet manipulation on microfluidic platform has gained significant importance
due to its applicability to various healthcare technologies, where larger
equipment can be reduced to portable handheld systems based on microfluidic
devices. Electrowetting-On-Dielectric (EWOD) has emerged as one of the most
promising techniques for droplet manipulation in microfluidic devices because it
is an easily programmable, cost effective, reconfigurable and reversible
technique. Droplet creation, actuation, merging, mixing, and splitting are the
fundamental operations which enable biochemical assays on EWOD based
microfluidic platform. In conventional EWOD the droplet is sandwiched between
two substrates. This however reduces the accessibility of the droplet to the
device edges only. Recently, it has been demonstrated that EWOD based droplet
actuation is also possible on single sided electrodes. Enhanced droplet
accessibility on such an open-chip microfluidic platform holds promise for
development of complex platforms with better integration of external sensors
and actuators. In the quest to improve the fundamental EWOD operations for its
open-chip version, we have studied the role of droplet interface oscillation in
enhancing mixing and enabling localized sensing.
The thesis work presented here focuses on two main aspects of interfacial
oscillations: i) non-axisymmetric modes of droplet which appear due to the
parametric coupling during oscillations. We have demonstrated the use of these
modes for applications in mixing; and ii) localized electrowetting where only a
part of droplet interface is actuated instead of full droplet by patterning the
actuation electrodes. We have demonstrated the use of localized interface
actuation for localized sensing application.
In non-axisymmetric oscillations, droplets contact line loses its symmetrical
shape during spreading and expands to form asymmetrical lobes. The number of
lobes represent different mode shapes. These mode shapes were analyzed using
image analysis. The extracted interface was fitted to a Fourier series to extract
amplitudes of different modes. Analysis of the extracted mode amplitudes
indicated that above a certain actuation voltage (force), the non-axisymmetric
modes grew at the expense of the axisymmetric modes. This indicated a coupling
between the axisymmetric and non-axisymmetric modes. Investigations revealed
parametric coupling leads to manifestation of the large non-axisymmetric mode
amplitudes. Further, the non-axisymmetric modes were identified to be
degenerate modes as given by the spherical harmonic functions. These nonaxisymmetric
parametric oscillations were modelled using the Mathieu equation
to identify the regime of actuation parameters where the parametric coupling is
obtained. These non-axisymmetric oscillations were applied to enhance mixing
(i.e. reduce mixing time) of reagents on an open-chip. In comparison to mixing by
pure diffusion, using non-axisymmetric modes leads to 37 times faster mixing of
droplets.
Manipulation of droplets containing biological samples is often hindered by
biofouling. To apply these oscillations to biological samples, an oil surrounding
was required. So, we propose use of compound droplets in open-chip
microfluidic platforms. Compound droplets are formed with sample (bio) as the
core and silicone oil as the surrounding shell medium. In this work, we studied
interface oscillations for different compound droplet configurations. For low
actuation frequencies, the aqueous-core responds to the actuation voltage
whereas the oil-shell is actuated by the oscillating core. Effect of varying oil-shell
volume on the oscillation of compound droplet was studied. The resonance
frequency of compound droplets decreased with increase in the oil-shell volume.
This reduction has been attributed to the increased mass loading and damping of
the increased oil volume. The regime of actuation parameters for attaining nonaxisymmetric
modes also changes with oil volume. These dynamics of compound
droplets were modelled using mass-spring-damper model and the Mathieu
equation. The mixing efficiency of these oscillations was also studied for
biological fluids (i.e. red blood cells (RBC) containing phosphate buffer saline
(PBS) solution). We observed enhanced droplet mixing using the non
axisymmetric modes in comparison to the mixing by pure diffusion. This
provides a technique for achieving faster mixing in biochemical assays on digital
chip. This mixing reduces the required chip space by removing the need of
external pumping or numerous electrodes.
Another interesting phenomenon pertaining to coalescence was observed while
studying mixing of oscillating compound droplets. For certain actuation
parameters prolonged non-coalescence was observed between the two core
droplets. Different regimes of coalescence and non-coalescence were obtained
based on amplitude and frequency of the core oscillations. The transition from
coalescing to non-coalescing regime was explained based on oscillation mode
amplitudes which led to periodic modulation of the entrapped oil bridge
between cores. We found that the role of electrostatic repulsion was limited to
the contact line and did not prevent droplet coalescence away from the contact
line. The capillary pushing of cores with time-period faster than the normal oil
bridge drainage time caused continuous modulation of the oil bridge width,
which was proposed as the reason for the observed non-coalescence of droplets
for certain range of frequencies and voltages. This study can be used to maintain
stable non-merging of droplets on substrate required for various applications
like compound lenses.
The last part of the thesis investigates droplet contact line as a micro-mechanical
resonator. Here, we reduced the dimensions of oscillating droplet interface by
actuating a small portion of droplet contact line using patterned line electrodes
of 50-450 μm width. By reducing the actuated interface length, its resonance
frequency given by
∝ ⁄ was expected to increase. We, however,
obtained completely damped oscillations in our experiments. This indicated the
dominant role of viscous forces. We used this damped localized electrowetting as
a sensing technique to study the liquid properties. The relaxation time of the
actuated interface was used as a measure of viscosity and surface tension of
liquid. The change in these relaxation dynamics during an on-going chemical
process in a droplet or a microfluidic chip, can tell us about the dynamic state of
reaction. This was demonstrated by monitoring the process of sugar dissolution
in water. This technique offers great potential to sense particles and determine
progress of fluid reactions in both droplet-based platforms and microfluidic
channels at different time instants and positions
3D Packaging for Integration of Heterogeneous Systems
No full textWith several new applications getting developed around wearable technologies for Internet of Things (IoT), there has been a growing need for development of the miniaturized systems. Emerging applications in healthcare, structural monitoring, consumer accessories, etc are fuelling the need for these miniaturized hybrid systems. Such micro-nano systems will be enabled through the development of heterogeneous integration technologies that will allow co-packaging of several chips with different functionalities in a single vertical 3D stack. Therefore, the consumer electronics industry has initiated development of 3D integration of CMOS devices in vertical stacks which are electrically interconnected using thru-silicon-via (TSV) technology. This technology is however not suitable for stacks having a complex combination of GaN-HEMT’s, MEMS, microfluidics, optical devices and CMOS. Moreover, due to the cross-contamination issues, most of these devices are never accepted in the standard silicon CMOS foundries. To address these issues, we have developed innovative processing technologies that would allow 3D packaging by the post fab vertical stacking technique, suitable for the packaging industry.
In the First Part of the thesis, we have developed processing technologies for the 3D stacking of the homogenous silicon systems. Using them, we have demonstrated a low temperature process to transfer MOS devices on ultra-thin silicon layers (1.5 μm) from a parent substrate to a foreign substrate or stack. In order to enable this transfer, we have analysed and resolved the associated stress issues. Furthermore, we demonstrate three-layer stacking of the ultra-thin silicon layers with functional MOSFET’s in each layer. We extensively characterize the changes in the device performance, which arise due to the transfer process.
In the Second Part of the work, we have demonstrated an approach for stacking the III-nitride-on-Si HEMTs and Si-MOSFETs on to a copper substrate. The developed process flow offers a significant improvement in the device behaviour due to the transfer to a thermally conducting substrate like copper. The functional AlGaN/GaN epi-layer stack from the HEMT-on-silicon wafer is lifted-off and bonded to a copper substrate using novel Cu-In bond. Next, an ultra-thin silicon layer (~1.5 μm) with functional NMOS transistors fabricated in-house, on an SOI wafer are separated from the parent SOI wafer
and then stacked over the GaN devices already bonded on the copper substrate, using cost-effective epoxy bonding approach. The devices are characterised to study the improvements in their performance.
In the Third Part, we have demonstrated a 3D integration method for miniaturisation of hybrid systems. Using this 3D packaging technique, a fluorescence-sensing platform consisting of (i) a silicon photodetector, (ii) plastic optical filters, (iii) commercial LED and (iv) a glass micro-heater chip is demonstrated. We have resolved several fabrication challenges related to planarization, stacking and interconnection of these divergent chips. The above process flow developed in this work, can be scaled to stack a larger number of layers for achieving more complicated systems with enhanced functionality and applications.
Finally, we have demonstrated interconnection methodologies using the nonconventional inkjet printing technique for via filling to enable identical die size stacking
Towards the development of open-chip digital microfluidics platform
No full textManipulating and utilizing fluid flows at microscale provides several opportunities towards technological advancement in different domains such as (but not limited to) lab-on-chip devices for mimicking biological laboratory settings in an automated manner, wearable devices for continuous health monitoring, body-on-chip devices towards personalized medicine goal, electronics cooling techniques for efficient thermal management of semiconductor devices. Engineering such microscale fluid flow devices comes under the study of microfluidics. There has been various development in continuous, droplet and digital microfluidics approach. The microfluidic technology has potential to automate the laboratory procedures while reducing the sample and reagent consumption during analysis. However, there is plenty of room towards a fully integrated lab-on-chip platform comprising of all the steps such sample preparation, analyte separation/enrichments, detection, and final readouts.
In this doctoral work, an attempt has been made towards development of open digital microfluidics platform that can be integrated with channel-based microfluidic devices. The digital microfluidics techniques provide solution for automated sample preparation by manipulating discrete droplets on a planar substrate. While the channel-based devices are suitable for downstream analysis of samples e.g., single cell analysis. Thus, bringing together these two techniques of manipulating fluid will help in the development of integrated sampling and analysis device. However, the conventional digital microfluidics devices comprise of squeezed droplets using cover slips. This prevents accessibility to droplets and integration of other sampling and detection devices. Thus, open digital devices provide an alternative solution in which droplet is not covered from top side. There are different techniques to manipulate droplets such
as optical, surface acoustic wave (SAW), magnetic actuation and electro actuation. In this work, electrowetting-on-dielectric (EWOD) based digital microfluidics devices has been used. Open-chip droplet manipulation using electrowetting enables micro-total-analysis systems with multiple sensor integration and re-routing capabilities. In literature, researchers have explored unit processes like droplet transport and mixing on open-chip digital microfluidics platform. But splitting of droplet has always been considered as bottleneck. The splitting of droplet is crucial for sample separation and creating dilution ratio.
Initially, the challenge in open-chip droplet splitting is explored. An energy-based simulation modes is developed using surface evolver. It shows that splitting a sessile water droplet is impossible on an open-chip configuration because of the low pad contact angle requirement. Low contact angles cannot be achieved due to contact angle saturation in electrowetting. Further, the splitting of surfactant-loaded single-phase sessile droplets is presented and explain it using a preferential surface charging phenomenon
Later, an alternative solution has been proposed for droplet splitting using compound droplet (droplet is engulfed in an oil shell). The planar electrode configurations and regime of electrowetting numbers for which splitting can be achieved are identified. It was observed that larger gaps and higher electrowetting numbers favour symmetrical splitting because the electrostatic force driving the actuation is significantly higher than the retarding interfacial forces. Conversely, asymmetrical splitting has been obtained when the actuation force is barely sufficient.
In the later part of the thesis, a scalable open EWOD device is presented that can be used for study of multi-droplets non-coalescence phenomenon using compound droplets. The droplet non-coalescence is an interesting phenomenon that is observed in nature. This phenomenon of non-coalescence is slightly counter-intuitive as we expect liquid interfaces of the same surface tension to merge when they come in contact. However, with the help of modulating oil film in between the liquid interface, non-coalescence is observed for long durations. In this work, we have achieved the non-coalescence of multiple compound droplets on a coplanar EWOD device. The effect of droplet volume on the non-coalescence phenomenon has been studied in two-droplet
systems. We have obtained the non-coalescence regime map for different operating parameters of applied voltage and frequency. We have also explored three-droplet systems and obtained a non-coalescence regime.
For developing an integrated platform there is a need for channel-based sampling and analysis device which can be integrated with digital microfluidic sample preparation platform. However, for controlled sampling, in-situ pressure measurement is very important. The pressure measurement in a microfluidic device is useful for several other purposes as well such as fluid flow effect study on cells, measurement of mechanical properties of cells, etc. This work presents a cleanroom-free and simple technique to integrate pressure sensor in microfluidic devices. In this work, we demonstrated a novel technique of patterning Ti3C2-MXene on PDMS membrane using inkjet printing. We showed the piezoresistive response of inkjet printed MXene that has high sensitivity and can detect low strain value of 0.0003. The response time of the sensor is around 200 ms. The printed layer has been tested for 9000 cyclic loading for durability test and it shows very consistent behaviour. The printed MXene layer has been used as pressure sensor in closed-chip microfluidic device. We developed a simple way to integrate sensors by transferring thin PDMS layer followed by sensor integration using inkjet printing. Later, we demonstrated the applicability of our process to print Wheatstone bridge on microfluidic device to measure pressure. We also demonstrated touch sensor, temperature sensor and ultra-sensitive pressure sensor by using 8 microns thick PDMS membrane. This technique provides a way for localized pressure sensing in the microfluidic device with simple electrical readout and opens further prospect to study strain effect on endothelial cells, deformability of cells in microfluidic flow cytometry, etc.
In the future work, the complete idea of integrated platform is presented that has been envisioned to have multiple robotic limbs each armed with different sampling and analysis device
Fabrication and Characterization of Nanostructured Antibacterial Surfaces
No full textBacterial antibiotic resistance is becoming wide spread due to the excessive and unregulated use of antibiotics in healthcare and agriculture. At the same time the development of new antibiotics has become slow. Adding antibiotics to surfaces result in poor long-term performance in preventing bacterial build-up. This also increases the risk of development of more drug resistant strains. Hence, approaches for realising antibacterial action through physical surface topography have become increasingly important and interesting to the community in recent years. The complex and strain dependent nature of the bacterial cell wall interactions with nanostructured surfaces leads to many challenges while the design of nanostructured antibacterial surfaces is concerned. First part of this work focuses on enhancing the antibacterial activity of the nanostructured surfaces by coating them with different chemistries. Using two different categories of coatings firstly metals (Cu and Ag) and secondly biocompatible polymer chitosan, we demonstrate efficient bactericidal activity against a range of bacteria. The second part of the work focuses on developing processing technology for demonstration of such nanostructured antibacterial surfaces for practical applications. The focus was to demonstrate a low-cost processing technology which can be easily scaled to large area. Finally, we test the bactericidal efficacy of the developed surfaces against the drug resistant strains obtained from a hospital.
In nature several insects such as cicada wing, dragonfly wing, dronefly wing possess sharp nanostructures on their wing which kill bacteria by contact killing mechanism. When a bacterium sits on such surfaces, they get stretched and deformed while trying to settle on maximum anchoring points. When the threshold of stretching is reached, cell wall is compromised, and cell lysis takes place. In this work a range of surfaces with distinct surface topography and chemistry has been studied. Initially, inspired from dragonfly wing, high-aspect ratio silicon nanostructured surface (NSS) was fabricated using a single-step deep reactive ion etching (DRIE) technique. The nanostructures were found to be random in both size (300-1100 nm) and spatial distribution (300-500 nm). Post fabrication the surfaces were coated with a thin layer of copper (NSS_Cu) and silver (NSS_Ag). The bactericidal efficacy of the NSS_Cu, NSS_Ag and NSS surfaces were tested and compared against Gram-negative bacterium E. coli. NSS_Cu was found to have the highest bactericidal efficacy killing 97% of the bacteria in just 90 minutes. The results from this study suggests that the addition of a surface chemistry to the physical nanostructures enhances the bactericidal efficacy. However, copper is not stable when exposed to environment and oxidises to form CuO, Cu2O etc. To overcome this problem, we replaced copper with a stable biocompatible polymer “Chitosan (CHI)”. Unlike copper coating where sputtering tool was used, CHI can be coated on any substrate by a simple dip coating technique making the process simpler and cost-effective. CHI was coated on flat silicon (Si_CHI) and NSS surfaces (NSS_CHI). The bactericidal efficacy of the surfaces was tested against Gram-negative E. coli and Gram-positive S. aureus. NSS_CHI surface was found to be the most efficient in killing bacteria as compared to the Si_CHI and NSS surfaces. Also, the antibiofilm characteristics of these surfaces was studied. NSS_CHI surface was found to have the least amount of bacterial bio mass on its surface after a period of 5 days of bacterial incubation in Luria Broth (LB) medium for both E. coli and S.
ii
aureus. Also, the CHI coating was found to be very stable when exposed to PBS for 7 days showing its durability for a longer period.
CHI coating was cost-effective and easy as compared to the sputtering technique. However, fabrication of NSS using DRIE still comes with a cost. To overcome this issue, we fabricated ZnO nanostructured surface using simple chemical synthesis method at near room temperature (~20O C). Neither sophisticated tool like DRIE nor clean room environment was required for this process. Sharp ZnO nanostructured surface was fabricated in an alkaline solution containing zinc nitrate hexahydrate and potassium hydroxide. The synthesis time was set to 12 hours (h). The ZnO nanostructures possess a length of 1.5-2 m, tip diameter ~20 nm, tip angle ~ 10O. Also, this technique was used to grow the ZnO nanostructures on a variety of substrates such as copper sheet, glass, polydimethylsiloxane (PDMS) showing the versatility of the fabrication technique. The antibacterial performance of the ZnO nanostructured surface was evaluated against Gram-negative E. coli. Flat silicon surface and silicon surface coated with 20 nm of ZnO thin film were taken as controls. Bacterial attachment was seen on the flat silicon and flat ZnO substrates after a 24 h of incubation. In contrary no bacterial colony was observed on the nanostructured ZnO surface showing its bacteriophobic behaviour. The simplicity and cost effectiveness of this process makes it possible for this surface to be used in practical applications. Also, large scale fabrication is possible using this technique.
Despite several advances in this area, it is well understood that the micro/nano structures are mechanically fragile. This reduces their reliability and hence increases the cost of use. Moreover, several applications such as aprons, gloves, temporary mats etc. require these surfaces to be flexible. The above requirements call for the development of flexible antibacterial surfaces with mechanical reliability. In addition, the surfaces should be low-cost so that they can be periodically replaced to address the issues with reliability. To achieve this, transferring of copper hydroxide nanostructures onto a curable silicone polymer, polydimethylsiloxane (PDMS), was carried out by a two-step process: (i) copper etching to form nanostructures and (ii) transfer of the copper based nanostructures onto the PDMS surface by mechanical tearing. This PDMS surface decorated with the copper nanostructures (PDMS_Cu) is unique in displaying two functionalities; superhydrophobicity preventing bacterial adhesion and a potent bactericidal effect from the copper nanowires as copper has been regarded as a very good antimicrobial agent from centuries. This process was scaled for large area fabrication for real world applications. Absence of a micro-fabricated template makes this process significantly cheaper and easily scalable as it is not limited by the size of the template. In addition, as the cured polymer strongly holds these nanowires in place, these surfaces showed reliability against abrasion, tape peel and solid weight impact. Also, the surface was superhydrophobic after dry heat, moist heat and UV exposure. The fabricated PDMS_Cu surface was tested against drug resistant E. coli, S. aureus and K. pneumoniae. The surface exhibited excellent antibacterial behaviour against all the drug resistant bacteria. Also, the PDMS_Cu surfaces were kept at several infectious places in the hospital. The flora count on the PDMS_Cu was lesser than the control surfaces showing its superior antibacterial property. The ability of the PDMS_Cu surface to support RAW Macrophage and HeLA cells proliferation was also evaluated using confocal microscopy by staining the cells with DAPI and tubulin. Both the Macrophage and HeLa cells attachment was found to be higher on the coverslip and PDMS substrates as compared to the PDMS_Cu surface which can be attributed to the superhydrophobic property of the PDMS_Cu
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surface. MTT assay method was also used to assess the cell metabolic activity. ~50% RAW macrophage and ~71% HeLa cells were found to be viable after an incubation period of 5 h. Taken together, these data confirm that the PDMS_Cu surface was not cytotoxic to the RAW macrophage and the HeLa cells. To demonstrate its application in healthcare, heartbeat sound recording was carried out via the PDMS_Cu surface. Good quality heart beat sound was recorded showing its plausible use as a thin covering on the stethoscope diaphragm to prevent the transmission of pathogenic flora from one person to another in the hospital.
Every surface studied in this work exhibits unique topography and surface chemistry and they can be used in several applications such as photovoltaic, high efficiency photo detector and sensors, water treatment, food packaging, health care etc
Fabrication of suspended Graphene microchannels and its application in studying graphene-liquid interface
No full textGraphene is a single layer of sp2 hybridized carbon atoms having extraordinary mechanical, optical, and electrical properties. These exotic properties make them attractive for applications in all fields of science and technology ranging from flexible electronics to multifunctional biomedical devices. Most of these applications require residue-free, and large-area graphene. Commonly, graphene is grown on copper using a self-limiting chemical vapor deposition process and then transferred to the required substrate. But there are still several fabrication challenges in realizing graphene-based devices. Both process steps cause corrugations and stress related defects in the transferred graphene. Further, graphene obtained on substrates suffer from many drawbacks like deteriorated electron mobility. Thus, platforms with suspended graphene has been pursued for its immense potential.
Graphene based devices has been primarily for applications in optical and electronic domains. Graphene based mechanical resonators have been demonstrated for several sensing applications. However, use of graphene in microfluidic applications has been limited. The use of graphene and its derivatives (graphene oxide) based layers have also been limited to biochemical sensing where these layers have been used as functionalised electrode surfaces. Flow of fluids with dissolved ions over graphene has been proposed as a technique for harvesting energy. However, there seems to be a lack of agreement between different experiments. This is primarily due to two reasons. Firstly, most studies have been performed on graphene transferred on various surfaces. Variability in underlying surface across studies effects the overall behaviour. Devices with graphene suspended over microfluidic channels have not been achieved yet due to fabrication challenges. Secondly, due to absence of these devices, studies pertaining to fundamental interaction of liquid with free graphene surface has been missing. This work addresses both these issues by first developing a process flow to suspend graphene over microchannels. Then, these devices are used to probe the fundamental nature of interaction between graphene and liquid.
Conventionally, polymer assisted transfer has been used by researchers to solve issues with stress and corrugation. The process has been modified with techniques like the inverted floating method or gradual solvent replacement to obtain suspended graphene on microcavities. But all these processes used multiple wetting and drying steps which alleviate the quality of graphene. Also, these techniques are detrimental in obtaining suspended graphene on microchannels due to the presence of liquid on both sides of graphene. To solve these challenges a novel modified direct transfer (MDT) process was developed. We eliminated multiple wetting and drying step and used a softer substrate to obtain suspended graphene over PDMS microchannels. This is a first demonstration of suspending graphene over polymer microchannels. It is a cleaner and gentler approach leading to good yield. The process was optimized, and graphene was repeat ably obtained on PDMS channels with 5 μm width and 10 μm depth.
The suspended graphene was characterized using scanning electron microscopy (SEM) and Raman spectroscopy. Suspended graphene was confirmed to be single layer using Raman spectroscopy. Defects such as corrugations, holes, cracks, wrinkles, and rolled up edges were characterized in suspended graphene. The stress in graphene due to the transfer process was analyzed using vector analysis of G and 2D peaks. The direct transfer process was used to suspend graphene on Si/SiO2 microchannels. Graphene suspended on PDMS microchannels using MDT process was compared with graphene suspend on Si/SiO2 microchannels. Less defects were observed on graphene suspended using MDT process, thus better quality.
Graphene-liquid interaction is a controversial problem and there have been multiple studies with contradictory results. Our platform can help us in gaining a better insight into such issues. We have demonstrated two key studies that can be done using our platform. The first study probes the graphene-liquid interface using dynamic atomic force microscopy. In this study, open microchannel with suspended graphene was filled with liquid using capillary forces. This led to a configuration with graphene interacting with liquid on one side and air on another side. Such a configuration allows us to mechanically probe the graphene-liquid interface using an AFM. We have probed this interaction and imaged the suspended graphene with water underneath graphene. As compared to suspended graphene, imaging with water underneath provides better quality due to the apparent physical support from the underlying water. Phase imaging was used to clearly distinguish the corrugated area and non-corrugated area in graphene. It is a unique and novel study that help us to understand the wettability of graphene.
Even though there have been several studies pertaining to the static contact angle of liquids on pristine graphene, contact line dynamics on graphene has not been studied before. It is not known whether, contact line dynamics shows conventional stick slip behavior on suspended graphene. The second study measures the dynamics of the contact line on suspended graphene. Microchannels with suspended graphene were filled using capillary wetting. Evaporation leads to drying. Liquid meniscus moves across the suspended graphene while drying. The meniscus gets pinned at various defects. The liquid dynamics were captured using a high-speed camera. The strength of various defects was compared using pinning time and meniscus speed.
Apart from suspended graphene on PDMS microchannels, a fabrication technique was developed to fabricate graphene sensors in microchannels. This is a unique method to transfer graphene on a flexible and soft substrate that eliminates the need for any polymer support or any process involving multiple wetting and drying processes. The copper on which graphene was grown were used as connection for electrical read out. The electrical read-out was used as etch stop for graphene transfer process. The same platform can be used as solution-gated graphene-based fields effect transistor
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