Arrested Dynamics of Droplet Spreading on Ice
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We investigate the arrested spreading of room temperature droplets impacting flat ice. The use of an icy substrate eliminates the nucleation energy barrier, such that a freeze front can initiate as soon as the droplet's temperature cools down to 0 C. We employ scaling analysis to rationalize distinct regimes of arrested hydrodynamics. For gently deposited droplets, capillary-inertial spreading is halted at the onset of contact line freezing, yielding a 1/7 scaling law for the arrested diameter. At low impact velocities (We < 100) inertial effects result in a 1/2 scaling law. At higher impact velocities (We > 100), inertio-viscous spreading can spill over the frozen base of the droplet until its velocity matches that of a kinetic freeze front caused by local undercooling, resulting in a 1/5 scaling law.
Highlight: Research published in Physical Review Letters. Paper link.
Evaporation-Induced Clogging of an Artificial Sweat Duct
Metal-based antiperspirants have been in use for centuries; however, there is an increasing consumer demand for a metal-free alternative. Here, we develop an artificial sweat duct rig and demonstrate an alternative, metal-free approach to antiperspiration. Instead of clogging sweat ducts with metal salts, we use a hygroscopic material to induce the evaporation of sweat as it approaches the outlet of the sweat duct. As a result, the sweat dehydrates almost completely while still inside of the duct, resulting in a natural plug that halts the flow. We show that the critical pressure gradient within the duct (~ 3 kPa), beneath which clogging occurs, can be rationalized by balancing the mass flow rates of the liquid (Poiseuille's law) and the evaporative vapor (Fick's law).
Highlight: Research published in ACS Applied Materials & Interfaces. Paper link.
In News: "Virginia Tech lab proves the concept of a natural approach to antiperspirants".
Coalescence and Spreading of Drops on Liquid Pools
Oil spills have posed a serious threat to our marine and ecological environment in recent times. Containment of spills proliferating via small drops merging with oceans/seas is especially difficult since their mitigation is closely linked to the coalescence dependent spreading. This inter-connectivity and its dependence on the physical properties of the drop has not been explored until now. Furthermore, pinch-off behavior and scaling laws for such three-phase systems have not been reported.
We investigate the problem of gentle deposition of a single drop of oil on a pool of water, representative of an oil spill scenario. Methodical study of 11 different n-alkanes, polymers and hydrocarbons with varying viscosity and initial spreading coefficients is conducted. Regime map, scaling laws for deformation features and spreading behavior are established.
The existence of a previously undocumented regime of delayed coalescence is reported. A novel application of the inertia-visco-capillary (I-V-C) scale collapses all experimental coalescence data on a single line while the early stage spreading is found to be either oscillatory or asymptotically reaching a constant value, depending on the viscosity of the oil drop unlike the well documented monotonic, power law late-time spreading behavior. These findings are equally applicable to applications like emulsions and enhanced oil recovery.
Highlight: Research published in Journal of Colloid and Interface Science. Paper link.
In News: "Researchers looking at oil and water interaction to prevent water contamination".
Increased Solidification Delays Fragmentation and Suppresses Rebound of Impacting Drops
The splat formed after drop impact on supercooled solid surfaces sticks to it. On the contrary, a sublimating supercooled surface such as dry ice inhibits pinning and therefore efficiently rebounds drops made of a variety of liquids. While rebound is expected at lower impact velocities on dry ice, at higher impact velocities the drop fragments leave behind a trail of smaller droplets. However, it is not known whether rebound can be entirely suppressed or fragmentation can be controlled on such surfaces and if it depends on the extent of solidification inside the drop. In this paper, we report on the role played by solidification within drops in modifying the outcomes of their impact on the supercooled ultralow adhesive surface of sublimating dry ice. We show that the solidification thickness depends on the impact velocity and is the primary driver in suppression of rebound and delay in fragmentation. Our findings imply that sublimating supercooled surfaces can present a broad spectrum of outcomes from complete bouncing to no rebound, which is not seen in drop impacts on supercooled superhydrophobic surfaces. We attribute this to thermo-elastocapillarity, which considers bending of the solidified layer and is used to demarcate regime boundaries and determine the coefficient of restitution during rebound.
Highlight: Research published in Physical Review Fluids. Paper link.
Bursting of Underwater Oil Drops
For decades, two main facets of underwater oil spills have been explored extensively - the rise of oil drops and resulting evolution of the oil slick at the air/water interface. We report on the bursting of rising oil drops at an air/liquid interface which precedes slick formation and reveal a counter-intuitive bulge}} reversal that releases a daughter oil droplet inside the bulk as opposed to upward-shooting jets observed in bursting air bubbles. By unraveling the underlying physics we show that daughter droplet size and bulk liquid properties are correlated and their formation can be suppressed by an increase in the bulk viscosity.
Highlight: Accepted in Physical Review Letters. Abstract Link.
Figure_1_Updated.pdfElectrostatic Defrosting
Electrification of ice has been studied for over half a century, mostly in the context of atmospheric science. Here, the polarizability and natural thermovoltage of a substrate-bound frost sheet are exploited by placing an actively charged electrode overhead for defrosting. This new technique, which we term electrostatic defrosting (EDF), can remove up to 75% of the frost's mass from its substrate over a time scale of only 10 min. A one-dimensional numerical model is developed to rationalize the effective electrostatic force exerted by the electrode on the warm end of the frost sheet. Experimentally, we show how the effectiveness of EDF depends on the applied voltage, relative humidity of the ambient air, the gap height between the frosted substrate and the electrode plate, and the type of substrate. EDF can effectively remove frost without the application of heat, chemicals, or mechanical forces, rendering it a promising new construct for defrosting. (Submitted)
Highlights: Research funded by NSF-GOALI in collaboration with Rolls-Royce.
Dynamics of Drop on Impregnated Fibers
Lubricant-impregnated surfaces (LIS) consist of a liquid lubricant stabilized within a textured surface by capillary forces. Droplets on LIS have the unique combination of a high lateral mobility (i.e., low contact angle hysteresis) and a high work of adhesion (i.e., difficult to detach). To date, virtually all reports characterizing the wettability of LIS have exclusively focused on the mobility of test droplets on planar substrates. Here, we pioneer the construct of lubricant-impregnated fibers (LIFs), which exhibit unique droplet dynamics due to the simultaneous exploitation of high mobility and high adhesion. Superhydrophobic fibers of differing diameters were impregnated with silicone oil and held horizontally. Immiscible test droplets were released onto the top of a LIF over a wide range of Weber numbers ranging from gentle deposition (We = 0) to high-speed impact (We > 500). Due to the low contact angle hysteresis, droplets always slid toward the bottom of the fiber’s cross section. (In preparation)
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