239 research outputs found
Effect of Poly(ethylene oxide) Molecular Weight on the Pinning and Pillar Formation of Evaporating Sessile Droplets: The Role of the Interface
No full textWe report on the drying process of
sessile droplets of aqueous
poly(ethylene oxide) (PEO) solutions studied by contact angle analysis.
Liquid samples were prepared with the same initial concentration of
four different molecular weights, <i>M</i><sub>w</sub>,
of PEO. Droplets with initial volumes of between 1 and 5 μL
were left to evaporate while temperature, pressure, and relative humidity
were kept constant. Residues were formed with either a disklike puddle
or a distinctive tall conical pillar shape. The latter occurred following
a four-stage deposition process: pinned drying, during which the contact
line is stationary; pseudodewetting, where the receding contact line
is induced by precipitation; bootstrap building, during which the
liquid droplet is lifted on freshly precipitated solid; and late drying.
Contact angle analysis allowed us to monitor all stages during drying
and consider transitions between stages for different molecular weights.
We illustrate the mechanisms taking place during the crucial stages
of pinning and depinning, revealing the effect of adhesion and contact
line friction for high molecular weights and its influence on the
final morphology of the dried PEO solute. To this end, we performed
PEO solution droplet evaporation on PEO and PTFE films demonstrating
the importance of interfacial interaction phenomena. We show that
the formation of disklike puddles for high molecular weights on glass
is associated with continuous droplet contact line pinning. This results
from the strong adhesion due to the interdigitation of the loops and
tails of a polymer layer (adsorbed on glass during evaporation) with
the polymer gel network inside the droplet that forms as water evaporates
Experimental and theoretical investigation of the interfacial phenomenon associated with wetting of trisiloxane surfactant solutions
No full textSurface active agents have been successfully employed in numerous industrial,
agricultural and biomedical applications for decades. Trisiloxane surfactants in
particular have proved to be exceptionally effective as wetting enhancers; hence the
name ‘superspreaders’. Since the early ‘90s these extraordinary surfactants have
become an irreplaceable component in various products and processes. However, the
true nature of their specific wetting behaviour has not been fully revealed and their
underlying wetting mechanisms are still poorly understood despite substantial
scientific interest during the last decades. In this thesis is an attempt to shed light on
specific wetting and spreading behaviour of trisiloxane solutions.
Commercial superspreader products were tested in various environments in order to
get further insight into their performance in specific practical applications.
Experimental investigation of wetting of superspreader solutions on surfaces of
different hydrophobicity and comparison to that of a conventional surfactant revealed
superiority of trisiloxanes. Exceptional interfacial activity was explained in terms of
the specific chemical structure and ‘T’-shape of the molecule. However, sensitivity
of the trisiloxane head to low pH and long-time ageing in aqueous environment was
revealed. Performance of binary mixtures of commercial superspreaders and
conventional surfactant was also assessed. Behaviour of trisiloxanes in the capillary
action was studied. Finally, a comprehensive mathematical model for trisiloxane
wetting, which incorporates diffusion as the governing factor of the wetting process,
was developed
Surface nano-patterning using the coffee-stain effect
No full textAddition of nanopacticles in a base solvent leads to suspensions with enhanced physiochemical
properties, compared to base solvent. This new type of suspensions is called
nanofluids, with applications ranging from biomedicine to automotives. As a
consequence extensive research is being conducted in the field, in particular, on the
evaporation of these fluids as it leads to well-defined and highly ordered coffee-rings.
However, the exact physics governing this phenomenon remain elusive. The goal of
this experimental investigation is to elucidate how various parameters affect the
progression of nanofluid coffee-stain formation.
Examination of the coffee-ring structuring, produced by the free evaporation of sessile
droplets containing nanoparticles, revealed an unexpected, disordered region at the
exterior edge of the ring. A self-assembly mechanism with two components, particle
velocity and wedge constraints, was proposed to describe the deposition of particles at
contact lines of evaporating drops.
Environmental pressure was used as a method to control particle crystallinity in the
coffee-rings. Essentially, evaporation rate and pressure were found to be inversely
proportional. Macroscopically, lowering pressure led to a transition from “stick-slip”
to constant pinning. Nanoscopically, lowering pressure promoted crystallinity.
Findings supported the proposed, in this thesis, particle self-assembly mechanism.
Particle aspect ratio and flexibility were subsequently examined. Pinning strength was
found to be a function of particle aspect ratio and rigidity, leading to constant pinning.
The proposed, in this thesis, particle self-assembly mechanism was found to be
applicable to a variety of aspect ratios and flexibilities. Lastly, particulate crystals grew
following different pathways depending on particle flexibility
Hydrodynamics, heat transfer and flow boiling instabilities in microchannels
No full textBoiling in microchannels is a very efficient mode of heat transfer with high heat and
mass transfer coefficients achieved. Less pumping power is required for two-phase
flows than for single-phase liquid flows to achieve a given heat removal.
Applications include electronics cooling such as cooling microchips in laptop
computers, and process intensification with compact evaporators and heat
exchangers.
Evaporation of the liquid meniscus is the main contributor to the high heat fluxes
achieved due to phase change at thin liquid films in a microchannel. The microscale
hydrodynamic motion at the meniscus and the flow boiling heat transfer mechanisms
in microchannels are not fully understood and are very different from those in
macroscale flows. Flow instability phenomena are noted as the bubble diameter
approaches the channel diameter. These instabilities need to be well understood and
predicted due to their adverse effects on the heat transfer.
A fundamental approach to the study of two-phase flow boiling in microchannels has
been carried out. Simultaneous visualisation and hydrodynamic measurements were
carried out investigating flow boiling instabilities in microchannels using two
different working fluids (n-Pentane and FC-72). Rectangular, borosilicate
microchannels of hydraulic diameter range 700-800 μm were used. The novel
heating method, via electrical resistance through a transparent, metallic deposit on
the microchannel walls, has enabled simultaneous heating and visualisation to be
achieved. Images and video sequences have been recorded with both a high-speed
camera and an IR camera.
Bubble dynamics, bubble confinement and elongated bubble growth have been
shown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodic
instabilities have been observed during flow boiling in the microchannel.
Analysis of the IR images in conjunction with pressure drop readings, have allowed
the correlation of the microchannel pressure drop to the wall temperature profile,
during flow instabilities.
Bubble size is an important parameter when understanding boiling characteristics
and the dynamic bubble phenomena. In this thesis it has been demonstrated that the
flow passage geometry and microchannel confinement effects have a significant
impact on boiling, bubble generation and bubble growth during flow boiling in
microchannels
Fundamentals of dropwise wetting and evaporation phase-change of binary mixture droplets on micro-decorated surfaces
No full textAlmost every aspect of our daily lives involves liquid-surface interactions and is intimately related to the physicochemical properties of the substrate as well as those of the liquid. Understanding the mechanisms taking place at the initial state of the evaporation process i.e., wettability, and during the evaporation of droplets is of great interest to many industrial, biological, and medical applications. This research investigates experimentally the initial wettability states and the evaporation modes for droplets of pure water, pure ethanol and their binary mixture, accessing a wide range of surface tensions, on hydrophobic and hydrophilic micro-pillared surfaces with fixed height and diameter whilst varying the spacing between the pillars.
On one hand, the initial wetting states of pure fluids and their binary mixtures on intrinsically hydrophobic micro-decorated surfaces are first studied and a wetting regime map is proposed. This regime map predicts the droplets’ symmetrical and asymmetrical shapes and wetting dependence on the fluid surface tension and the surface structure on the hydrophobic microstructured surfaces, which in turn govern the subsequent evolution of the droplet contact angle and contact radius. Four different evaporation modes have been observed which are consistent with the literature and further two evaporation modes have been revealed here for the first time, namely, increasing contact angle mixed-sick- slip mode and decreasing contact angle mixed-stick-slip mode.
On the other hand, on intrinsically hydrophilic surfaces, the same systematic experimental study is applied using the same fluids and the same microstructured surfaces. It is remarked that the wettability and evaporation on hydrophilic structured surfaces can be affected by ambient exposure after subjecting the surfaces to air plasma cleaning, which eventually removes any deposition of hydrocarbons ever present in the ambient. Unlike the hydrophobic surfaces, the hemi-wicking and spreading regimes are further observed on these surfaces which, consequently, affect the evaporation process. The same six evaporation modes have been observed on these surfaces with different durations though. Investigating the initial wetting and the evaporation modes can lead to a better understanding of choosing the proper structure and wettability (and/or ambient exposure) combined with the correct binary mixture concentration to be specifically tailored to different applications
From wetting to withering: the life and times of droplets on microstructured terrains
No full textThis thesis presents a detailed exploration into the dynamics of droplet wetting and evaporation on microstructured surfaces, bridging the interdisciplinary fields of physics, chemistry, and surface engineering. Each chapter methodically delves into a distinct facet of this complex interplay, ranging from non-spherical droplet morphologies and nanoparticle deposition patterns to the novel phenomenon of droplet spawning within binary fluid mixtures. The investigation begins with pure fluids and progressively extends to nanofluids and binary mixtures, offering a comprehensive view of fluid behaviour on structured surfaces.
Initially, the research is anchored in understanding the morphological control of droplets on textured surfaces. It places significant emphasis on the influence of surface roughness and microscale topography in shaping droplet wetting and evaporation dynamics. A pivotal finding is how micropillar spacing and geometry critically affect droplet shapes, inducing distinct morphologies and modulating evaporation rates, thereby revealing the nuanced interaction of fluid physics with surface characteristics.
As the thesis progresses, the focus shifts to complex fluids, particularly the behaviour of aluminium oxide nanofluid droplets. Here, the role of interpillar spacing, geometry, and nanofluid concentration is systematically examined. The research uncovers the formation of sophisticated nanoparticle networks, highlighting concentration-dependent self-assembly processes significantly influenced by the geometric constraints of microstructured surfaces.
The study concludes with an innovative investigation into the behaviour of binary ethanol-water mixtures in a hemiwicking state on microtextured surfaces. This research unveils the spontaneous emergence of mini-droplets, a phenomenon driven by the differential volatility of the fluid components and the resultant changes in local surface tension.
Collectively, this thesis offers novel insights into droplet behaviour on microstructured surfaces and lays the groundwork for potential technological applications. By bridging theoretical principles with experimental observations, the research opens new avenues for technological advancements and sets a foundation for future explorations in the evolving landscape of fluid dynamics
Mathematical models for the evaporation of sessile droplets
No full textDespite its intrinsic complexity and multidisciplinary nature, considerable insight can be gained into many aspects of the evaporation of sessile droplets by using relatively simple mathematical models. In this chapter we describe some of these models and some of the insights they bring to our understanding of this fascinating problem, focusing almost entirely on models that can be solved analytically either fully or, in some cases, up to quadrature of known functions
A study on the evaporation and desiccation patterns of bio-drops for the development of a disease diagnostic tool
No full textEarly diagnosis and treatment are crucial issues in medicine. Disease leads to alterations in the properties of biological fluids, which in turn, lead to changes in the final desiccation patterns arising from droplet evaporation on surfaces. Hence, investigating the drying patterns of bio-droplets could provide the means for rapid medical diagnosis. The aim of this thesis is to probe the effect of various parameters on the evaporative behaviour and final desiccation patterns of micro-litre bio-fluid droplets.
Initially, the evaporation of aqueous saline drops is studied, as ions are present in human serum and urine. The study reveals the development of a crystallisation-driven flow at the final drying stages, manifested by strong jets of flow towards the growing crystals and accompanied by the existence of vortices in each side of the growing crystals. This flow is attributed to the interplay between continuity and Marangoni convection.
The effect of substrate temperature on the evaporation and dried patterns of Foetal Bovine Serum (FBS) drops is then investigated and found to affect the number of cracks and the mechanism of crack formation, as well as the crystallisation patterns in desiccation deposits. Mechanisms affecting crack formation are examined, indicating that for high substrate temperatures protein denaturation must be taken into account. Additionally, the morphology of crystallisation patterns is probably related to thin film phenomena occurring at the final stages of drying.
The addition of salts (NaCl or CaCl2) to FBS drops is probed, leading to various morphological features that depend on the salt type and initial ionic strength. NaCl is found to cause faster dehydration compared to CaCl2. Interestingly, the deposits occurring from evaporation of FBS drops, that consist of high CaCl2 concentrations, are sensitive to relative humidity (RH) changes and manifest the ability to deliquesce (upon increase of the RH) and re-crystallise (upon decrease of the RH). The addition
of NaCl to FBS-CaCl2 mixtures promotes droplet dehydration and exhibits distinct evaporative dynamics.
The effect of urea concentration on the dried patterns of aqueous saline drops is subsequently examined under polarised light, revealing that urea selectively deposits upon the already formed salt crystals towards the end of drying. The final crystalline patterns initially show up as isotropic structures, but birefringence manifests shortly after pattern formation, indicating the anisotropy caused by urea deposition
3D Electrospinning: the combination of electrospinning and 3D-printing for the fast fabrication of designed 3D polymeric macrostructures made of nanofibres
No full textNanofibrous structures, due to their unique morphology boasting a high surface areato-volume ratio, make them interesting in several research fields. Added to that, recent
studies have highlighted the important benefits of assembling 3D structures with
nanofibrous features. For example, 3D scaffolds made of nanofibres have been shown
to have better cell attachment and growth, because of their close resemblance to the
natural extracellular matrix. 3D nanofibrous structures have also been used in high
filtration processes.
Electrospinning is a good candidate for building these 3D nanofibrous structures. The
high versatility of electrospinning allows it to change the morphology of the electrospun
fibres easily (porosity, diameter, surface roughness, fibres alignment, etc…). The
nature of the polymer used in electrospinning is flexible as well, and with the existing
possibility for further functionalizing electrospun fibres, electrospinning is applicable to
a wide range of research fields. Furthermore, several methods of inducing 3D build-up
via electrospinning have been investigated, however, these methods have several
disadvantages such as being time-consuming, made of several steps, requiring an
extra support material, or having no control over the shape of the final 3D structures.
In this thesis, a device combining the versatility of electrospinning with the
manoeuvrability of 3D printing is studied. By inserting specific additives to a polymer
solution, self-assembly of 3D structures via electrospinning is possible. The precise
control of the movement of the nozzle head during electrospinning, as well as the
setting of the collector height allow to direct the position of the deposition area during
the whole electrospinning process.
By combining these two features together, it is possible to fabricate a designed 3D
polymeric macrostructure made of nanofibres, from a simple computer-aided design
(CAD) file. Thus, this technology is named “3D electrospinning”.
The first aspect of this thesis is to have an in-depth look at the formation mechanism
of 3D electrospun structures. The process parameters of 3D electrospinning have been
identified and investigated to better understand the formation mechanism of the 3D
build-up for polystyrene (PS), the model polymer. It is shown that the crystal phase of
the polymer itself, the viscosity and the conductivity of the polymer solution have no
influence on the 3D build-up of the electrospun structures. It was instead shown that
the rapid solidification of the fibres as well as the in-situ charge induction and
polarization of the fibres are inducing the 3D build-up. Overall, it is possible to build a
3~4 cm high macrostructure in a single step in 10 minutes of electrospinning.
A thorough study of the experimental parameters of 3D electrospinning allows to
optimise the shaping of the 3D electrospun structures, in terms of wall resolution and
fibres’ morphology. It is shown that the improper adjustment of any parameters such
as polymer concentration, applied voltage, working distance, flow rate or nozzle
moving speed can have detrimental effects on the 3D build-up and instead leads to an
electrospun flat 2D mat. After identifying the optimal experimental parameters, several
shapes such as triangle, square, star or smiley face, are electrospun to showcase the
versatility of the 3D electrospinning process. Other polymers such as polyacrylonitrile
(PAN) and polyvinylpyrrolidone (PVP) are then 3D electrospun to extend the range of
usable polymers, and thus potential applications, for 3D electrospinning. The
differences in shape induced by each polymer are identified.
Different types of electrodes are then used to change the electric field profile and alter
the path of the electrospun jet. Base electrodes and steering electrodes are shown to
have detrimental effects on the 3D build-up leading to either poor drying of the fibres
or poor shaping of the 3D structures. Guiding electrodes (electrode at the collector
level) were able to further enhance the shaping of the 3D electrospun structures, with
an increased wall resolution and no noticeable drawback. However, this beneficial
effect was only shown for polystyrene. A pillar support was used as a guiding electrode
to force the fabrication of an electrospun 3D structure with radial alignment. The effect
of the pillar height, pillar thickness, applied voltage and working distance on the 3D
structure and fibres alignment is shown.
Finally, the long-term stability and the mechanical stability of the 3D electrospun
structures have been investigated. While both PS and PAN structures show high shelflife in ambient conditions, only 3D PS structures demonstrate some shape recovery
after compression.
Upscaling of the 3D structures was then achieved with both the 3D electrospinning
device and a nozzle-free electrospinning setup. Similar to extrusion-based 3D printing,
it is possible to raise the working distance during 3D electrospinning to increase the
final height of the 3D structure. 3D electrospinning with a nozzle-free electrospinning
setup is possible via precise control of the rotation speed during 3D build-up. This
opens up the possibility to fabricate electrospun 3D structures on a commercial scale.
Carbonization of 3D PAN structures is also demonstrated, to fabricate carbon fibrous
structures with 3D features, which can have applications in energy-related fields.
The work conducted in this thesis has successfully expanded upon the field of 3D
material fabrication. 3D electrospinning is a simple, cost-effective and fast process to
build designed 3D structures. It is a versatile process not limited to a single type of
polymer. As such, 3D electrospinning is a viable technique for several applications. 3D
PS and 3D PVP could both be used as a cell culture material. 3D PAN, which is a
typical precursor for carbon fibres, could be used as an electrode material for Lithiumion batteries
Experimental investigation into the evaporating behaviour of pure and nanofluid droplets.
No full textIn this experimental investigation the evaporative behaviour of liquid droplets of both pure fluids and fluids containing nanoparticles was studied. Initial tests were conducted on drops of pure volatile liquids using IR thermography, and the effect of substrate material, drop composition, and substrate temperature was investigated. The effect of the addition of nanoparticles to the liquid drops was then investigated using a contact angle analyser which could record the drop profile in time. The effects of liquid composition, nano-particle composition, nanoparticles concentration, substrate hydrophobicity, and substrate temperature were all studied.
Results obtained from IR thermography showed that there exists interfacial temperature instabilities in evaporating volatile drops, the appearance of these fluctuations was found to be dependent on the liquid and substrate in question and are self generated temperature gradients resulting from non-uniform evaporation. A stability analysis was conducted and the results give a good agreement with experimental results.
The addition of nanoparticles to a liquid drop was found to alter the evaporative behaviour by enhancing pinning of the drop contact line and preventing the drop radius from shrinking. By manipulating the concentration of the particles suspended in a drop, a stick-slip evaporative process was achieved, leading to rings of particulate material formed upon total evaporation. By varying parameters such as substrate hydrophobicity, nanoparticle concentration, liquid composition, and substrate temperature, many distinct nanoparticle deposit patterns were observed upon total evaporation. It was shown that by varying these parameters, many different patterns could be achieved, and that inside these deposit patterns regular formations such as particulate rings, radial lines, and cellular structures were present
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