1,721,117 research outputs found
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
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
Surface nano-patterning using the coffee-stain effect
Full text linkAddition 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
A study of passive and active driven motion of droplets on engineered substrates
Full text linkDroplet motion is an everyday phenomenon with potential benefits to multiple
industrial and biological applications. It can be achieved via various methods,
and the understanding and altering of the underlying mechanism are
important to the accurate control of the droplet behaviour and motion. This
thesis focuses on three different mechanisms that induce the droplet motion:
roughness gradient by micro-structure fabrication, thermocapillary motion
with self-rewetting fluid and vapor-mediated droplet motion.
Firstly, the motion of microscale water droplets on the hydrophobic microstructured
surfaces with structural wettability contrast has been studied. The
velocity and displacement of the droplets moving across the wettability
contrasts have been monitored and their relations to the morphological
parameters of the micro-structure have been systematically investigated.
Besides, the dynamic behaviour of the droplets has been investigated and
explained by the mathematical mode proposed.
Secondly, the thermocapillary motion of self-rewetting droplets has been
reported. The behaviour of self-rewetting droplets departed greatly from the
droplets of ordinary mixture and pure fluids. A unique oscillatory behaviour
was observed for self-rewetting droplets, which was related to the nonmonotonic
dependence of surface tension on temperature. Influencing
parameters were studied and IR thermography assisted to reveal the internal
convection.
Last, the motion of sessile mixture or pure droplets induced by vapour was
investigated. The spatial concentration change via the mass transfer through
the liquid-vapour interface near contact line leads to unbalanced surface
tension, which leads to droplet motion. Depending on the concentration of
both droplets and the vapour, repulsive or attractive motion can be observed.
A phase map as well as a critical concentration boundary was proposed for
the mixture of PG and water droplets, which can help to predict the direction
of droplet motion
Wetting and evaporation of nanosuspension droplets
No full textL’évaporation de gouttes de liquides contenant des particules non volatiles représente un phénomène largement présent dans la vie quotidienne, à l’image des traces laissées par le marc de café après séchage. L’étude de la morphologie des dépôts de particules présente un grand intérêt dans les domaines de la biologie et trouve de nombreuses applications dans l’industrie. De ce fait, elle a fait l’objet de nombreuses recherches durant les dernières décennies. Malgré les nombreuses récentes recherches sur les morphologies des dépôts de particules, les mécanismes les contrôlant restent encore non complétement expliqués. Certains facteurs influençant les morphologies des dépôts sont nombreux (température de substrats…) mais restent encore peu documentés dans la littérature. Cette étude expérimentale s’intéresse à l’influence de la température du substrat sur la morphologie des dépôts de nanoparticules après séchage de gouttes sessiles de liquides. L’augmentation de la température du substrat accélère le processus d’évaporation et entraine des morphologies de dépôts très différentes de celles obtenues sur des substrats à température ambiante. Dans cette étude, la microscopie combinée à la thermographie infrarouge et à l’interférométrie ont permis d’expliquer la dynamique de formation de dépôts. De plus, l’étude a permis d’analyser les effets d’autres paramètres sur la morphologie des dépôts, tel que la composition chimique du liquide composant les gouttes.Evaporation of liquid droplets containing non-volatile solutes is an omnipresent phenomenon in daily life, e.g., coffee stains on solid surfaces. The study of pattern formation of the particles left after the evaporation of a sessile droplet has attracted the attention of many researchers during the past two decades due to the wide range of biological and industrial applications. Despite the significance of controlling the deposition morphology of droplets, the underlying mechanisms involved in pattern formation are not yet fully understood. There is a varied range of factors that affect the final deposition patterns and some, e.g., substrate temperature, are poorly studied in the literature. This experimental study investigates the effect of a wide range of substrate temperatures on the deposition patterns of nanoparticles from drying sessile droplets. Increasing substrate temperature and accelerating the drying process lead to the formation of the patterns not observed on non-heated substrates. This research elucidates the formation mechanisms of these patterns by optical microscopy, infrared thermography, and white light interferometry techniques. Furthermore, the combined effects of substrate temperature and other factors such as chemical composition of base fluid and particle size on the dried patterns are studied. The underlying mechanisms involved in the formation of the patterns influenced by the combined factors are also discussed and presented
Effect of Poly(ethylene oxide) Molecular Weight on the Pinning and Pillar Formation of Evaporating Sessile Droplets: The Role of the Interface
Full text linkWe 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
Full text linkSurface 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
On passive and active drag reduction of free-falling bodies in quiescent viscous fluid
Full text linkModes of drag reduction on surfaces interfacing with liquids have received a considerable amount of attention from researchers and industries due to the significantly associated advantages in terms of energy-savings and power consumption associated with various applications such as ships, underwater vehicles, piping infrastructures and microfluidic devices. One revolutionary technique to accomplish drag reduction on moving objects in liquids is to introduce a lubricating gas or vapour layer between the object surface and the ambient liquid via different strategies such as surface modification and by inducing the Leidenfrost effect. However, there are many open questions regarding the understanding, effectiveness and implementation of these drag reduction techniques. The main aim of the present study is to investigate the effect of various surface treatment techniques on the drag coefficient of a solid sphere and drag reduction by Leidenfrost effect on deformable liquid droplets in a free-falling experiment. This was accomplished by a newly designed and constructed experimental setup to facilitate the capture of the free-falling motion of both a solid sphere and a liquid droplet through a quiescent continuous viscous fluid phase in a vertical tank. The solid spheres used in the experiments were stainless-steel spheres with a diameter ranging from 4 mm to 7 mm. The spheres were surface-modified by a perflourodecyltrichlorosilane (FDTS) coating, roughened via an etching process and dry ice coating. No significant differences were found for the etched spheres compared with the unmodified spheres. Surprisingly, the drag coefficient of the FDTS sphere was increased by 13%. The dry ice coating successfully produced a substantial gas layer surrounding the free-falling spheres. However, due to issues with the uniformity of the coating, this method was abandoned. Following these, liquid gallium was used as the dispersed phase in free-falling deformable droplet experiments. Firstly, the effect of shape and deformation on the velocity and the drag coefficient of free-falling liquid gallium droplets in water were investigated for droplet diameters (spherical volume-equivalent) ranging from 2.67 mm to 5.56 mm under isothermal conditions with temperatures in the range of 30◦C to 70◦C. The initial shape of the droplets after detachment was found to be spherical. Spherical-oblate oscillations began immediately after the detachment of the droplet prior to the dampening of the oscillations into a final shape of an oblate-spheroid except for the smallest droplet size which remained spherical without any notable change in shape. It was found that the rhythmic change in shape induced the falling velocity to oscillate at a frequency double that of the aspect ratio. Moreover, increasing the viscosity ratio enhanced the amplitude of the oscillations. However, the oscillation frequencies were sensitive to the droplets’ size rather than their associated viscosity ratio. The experimental results reveal that for a deformed liquid gallium droplet with a terminal Reynolds number that varied in the range of 103 to 104, the drag coefficients were found to be larger than those associated with a solid sphere in the same Reynolds number range. Furthermore, the deformation is highly dependent on interfacial surface tension and inertial force, while the viscosity ratio and pressure distribution have negligible effect. Subsequently, the continuous phase was changed to a low boiling point perfluorinated liquid (FC-72) in order to investigate the drag reduction by Leidenfrost effect. The liquid gallium temperature was varied in the range of 40◦C to 170◦C to induce an inverted Leidenfrost effect. The fully-developed Leidenfrost regime was stable at a droplet temperature of 130◦C, and was illustrated by the vapour layer stream moving upward on the droplet surface. Unlike in water, the liquid gallium droplets in FC-72 formed a tear-drop shape. The drag coefficient calculated based on the maximum velocity achieved by the droplets revealed a drastic drag reduction of about 57% for the highest temperature droplet compared with the lowest temperature droplet. Numerical simulation based on the two-dimensional lattice Boltzmann model (LBM) was also carried out to study the velocity field and pressure distribution around a deformable droplet falling through an immiscible quiescent viscous liquid
Mathematical models for the evaporation of sessile droplets
Full text linkDespite 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
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