Session L12: Drops: General II

Fabrication of solid-liquid composites with desired droplet size and volume fraction by adopting thermoreversible phase transition

Solid-liquid composites (SLCs) have recently attracted many researchers as a way to overcome soft polymer material's limitations, realize new properties, and fabricate the porous structure. Here, we develop a new fabrication method for SLCs, where thermoreversible hydrogel droplets with the desired size are uniformly distributed in the silicone matrix. Using a three-dimensional axisymmetric flow-focusing device, we successfully generate uniform size droplets and control the volume fraction of droplets with a highly viscous continuous phase. Since the continuous phase is a mixture of PDMS base and catalyst, adding a crosslinker to the continuous phase causes the PDMS to be cured and become a silicone matrix. As droplets distributed in silicone matrix go through sol-gel transitions, the SLCs sensitively change mechanical properties according to temperature. In addition, since the developed method can be used in fabricating designable porous structures by evaporating liquid droplets in SLCs, it has potential applications in diverse fields ranging from gas separation systems to tissue engineering.   

Effect of deposition pattern of water on fog-harvesting performance of Janus mesh

The effect of different deposition patterns of water on the fog-harvesting performance of Janus meshes is investigated with a systematic adjustment of mesh geometry for four different Janus configurations. In the present work, a dropwise deposition pattern of water is observed to deteriorate fog-harvesting yield by locally disturbing fog flow around a water drop, which weakens impact inertia of fog particles. A super-hydrophilic mesh surface, on the other hand, provides the most aerodynamically favorable environment via filmwise deposition of water. Besides these effects from local aerodynamics, water transport across Janus mesh via wettability gradient is also found to be an important factor determining fog-harvesting yield by influencing drainage efficiency. Water drainage can be maximized when the directions of fog flow and wettability gradient of Janus mesh are both the same, which is suitable for transporting water via air flow.   

Heat transfer in drop-laden turbulent channel flow

In this work, we study the heat transfer between a dispersed phase of large deformable drops and a carrier fluid in wall-bounded turbulence. We perform a campaign of direct numerical simulations of the Navier Stokes equations, coupled with a Phase Field Method, which describes the behavior of the dispersed phase and coupled with the energy equation, which takes into account the heat transport. We consider a system in which drops are free to deform, break and coalesce, while heat is exchanged between the two phases and the two walls. Several values of Weber number (We, ratio of inertial forces to surface tension forces) and Prandtl number (Pr, ratio of momentum diffusivity to thermal diffusivity) are evaluated, while the shear Reynolds and the volume fraction are fixed. First, we characterize the dispersed phase dynamics by analyzing the coalescence and breakage dynamics of the drops and the corresponding drop size distribution. Second, we investigate the role of the drops internal flow in the modulation of heat transport. In particular, we analyze the characteristics of the flow structure inside the drops and we study how turbulent mixing influences the rate with which heat is diffused inside the drops, and transferred across the interface of the drop towards the carrier fluid.   

Droplet Field Sensing in a Low Speed Windtunnel

With the continued operational and technological advancements of small unmanned aerial systems (sUAS), they are capable of contributing to scientific observations. Flights from a sUAS allow more frequent and closer transects than manned aircraft with the small aircraft size and lesser degree of cloud disruption. The tighter grid observations in smaller amount of time will allow better geographic and temporal resolution. Airborne microphysical measurements have shown great sampling zone variability. Optical particle sensor calibration for sUAS flight is performed by applying a droplet field through a wind tunnel at the desired flight speeds.

The preliminary examination is of the sensor's dry air observations under increasing velocity conditions. These are compared with direct imaging observations as an initial test of varied sensor options. Pulsed high-power LED illumination will be used for particle image velocimetry. For sUAS sensor calibration, it is imaging will provide the amount of water in the test volume, and droplet count and size distribution. For a wider physical significance, the droplet field will also be examined for homogeneity, turbulent structures, and sensor impingement characteristics.    

The interplay of dropwise condensation and drop shedding mechanism on biphilic patterned surfaces.

The optimization of the dropwise condensation region on a patterned surface is important for applications such as heat transfer performance, water harvesting efficiency, and self-cleaning surfaces. Previous research found that narrowing the dropwise region increased drop shedding frequencies, which continuously renewed the condensate surfaces and resulted in better heat transfer coefficients. In this study, we looked at the area that condensate drops covered, the volume of departure drops, and the drop size distribution on biphilic surfaces' dropwise condensation region. In addition, we discussed the connection between drop shedding mechanisms and ideal dropwise condensation surfaces. Because the number of drops increased, the wider dropwise region width provided more condensate region and increased the number of drop shedding mechanisms. We observed greater drop area coverage at lower area fractions while adjusting the ratio of dropwise region area to total surface area. As a result, the maximum volume of departure drop was found to be optimal at 30% area fraction.

    

Turbulent Droplet Breakage in a von Kármán Flow Cell

Breakage of dispersed phase liquid droplets immersed in a second immiscible continuous liquid phase is a common phenomenon in the production of petrochemicals, polymers, metals, foods, and pharmaceuticals. It is also vital for environmental management, such as cleaning up oil spills and leaking underground storage tanks.  Although several mathematical models have been developed to describe droplet breakage in agitated liquid emulsions, the applicability of these models is limited by the fact that they incorporate multiple fitting parameters that must be determined empirically for specified fluid pairs and flow conditions. Here, a semi-empirical equation has been developed fo determinng the fitting parameters for two well-known breakage models without the need to perform experiments. This equation was derived using dimensional analysis and a hypothesis regarding breakage probability based upon competition between disruptive and restorative stresses on droplets. The data used for model validation was obtained from both previously published experiments of droplet breakage in stirred tanks and also experiments performed in a von Karman box designed to produce homogeneous turbulence. The model shows a high degree of fit with the experimental data from both sources.

    

Magnetic field altered ferrofluid droplet deformation in the uniaxial extensional flow

Ferrofluid droplets find several applications in various biomedical and process engineering systems related to magnetohydrodynamic problems. This study deals with the ferrofluid droplet deformation dynamics under the joint influence of the uniform magnetic field and uniaxial extensional flow. Here, using the phase-field method, we have investigated an extensive range of governing parameters to analyze the droplet deformation in uniaxial extensional flows under uniform magnetic fields in the Stokes flow regime. Our model was validated against the seminal work of Stone and Leal (Stone, H. A. & Leal. L. G., 1989, J. Fluid Mech., 198, 399-427) for a system without an external magnetic field, and also with the experimental results of Afkhami et al. (Afkhami, S., Tyler, A. J., Renardy, Y., Renardy, M., ST. Pierre, T. G., Woodward, R. C., & Riffle, J. S., 2010, J. Fluid Mech., 663, 358-384) that included the presence of a magnetic field. The results reveal that the droplet deformation depends on the strength of the magnetic field relative to the background flow and its orientation. Additionally, this work elaborates on the influence of magneto-rheological properties of the fluids on droplet deformation.   

Computational study on the breakup of FENE-P drop migrating through microconfinement with gradual entry and exit.

In the present study, we deciphered the breakup dynamics of a viscoelastic Finitely Extensible Nonlinear Elastic – Peterlin (FENE-P) drop migrating through confinement with gradual entry. Drop breakup considerably depends upon the channel orientation and nature of drop entry inside the confinement. Our previous computational study reported the breakup phenomenon of FENE-P drop inside confinement with sudden entry.  We observed that the critical capillary number for drop breakup in confinement with a gradual entry differs from that with sudden entry. Drop breakup was accessed at different Deborah numbers and viscosity ratios. The critical capillary number variation with Deborah number and viscosity ratio was also discerned in the study. Drop breakup with the formation of daughter and satellite drops was represented with a map showing drop interface position at different axial positions of the microchannel. The interface tracking for the viscoelastic drop was done using the Volume of Fluid (VOF) method, while the simulations were performed on open source solver Basilisk.  FENE-P viscoelastic drop exhibits a shear-thinning behavior, and it can represent the flow of cells and drug carriers migrating in blood vessels. Thus the drop flow inside confinement can replicate the flow of anti-cancer drug carriers like hydrogel inside minute blood capillaries.  Further, a drop breakup study will help get deeper insights into finding critical conditions for nano-carrier disintegration.  The nano-carrier disintegration is required to achieve on-demand burst drug release at the cancer site.

    

Simultaneous Impact of two Droplets on a Solid Surface: Central Sheet Evolution and Splashing

Simultaneous impacts of two droplets on dry substrates can lead to lamella interaction that generates a central uprising sheet, modifying the outcomes of equivalent single droplet impacts. The central sheet either breaks into droplets or deposits on the impact surface depending on droplet impact conditions. We investigate the sheet evolution through a combined experimental and theoretical study. A new micro-controlled droplet generator released two droplets simultaneously on-demand for varying impact Weber number and inter-droplet spacing. Simultaneous high-speed imaging from different views recorded the sheet morphology, including its rim-bounded semilunar shape, surface waves, finger formation and deposition or splashing. The maximum height of the sheet scales with the lamella-lamella collision force at the start of their interaction. Considering geometrical constraints and mass balance, we theoretically evaluate the temporal evolution of the base width and thickness of the semilunar uprising sheet. Secondary atomisation during splashing generates larger droplets than sizes from single droplet splashing. The results provide new insights into the splash/non-splash outcomes of simultaneous multi-droplet impacts and indicate good agreement between theory and experiments.   

Droplet swimming in a pipe

Self-solubilising droplets spontaneously self-propel in surfactant-saturated solutions as a result of the non-linear transport of surfactant molecules and micelles by their interfacial flows. While much is now known on the onset of propulsion and subsequent nonlinear dynamics in bulk fluids, the effect of confining boundaries is in general overlooked despite their ubiquitous relevance for experiments.  Recent experiments have indeed demonstrated the droplets' ability to swim inside small capillaries despite the increased resistance of lubrication flows.

Using a combination of numerical simulations and asymptotic analysis, we will show how the confining geometry profoundly modifies chemical transport and may in fact promote rather than prevent the swimming motion of chemically-active droplets.