Capillarity and dynamic wetting of non-Newtonian fluids are important in many natural and industrial processes, examples cover from a daily phenomenon as splashing of a cup of yogurt to advanced technologies such as additive manufacturing. The applicable non-Newtonian fluids are usually viscoelastic compounds of polymers and solvents. Previous experiments observed diverse interesting behaviors of a polymeric droplet on a wetted substrate or in a microfluidic device. However, our understanding of how viscoelasticity affects droplet dynamics remains very limited. This work intends to shed light on viscoelastic effect on two small scale processes, i.e., the motion of a wetting contact line and droplet splitting at a bifurcation tip.   Numerical simulation is employed to reveal detailed information such as elastic stresses and interfacial flow field. A numerical model is built, combining the phase field method, computational rheology techniques and computational fluid dynamics. The system is capable for calculation of realistic circumstances such as a droplet made of aqueous solution of polymers with moderate relaxation time, impacting a partially wetting surface in ambient air.   The work is divided into three flow cases. For the flow case of bifurcation tube, the evolution of the interface and droplet dynamics are compared between viscoelastic fluids and Newtonian fluids. The splitting or non-splitting behavior influenced by elastic stresses is analyzed. For the flow case of dynamic wetting, the flow field and rheological details such as effective viscosity and normal stress difference near a moving contact line are presented. The effects of shear-thinning and elasticity on droplet spreading and receding are analyzed, under inertial and inertialess circumstances. In the last part, droplet impact of both Newtonian and viscoelastic fluids are demonstrated. For Newtonian droplets, a phase diagram is drawn to visualize different impact regions for spreading, splashing and gas entrapment. For viscoelastic droplets, the viscoelastic effects on droplet deformation, spreading radius and contact line motion are revealed and discussed. / <p>QC 20160329</p>

Current problems on the interaction of complex liquids (i.e. droplets or foams) with complex surfaces (i.e. soft deformable or porous surfaces) are addressed in the following areas: (1) wetting of deformable substrates and surface forces, (2) kinetics of wetting and spreading of non-Newtonian liquids over porous substrates, (3) kinetics of spreading of non-Newtonian solutions over hair, (4) free drainage of foams produced from non-Newtonian solutions, and (5) foam drainage placed on porous substrates. Equilibrium of liquid droplets on deformable substrates was investigated and the effect of disjoining pressure action in the vicinity of the apparent three phase contact line was taken into account. It was proven that the deformation of soft solids is determined by the action of surface forces inside the transition zone. Spreading/imbibition of blood, which is a power law shear thinning non-Newtonian liquid, over a dry porous layer was investigated from both theoretical and experimental points of view. It was found that blood droplet spreading/imbibition over porous substrates shows two different behaviours: (i) partial wetting case with three subsequent stages: initial fast spreading, constant maximum droplet base and the shrinkage of the droplet base; (ii) complete wetting case with only two stages: initial fast spreading and the shrinkage of the droplet base. The wetting of hair tresses by aqueous solutions of two commercially available polymers, AculynTM 22 (A22) and AculynTM 33 (A33) was investigated experimentally. Both A22 and A33 solutions demonstrate well pronounced shear thinning behaviour. Initial contact angle of the A22 and A33 solutions on hair tresses was about 100o. The A22 droplets remained on the hair tress after spreading for at least half an hour. However, a fast penetration of the A33 droplets inside the hair tresses was observed when advancing contact angle in the course of spreading reached a critical value of about 60o. This could be explained by Cassie-Wenzel wetting transition which is caused by filling the pores inside the porous media by liquid. The influence of non-Newtonian rheology of A22 and A33 solutions on foam drainage was also investigated experimentally and a new theory of foam drainage was presented for the case of free drainage. For lowly viscous polymeric solutions and under the assumption of rigid surface of the Plateau border, the predicted values of the time evolution of the foam height and liquid content were in good agreement with the experimental data. However, in the case of highly viscous solutions an interfacial mobility at the surface of the Plateau border has to be taken into account. A completely new theory of foam drainage placed on porous substrate was developed. It was found that there are three different regimes of the process: (i) a rapid imbibition, the imbibition into the porous substrate dominates as compared with the foam drainage; (ii) an intermediate imbibition, that is, the imbibition into the porous substrate and the rate of drainage are comparable; (iii) a slow imbibition, the rate of drainage inside the foam is higher than the imbibition into the porous substrate for a period of time and a free liquid layer is formed over the porous substrate.

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<p>This thesis describes the experimental results of liquid polymer droplets levelling on thin films of identical polymer liquid. Through varying the thickness of the underlying polymer film relative to the size of the droplet height, we have observed a crossover in the dynamics between droplets spreading on very thin films to droplets levelling on films thicker than the droplet itself. In the thin film regime, the underlying film behaves as a pre-existing precursor film and the droplet spreads according to the well-known Tanner's law where the droplet height, d₀, decreases in time as d₀ ~ t^-1/5. In the opposite regime, when the film thickness is much greater than the initial droplet height, the droplet levels with a much stronger time dependence compared to Tanner's law spreading, d₀ ~ t^-1/2. Not only have we observed the two extreme cases, we have also experimentally observed levelling behaviour of intermediate systems, where the droplet height and film height are on the same order. We have captured experimentally the crossover behaviour of droplets spreading on thin films to droplets levelling on thick films. In addition, we have developed a theoretical model that accurately captures the physics of this crossover using the lubrication approximation for thin film flows. The relevant background information will be presented as well as a detailed description of the sample preparation techniques required to fabricate spherical caps atop thin films of identical material.</p> / Master of Science (MSc)

The first step on the way to understanding the complicated dynamics of spray is to study the behavior of isolated droplets. In many industrial and natural processes such as turbulent combustion, agricultural sprays, spray cooler, falling raindrops and cloud evolution the droplet is subjected to a chaotic unsteady external flow field. The interaction between the liquid and gaseous phases results in very intricate droplet dynamics like capillary instabilities, atomization, droplet collision and coalescence and vaporization, to name a few. In this dissertation, the focus is on shape oscillations, atomization and vaporization dynamics of pendant and sessile droplets. A droplet residing on a substrate which vibrates vertically at ultrasonic frequency will exhibit different modes of shape oscillation. The competition between capillary forces and inertia forces is basically responsible for these oscillations. However, when an acoustic force field is introduced asymmetrically around the droplet, we discover with the help of ultra high-speed imaging, a new droplet spreading phase. This new method of droplet manipulation could nd application in micro fluidics and lab-on-a-chip systems. By lading the droplet with nanoparticles, the spreading rate can be easily controlled. The spreading phase is followed by an atomization phase where surface ligaments grow to disintegrate into daughter droplets; the intensity of atomization is found to decrease with increase in fluid viscosity. The ability to control atomization characteristics of droplets by lading them with nanoparticles is a powerful technique that may be implemented in spray coolers and combustors to control the spray characteristics or combustion efficiency. Both the spreading and ligament dynamics have been theoretically simulated and the physics behind the observed trends is explained. The growth rate of the ligaments is found to be governed by Weber number modified to include the acoustic pressure level of the standing wave. The frequency of ligament breakup is found to decrease with fluid viscosity and this observation is adequately supported by a theory developed based on the evolution crater on the droplet surface. Turning now to the pendant droplets, by decomposing the droplet shape into Legendre modes, the shape oscillations exhibited by a droplet hanging from the junction of cross-wire placed at the center of an air jet is studied. Both high-speed imaging and hot-wire anemometry are employed. The driving force of oscillation of droplets subjected to the air jet is proved to be the inherent pressure fluctuations in the jet. The effect of surface tension, viscosity and Reynolds number on the shape oscillation level has been examined. The first experimental evidence of viscous attenuation of lower frequencies in a particular mode in glycerol/water mixture is reported. A theoretical model was developed to simulate the droplet shape oscillations induced by different ambient flow fields like pulsatile flow, vortical flow and flow with broadband energy spectrum. The time of interaction of the droplet with an eddy in the flow is found to be very crucial in determining the amplitude of oscillation of the droplet. The shorter the interaction time, the higher are the chances of the droplet oscillation being pushed into resonance. Finally, the heat transfer and droplet regression dynamics of pendant droplets in a hot air stream of various chemical compositions (like conventional fuels, alternative fuels and nanosuspensions) have been experimentally analyzed using high speed imaging. The droplet is deployed at the junction of cross-wire at the centre of a vertical air jet. A hybrid timescale has been proposed which incorporates the effects of latent heat of vaporization, saturation vapor pressure and thermal diffusivity. This timescale in essence encapsulates the different parameters that influence the droplet vaporization rate. The analysis further permitted the evaluation of the effect of various parameters such as surrounding temperature, Reynolds number, far-field vapor presence, impurity content and agglomeration dynamics (nanosuspensions) in the droplet.

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