735 research outputs found
INTERFACIAL DYNAMICS OF DROP COALESCENCE AND IMPINGING LIQUID JETS: EFFECT OF VISCOUS, MARANGONI AND SHEAR-THINNING STRESSES
The free-surface dynamics of drop coalescence and that of impinging liquid jets have both fundamental interest and practical importance in processes ranging from crop spraying and the processing of food emulsions to the atomization of fuel and pro- pellants in combustion and propulsion engines. These processes frequently involve the use of complex fluids -- shear-thinning or viscoelastic non-Newtonian liquids, which often contain surfactants either as additives or as contaminants. This Thesis reports high-fidelity simulations of unsteady free-surface flows of complex fluids. The objec- tive is to advance the understanding of the free-surface dynamics of drop coalescence and liquid jet impingement when viscous, surfactant and shear-thinning e↵ects are important. Simulations in this Thesis enabled, for the first time, a comprehensive nu- merical analysis of the coalescence of surfactant-laden drops after the merging drops make contact. The analysis reveals how interfacial (Marangoni) stresses induced by uneven accumulation of surfactant control the rate at which the drops coalesce by modulating the pull of surface tension on the tiny meniscus bridge joining the drops. Simulations also enabled the analysis of the unsteady free-surface dynamics of im- pinging viscous and shear-thinning liquid jets. Results demonstrate that viscous and shear-thinning stresses profoundly a↵ects the impingement dynamics -- in particu- lar the velocity and thickness of the resulting radially expanding liquid sheet -- by modifying the pressure developed in the impact region
Coalescence of surfactant-laden droplets
Droplet coalescence is an important process in nature and various
technologies (e.g. inkjet printing). Here, we unveil the surfactant
mass-transport mechanism and report on several major differences in the
coalescence of surfactant-laden droplets as compared to pure water droplets by
means of molecular dynamics simulation of a coarse-grained model. Large scale
changes to bridge growth dynamics are identified, such as the lack of multiple
thermally excited precursors, attenuated collective excitations after contact,
slowing down in the inertial regime due to aggregate-induced rigidity and
reduced water flow, and a slowing down in the coalescence rate (deceleration)
when surfactant concentration increases, while at the same time we also confirm
the existence of an initial thermal, and a power-law, inertial, regime of the
bridge growth dynamics in both the pure and the surfactant-laden droplets.
Thus, we unveil the key mechanisms in one of the fundamental topological
processes of liquid droplets containing surfactant, which is crucial in
relevant technologies.Comment: 23 pages, 7 figure
Morphology of clean and surfactant-laden droplets in homogeneous isotropic turbulence
We perform direct numerical simulations of surfactant-laden droplets in
homogeneous-isotropic turbulence with Taylor Reynolds number
. Effects of surfactant on the droplet and local flow
statistics are well approximated using a lower, averaged value of surface
tension, allowing us to extend the framework developed by Kolmogorov (1949) and
Hinze (1955) for surfactant-free bubbles to surfactant-laden droplets. We find
the Kolmogorov-Hinze scale () is indeed a pivotal length scale in the
droplets' dynamics, separating the coalescence-dominated and the
breakage-dominated regimes in the droplet size distribution. We see that
droplets smaller than have spheroid-like shapes, whereas larger droplets
have long convoluted filamentous shapes with diameters equal to . As a
result, droplets smaller than have areas that scale as , while
larger droplets have areas that scale as , where is the droplet
equivalent diameter. We further characterise the filamentous droplets by
computing the number of handles (loops of the dispersed phase extending into
the carrier phase) and voids (regions of the carrier phase enclosed by the
dispersed phase) on each droplet. The number of handles per unit length of
filament () scales inversely with surface tension, while the
number of voids is independent of surface tension. Handles are indeed an
unstable feature of the interface and are destroyed by the restoring effect of
surface tension, whereas voids can move freely inside the droplets.Comment: 31 pages, 13 figure
Liquid-liquid Dispersion in Batch and In-line Rotor-Stator Mixers
This two-part dissertation investigates the behavior of batch and in-line rotor-stator mixers separately. In the first study, water was dispersed into viscous oil using a batch Silverson L4R rotor-stator mixer. The flow regime was determined by reference to published Power number data and by qualitative differences in drop size data. Drop breakup in laminar flow was analyzed by comparison to published single drop breakup experiments in idealized flow fields. The breakup mechanism in laminar flow was similar to that for simple shear flow and equal to about twice the nominal shear rate in the rotor-stator gap. Drop breakup in turbulent flow followed a mechanistic correlation for mean drop size for drops less than the Kolmogorov microscale, but still large enough that both inertial and viscous effects were manifest. Surfactants decreased drop size with Marangoni effects observed near the CMC for laminar, but not for turbulent flow. Below phase fractions of 0.05, d32 increased in a log-linear fashion with phase fraction for all conditions tested including: laminar and turbulent flow, presence of surfactant, and hydrophobically treated high-shear surfaces. The significant effect of phase fraction was caused by the flow structure being locally laminar near the drops, and was permitted by sufficiently low fluid viscosities which promoted film drainage. Above phase fractions of 0.1, drop sizes plateaued. This was attributed to decreasing coalescence rate and efficiency, along with increasing breakup. In the second study, the power consumption of an IKA 2000/4 in-line pilot scale rotor-stator mixer was measured with a purpose-built torque meter. The power spent by the mixer on pumping was insignificant compared to viscous dissipation. A constant power number was obtained for turbulent flow using constant power per stage with an empirically determined effective diameter for each generator type. For conditions where mean drop size was close to equilibrium, as determined by flowrate independence, previously reported mean drop size data were calculated using the well-known inertial subrange scaling law along with the power draw measurements of the present study. The maximum local energy dissipation rate was found to be nine times the average energy dissipation rat
Collodial particles at a range of fluid-fluid particles
The study of solid particles residing at fluid-fluid interfaces has become an established area in surface and colloid science recently experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes like anti-foam formulations, crude oil emulsions, aerated foodstuffs and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also and the area is now multi-disciplinary involving research in the fundamental science and potential applications. In this Feature Article, a flavour of some of this interest is given based on recent work from our own group and includes the behaviour of particles at oil-water, air-water, oil-oil, air-oil and water-water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilised emulsions, particle-stabilised aqueous and oil foams, dry liquids, liquid marbles and powdered emulsions
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