Research

Our research objective is to address global-scale challenges pertaining to energy, environment, and health. We work at the cross-roads of surface science, fluid and thermal transport, and interfacial interactions. The focus of our research group is to use the understanding of micro-scale phenomena to devise solutions for areas related to thermal management, biomedicine, and fouling.

Droplets

Impact Dynamics

Impact dynamics of hollow droplets

Compound droplets are utilized in applications ranging from preparation of emulsion to biological cell printing and additive manufacturing. We study the impact dynamics of an air-in-liquid hollow compound droplet on a solid substrate. Contrary to the impact of pure droplets and compound droplets with liquids of similar densities, a compound droplet with an encapsulated air bubble demonstrates the formation of a counterjet in addition to the lamella. We experimentally investigate the influence of the size of the air bubble, liquid viscosity, and height of impact on the evolution of counterjet and the spreading characteristics of the lamella.

Evaporation

Evaporation-induced convective transport in confined saline droplets

We study the convection pattern inside a confined saline droplet before crystallization (prenucleation) and at the onset of crystallization (postnucleation). The flow field in the prenucleation and the postnucleation stages is attributed to the density gradient established inside the droplet during evaporation and subsequent crystallization. A stark contrast in the convective flow patterns and the flow strength is observed between the two stages of evaporation. While the prenucleation regime is marked by an axisymmetric toroidal vortex pattern, the symmetry breaks in the postnucleation regime with also an increase in the flow strength. This increase in flow strength is attributed to the large difference in the local concentration arising because of nucleation.

Heat Transfer

Influence of the substrate permeability on Leidenfrost temperature

Leidenfrost phenomena adversely affect the transfer of heat from the substrate to the droplet during cooling applications. The introduction of micro-textures alters the Leidenfrost transition temperature. It is observed that tall and sparse micropillar arrays increase the Leidenfrost temperature to ~ 500 0C compared to short dense pillars whose Leidenfrost temperature is on par with that on a smooth surface which is 270 0C.

Dynamic of unconstrained droplet in transition boiling regime

The Leidenfrost temperature and the transition boiling regime depends on the surface morphology. An unconstrained droplet shows trampolining in the transitional boiling regime on all the test substrates. Droplet shows high trampolining heights which reduces with increase in temperature. The mechanism of droplet trampolining is attributed to bubble growth and rupture hypothesis.The universality of the trampolining phenomenon is observed in the all the substrates with transitional boiling regime.

Thin film dynamics

Thin film drainage and coalescence of interacting bubbles

Stability of the fluid film between bubbles and droplets determine the life of the components that are made with them. Thinning and subsequent rupture of the film leads to the coalescence of the bubbles and droplets. Thus, understanding the mechanism of rupture of the intervening film and the parameters that inhibit and accelerate coalescence is important. Whether it be interacting bubbles or droplets, vapor condensation on a solid surface or flotation of foams and emulsions, what determine the state of the system at a later time is the stability of the intervening film. There are applications which requires the intervening film to be stable and those which require the film to break. For example, in microfluidics, the higher transport rate of biological cells, drops or bubbles through microchannels which is crucial for the efficient working of the microscale devices is achieved through a stable liquid film that separates the cells, drops or bubbles from the wall during the entire process. Film rupture leads to the malfunction of the device. On the other hand, rupture of the film is essential to enhance the performance of a boiler or condenser etc. Ruptured film during these heat transfer process also makes sure of the absence of heat flux fluctuation if the film ruptures suddenly. In order to understand how the film of continuous medium evolve with time during the interaction of bubbles or droplets, evolution equation of the film is solved numerically and Color interferometry, a recently emerged technique in the field of interferometry is used to understand the evolution of thin film which makes use of white light instead of conventional single or three monochromatic light sources.

Thermal Management and Energy

Role of extended surface on Quenching Boiling

Boiling is an effective mode of heat transfer because of the high latent heat of vaporization associated with liquid to vapor conversion. High heat transfer rate is essential in several industrial applications, including, nuclear reactors, storage and transport of cryogenic liquid, and metal forming, which deal with the rapid cooling of various materials. Rapid cooling of surfaces at temperatures significantly higher than the boiling point of the coolant is often restricted by formation of a vapor layer around the surface that retards the rate at which heat can be dissipated. The current research work is aimed at investigating the effect of macro-scale surface enhancements (fins) on the enhancement of heat transfer performance during quenching of vertical cylinders at different pool temperatures. Cylinders with annular and pin fins with varying geometrical parameters in terms of fin length and cross section are quenched to observe the corresponding change in the boiling curve.The experimental results indicate the geometry of the fins plays a significant role in the vapor flow dynamics, influencing the heat transfer in the film, transition and nucleate boiling regimes. The hydrodynamic rupture of the vapor film due to the protruding fins results in the termination of the film boiling regime; reducing the overall quench duration significantly.

Image of a multistage desalination system

Passive thermal desalination system

Desalination promises to be sustainable choice to resolve fresh water shortages increasing day by day. We present a compact multistage thermal desalination system based on interfacial evaporation and latent heat recovery. We develop a textured metallic evaporator substrate that serves as a wick and reduces the thermal resistance for interfacial evaporation. However, low water collection efficiency is still major concern for majority of the systems developed. Therefore, we demonstrate that the rate of evaporation adn condensation can be increased by tuning the wettability of the condenser surface. Patterned wettability-contrast surfaces are shown to enhance the rate of condensation compared to an unpatterned substrate with spatially uniform wettability.

Food Safety

Evaporation-based low-cost method for the detection of adulterant in milk

We propose a simple evaporative deposition-based technique to detect the presence of two of the most common adulterants – water and urea in milk. In pure milk, we observe a specific pattern formation of the non-volatile milk solid deposited at the end of evaporation. These patterns change or diminish with the addition of water. In the case of diluted milk-urea, the crystallization pattern of urea in the droplet is used as a tool to detect the addition of urea in milk. The detection of urea can be improved by the dilution of milk. Based on the detection level of urea, we present a regime map that shows the concentration of urea that can be detected at different extents of dilution of milk. While milk is a complex system and adulteration may not be restricted to only water or urea, this evaporation-based method opens up an avenue to explore the dependence of evaporative deposition pattern on the composition of milk and use it as a physical detection tool for detecting adulterants at home or any remote location.

Micro/Nano-engineered surfaces

Capillary-fed Microthruster

Small satellites offer a unique platform for short-term and low-cost communications and surveillance missions. Such miniaturized satellites require compact and lightweight propulsion systems that can provide precise reaction and attitude control and can be easily integrated with the satellite. We have design and development of a capillary-fed microthruster that relies on localized evaporation of a thin water film obtained using micro-engineered surfaces. Two generations of the device designed are analyzed. In first gen device wicking was observed till the nozzle which was removed in second gen. A resistive thin-film heater is used for heating of the thin liquid film to generate vapor that exits through the nozzle to provide a thrust. The experimental characterization of the first-generation device is done in terms of wicking of the liquid and liquid-vapor phase change on application of heating power and subsequent ejection of vapor through the nozzle. The evaporative mass flow rate obtained using the first-generation of the device is experimentally estimated at an ambient pressure of 500 Pa. The second generation of the device is expected to eliminate ice formation and boiling induced instabilities observed in the first generation of the device. A combined analytical-numerical model is developed to predict the performance of the second-generation device in terms of the evaporation rate and expected thrust.

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