1,721,011 research outputs found
Optically induced electrokinetic concentration and sorting of colloids
No full textWe demonstrate an optically induced ac electrokinetic technique that rapidly and continuously accumulates colloids on a parallel-plate electrode surface resulting in a crystalline-like aggregation. Electrothermal hydrodynamics produce a microfluidic vortex that carries suspended particles toward its center where they are trapped by local ac electrokinetic hydrodynamic forces. We characterize the rate of particle aggregation as a function of the applied ac voltage, ac frequency and illumination intensity. Hundreds of polystyrene particles (1.0 mu m) suspended in a low conductivity solution (2.4 mS m(-1)) were captured at a range of voltages (5-20 V-pp) and frequencies (20-150 kHz) with an optical power of approximately 20 mW. This technique was not restricted to near infrared (1064 nm) illumination and was also demonstrated at 532 nm. The sorting capability of this technique was demonstrated with a solution containing 0.5 mu m, 1.0 mu m and 2.0 mu m polystyrene particles. This dynamic optically induced technique rapidly concentrates, sorts and translates colloidal aggregates with a simple parallel-plate electrode configuration and can be used for a variety of lab-on-a-chip applications
3D-Printing of Lunar Soil Simulant by Direct-Extrusion method.
No full textThe extrusion-based additive manufacturing (EAM) technique is recently being widely employed for the 3D printing of complex-shaped components made of ceramic powder (containing irregular-shaped particles) when it is cast in the form of a slurry/ink. In this work, we utilize a direct extrusion method for printing structures from extra-terrestrial soil simulants using a piston-based extruder. Printing is demonstrated using a slurry composed of lunar soil simulant (LSS) variant ISAC-1 (avg. particle size ~ 90µm) mixed with biopolymer guar gum as a sustainable binding agent and DI water as a solvent. Parts were printed using a 2 mm diameter nozzle by optimizing print speed, nozzle height, inter-layer drying time, and build temperature, to ensure shape retention post-printing. The final green parts were dried in a hot air oven (50°C) for 48hrs, followed by sandpaper polishing. The strengths of the printed specimens were evaluated using compression and flexure tests and were found to be comparable to that of bio-consolidated structures. Unlike solid geometries, the well-known shell-infill type area-filling strategy generated several travels and re-tracings in the toolpath for cellular geometries. Owing to the yield stress of slurry, the travels and re-tracings resulted in discontinuous print and poor dimensional accuracy respectively. This necessitated a toolpath with increased continuity in the extrusion path. The customized toolpath is generated by defining a continuous nodal path over a lattice structure corresponding to the cellular frame. The extrusion flow rate is tuned according to the nodal path and the requirement of material deposition. Qualitatively the increased extrusion continuity in the customized toolpath resulted in continuous print with improved dimensional accuracy, whereas quantitatively a significant (~ 60%) reduction in print time is observed. These results show the potential for using the direct extrusion 3D printing method in remote extra-terrestrial environments to obtain lightweight load-bearing structures like cellular frames
Sub-Newtonian Coalescence in Polymeric Fluids
No full textDroplet coalescence is a thermodynamic equilibration process driven by surface energy
minimization. The physics of this phenomenon is characterized by the temporal evolution
of a liquid bridge formed upon the proximate approach of two droplets. This phenomenon
is ubiquitous, manifesting in processes linked to life like those in raindrop formation,
growth of tumor cells, and industrial processes like those in combustion, spray paintings,
and coatings. Despite these universal occurrences, studies on the coalescence of complex
fluid droplets remain scarce in the literature. Unlike Newtonian fluids, complex fluids
have signature micro-structures that can result in a wide range of responses depending
on external perturbations making a unified model elusive. Such diversity in micro structures and flow behaviors have resulted in classifying these in sub-classes ranging
from polymers to suspensions. But there has been a recent surge in studies investigating
coalescence dynamics in macromolecular-based micro-structure fluids, i.e., polymeric
fluids. However, a detailed work on developing a theoretical framework for coalescence
in complex fluids remain unknown. Here in this thesis we propose such frameworks for
polymers, suspensions and dispersions. Further based on our observations for polymeric
fluids, we propose the existence of new coalescence regime, namely the sub-Newtonian
regime with arrested coalescence as its limiting case.MHR
Coalescence of Polymeric Droplets
No full textCoalescence is an energy minimization phenomenon in which two equilibrium droplets undergo
a transition to attain a final equilibrium state, i.e., a coalesced state. Coalescence begins
with a point contact between the two drops followed by a liquid bridge of size comparable to
the diameter of the droplets. This phenomenon is more complex for macromolecular fluids
like polymeric solutions than its counterpart Newtonian fluids due to molecular relaxations
and chain entanglements. Under experimental conditions, coalescence can be achieved in
three different configurations: sessile-pendant, sessile-sessile and pendant-pendant. This study
demonstrates the coalescence dynamics of polymeric droplets in sessile-pendant and sessilesessile configurations. To probe this phenomenon in various configurations, we quantify the
growth of liquid neck.
The dynamics of the sessile drop coalescing with the pendant drop is governed by the growth
of neck radius R with time t. In this configuration, we unveil the existence of three regimes
based on concentration ration c/c
∗
, namely, inertio-elastic (IE) c/c
∗ < ce/c
∗
, viscoelastic (VE)
ce/c
∗ < c/c
∗ < 20 and elasticity dominated (ED) regimes c/c
∗ > 20. Our results suggest that
the neck radius growth follows a power-law behaviour R ∼ t
b
. The coefficient b is constant in
IE, VE and with a monotonic decrease in ED. Based on the variation of b in ED, we propose a
new measurement technique named Rheocoalescence, which can possibly predict the relaxation
times of the fluids in elasticity dominated regimes.
The constant value of b in IE and VE regimes is found to be 0.37 and is distinct from the
value of 1, 0.5 in viscous and inertial regime respectively of Newtonian fluids. Further, we reveal
the existence of universality in the neck radius evolution following a scale of R ∼ t
0.36. This
universal behaviour is probed across various polymers like polyacrylamide (PAM), polyethylene oxide (PEO), Polyvinyl alcohol (PVA) and polyethylene glycol (PEG) of different molecular
weights using high-speed imaging. Our findings are substantiated by a theoretical model using
the linear Phan-Thein-Tanner (PTT) constitutive equation.
In comparison, coalescence in sessile-sessile configuration is relatively more complex due
to the contact line motion and energy interaction between the solid and liquid interface. In such
a configuration, coalescence can be triggered by volume filling (VFM) or droplet spreading
method (DSM). Coalescence of sessile polymeric fluid drops on a partially wettable substrate via
DSM exhibits a transition from inertio-elastic (IE) to viscoelastic (VE) regime at concentration
ratio c/c
∗ ∼ 1. Our findings unveil that the temporal evolution of the bridge height follows
a power-law behaviour t
b
, such that the coefficient b continuously decreases from 2/3 in the
inertial regime (c/c
∗ < 1) to an asymptotic value of 1/2 in the viscoelastic regime (c/c
∗ > 1).
To account for fluid elasticity and characteristic timescale in the viscoelastic regime, a modified
thin film equation under lubrication approximation has been proposed using the linear PhanThien-Tanner constitutive equation. The temporal evolution of the droplet has been evaluated
by solving the modified one-dimensional thin-film equation using a marching explicit scheme.
The initial droplet shapes are obtained by resorting to energy minimization. A good agreement
between numerical and experimental results is obtained.
The coalescence of two droplets on a solid substrate via the volume filling method (VFM)
has a contrasting behaviour compared to DSM. Similar to DSM, we identify four different
regimes, namely, inertial dominated (ID), inertio-elastic (IE), viscoelastic (VE) and elasticity
dominated (ED) regimes on the basis of c/c*. Our results reveal that the temporal evolution of
bridge height for VFM follows a power law behaviour, such that b remains constant at 2/3 in
ID, IE, VE, followed by a monotonic decrease in ED. Our study unveils that the coalescence
dynamics of polymeric drops are not universal and, in fact, are contingent on the method by
which the coalescence is triggered. Additionally, we demonstrate the spatial features of the
bridge at different time instants by similarity analysis. We also theoretically obtain a universal bridge profile by employing the similarity parameter in a modified thin film lubrication equation
for polymeric fluids
A simple, optically induced electrokinetic method to concentrate and pattern nanoparticles
Full text linkWe demonstrate an optically induced electrokinetic technique that continuously concentrates nanoparticles on the surface of a parallel plate electrode that is biased with an AC signal. A highly focused beam of near-infrared light (1064 nm) was applied, inducing an electrothermal microfluidic vortex that carried nanoparticles to its center where they were accumulated. This technique was demonstrated with 49 nm and 100 nm fluorescent polystyrene particles and characterized as a function of applied AC frequency and voltage. With this technique the location and shape of colloidal concentration was reconfigured by controlling the optical landscape, yielding dynamic control of the aggregation. Colloidal concentration was demonstrated with a plain parallel plate electrode configuration without the need of photoconductive materials or complex microfabrication procedures
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
An optoelectrokinetic technique for programmable particle manipulation and bead-based biosignal enhancement
No full textTechnologies that can enable concentration of low-abundance biomarkers are essential for early diagnosis of diseases. In this study, an optoelectrokinetic technique, termed Rapid Electrokinetic Patterning (REP), was used to enable dynamic particle manipulation in bead-based bioassays. Various manipulation capabilities, such as micro/nanoparticle aggregation, translation, sorting and patterning, were developed. The technique allows for versatile multi-parameter (voltage, light intensity and frequency) based modulation and dynamically addressable manipulation with simple device fabrication. Signal enhancement of a bead-based bioassay was demonstrated using dilute biotin–fluorescein isothiocyanate (FITC) solutions mixed with streptavidin-conjugated particles and rapidly concentrated with the technique. As compared with a conventional ELISA reader, the REP-enabled detection achieved a minimal readout of 3.87 nM, which was a 100-fold improvement in sensitivity. The multi-functional platform provides an effective measure to enhance detection levels in more bead-based bioassays
Insights into evaporation, atomization and precipitate formation of polymer droplets
No full textEvaporation of polymer droplets is an active area of interest due to its applications in systems such as ink jet printing, thin films, spray combustors to name a few. Thus, understanding complex dynamics of evaporating polymer droplets in different experimental configurations is crucial to cater to the wider industrial applications. This study investigates evaporation and subsequent dynamics of two classes of polymeric droplets-low viscoelastic (PAM) and high viscoelastic (PEO) in two different experimental configurations: Laser induced evaporation of droplet under acoustic levitation, natural evaporation of droplet on hydrophobic surfaces.
In the first part, we investigate the interaction of an aqueous low viscoelastic polymer droplet (PAM) with a tunable continuous laser in an acoustically levitated environment. Depending on the laser irradiation intensity and polymer concentration, we observe four temporal phases: droplet evaporation, vapor bubble growth followed by membrane inflation, bubble/membrane rupture through hole nucleation, and droplet breakup. During the initial droplet evaporation phase, concentration build-up at the droplet surface beyond a critical limit lead to the formation of a skin layer. It is revealed that at a given location inside the droplet, hot spots occur, and the maximum temperature at the hot spots scales linearly with irradiation intensity until a bubble nucleates. The low-intensity laser interaction leads to symmetric membrane inflation that eventually forms holes at droplet poles and cracks on the shell surface. On the contrary, high intensity causes early bubble nucleation followed by asymmetric membrane inflation that eventually ruptures through multiple hole formation. Furthermore, the growth and rupture of the membrane is followed by a catastrophic breakup of the droplet. Two dominant atomization modes are reported at significantly high irradiation intensities: stable sheet collapse and unstable sheet breakup. The evolution of droplets into a stable/unstable sheet follows universally observed ligament and hole dynamics. A regime map is shown to describe the influence of polymer concentration and irradiation intensity on the strength and mode of droplet atomization.
In the second part, we investigate the interaction of a aqueous high viscoelastic polymer droplet (PEO) with a tunable continuous laser in an acoustically levitated environment. Depending on the laser irradiation intensity, we observe nucleation of a bubble in the dilute regime of polymer concentration, contrary to the previously observed bubble nucleation in a semi-dilute entangled regime for low viscoelastic modulus polymer droplets. After the bubble nucleation, a quasi-steady bubble growth occurs depending on the laser irradiation intensity and concentrations.
Our scaling analysis reveals that bubble growth follows Plesset-Zwick criteria independent of the viscoelastic properties of the polymer solution. Further, we establish that the onset of bubble growth has an inverse nonlinear dependence on the laser irradiation intensity. At high concentrations and laser irradiation intensities, we report the expansion and collapse of polymer membrane without rupture, indicating the formation of an interfacial skin with significant strength. The droplet oscillations are primarily driven by the presence of multiple bubbles and, to some extent, by the rotational motion of the droplet. Finally, depending on the nature of bubble growth, different types of precipitate form contrary to the different modes of atomization observed in low viscoelastic modulus polymer droplets.
In the third part, we experimentally report the concentration and molecular weight dependence of the deposit patterns of low viscoelastic evaporating polyacrylamide (PAM) droplets on hydrophobic surfaces. We find that with an increase in non-dimensional concentrations c⁄c^* ranging from 0.16 (dilute) to 66.66 (semi-dilute entangled) there is a gradual transition from ring to uniform precipitates. However, with a decrease in the molecular weight of the polymer by one order, the coffee ring formation was not suppressed for the reported range of concentration. We attribute these results to the role played by the critical overlap concentration (c^*) and diffusion coefficient of polymer along with the evaporation modes.
Lastly, the authors report the experiments on the precipitate formation of evaporating (PEO) droplets on hydrophobic surfaces. We observe the final precipitates to be deformed with the formation of central dip over the concentrations ranging from semi-dilute unentangled to semi-dilute entangled.
Overall, this study provides valuable insights into the complex phenomenon of evaporation of polymer droplets in different configurations and its importance in various industrial and natural processes. The findings can help optimize these processes and improve our understanding of them
Experimental and computational approaches to understand collective behaviors of bacterial pathogen Pseudomonas aeruginosa
No full textPseudomonas aeruginosa is a Gram-negative opportunistic pathogen, estimated to account for 15-20% of hospital-acquired infection-related deaths around the globe in a year. This bacterium exhibits two distinct collective behaviors, biofilm formation and swarming motility. During biofilm formation, the P. aeruginosa population is comprised of sessile cells - covered in a self-produced polysaccharide matrix. Swarming, on the other hand, is group motility facilitated by flagella and bio-surfactant. Such cooperative behaviors help P. aeruginosa to thrive in hostile environmental conditions. However, the triggers that regulate these behaviors remain unknown and form the focus of my Ph.D. thesis.
P. aeruginosa forms biofilms on indwelling medical devices such as endotracheal tubes (ETTs), urinary catheters, vascular access devices, tracheostomies, and feeding tubes, often leading to hospital-acquired infections. Pseudomonas aeruginosa is one of the four frequently encountered bacteria causing pneumonia. In the current work, we have established an in vitro model mimicking the biofilm formation on the endotracheal tube. We have identified two-component system (TCS) genes contributing to this process. The TCS comprises a membrane-associated sensor kinase and an intracellular response regulator. We have found that out of 112 TCS mutants studied, 56 had altered biofilm biomass on ETTs. Some of these are novel ETT-specific TCSs that could serve as targets to prevent biofilm formation on indwelling devices frequently used in clinical settings.
Swarming in P. aeruginosa is a collective movement of the bacterial population over a semisolid surface, but specific signals that trigger this motility are unclear. We hypothesized that specific environmental signals could induce swarming in P. aeruginosa. Our data show that a low ethanol concentration under nutrient-limiting conditions provides a strong ecological motivation for swarming in P. aeruginosa PA14. Ethanol serves as a signal and not a carbon source under these conditions. Moreover, ethanol-driven swarming relies on the ability of the bacteria to metabolize ethanol to acetaldehyde using a periplasmic quinoprotein alcohol dehydrogenase, ExaA. We found that ErdR, an orphan response regulator linked to ethanol oxidation, is necessary for the transcriptional regulation of a cluster of 17 genes, including exaA, during swarm lag. Finally, we show that as a volatile, ethanol could induce swarming in P. aeruginosa at a distance, suggesting long-range spatial effects of ethanol as a signaling molecule.
P. aeruginosa exists in multispecies consortia in the environment and during the infection of various hosts, including humans. The physicochemical properties which mediate interactions between P. aeruginosa and its neighbors remain elusive. We began our study using P. aeruginosa and Cryptococcus neoformans, a pathogenic yeast species, in a surface-based co-culture assay. We found that the P. aeruginosa colony spread more on the lawn of C. neoformans. Upon microscopic investigation, we found that P. aeruginosa shows exploratory behavior in proximity to C. neoformans cells, and this exploratory behavior does not require metabolic active yeast cells. We hypothesize that the fluid accumulation near C. neoformans cells plays an essential role in the microscopic interaction leading to the macroscopic growth of the P. aeruginosa colony. To test this hypothesis, we have developed an individual-based model with experimentally motivated constraints such as microbial division time, fluid accumulation near yeast cells, and the motility of individual P. aeruginosa and scaled the parameters to simulate macroscopic behavior using the cellular automata modeling approach. We found that the presence of yeast lawn allowed P. aeruginosa to cover more area by utilizing the fluid accumulated around yeast cells
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