Compound 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. 

Cassie-Wenzel transition to lift-off

We investigate Cassie–Wenzel transition on heated substrates and how it leads to striking lift-off behavior near the Leidenfrost regime. At temperatures below saturation, the transition occurs at a characteristic droplet volume determined by surface morphology and temperature. As the substrate approaches its Leidenfrost temperature (≈140–170 °C), partial impalement and elevated vapor pressure beneath the droplet generate a violent out-of-plane lift-off. The substrate geometry plays a key role by modulating vapor-flow-induced lubrication pressure, which governs the critical lift-off volume. Additionally, localized bubble nucleation along the liquid–vapor interface produces capillary waves that can trigger detachment. 

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. 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. 

Substrate permeability effect 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.  

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.

Trampolining in a Leidenfrost droplet

The levitating Leidenfrost (LF) state of a droplet on a heated substrate exhibits dynamic behaviors such as star-shaped oscillations, self-propulsion, bouncing, and trampolining. Between the trampolining events, the droplet exhibits decreased amplitude bouncing or quiescent hovering. We show that the reappearance of trampolining at particular radii arises from parametric resonance between oscillations of the vapor layer and the droplet, where subharmonic droplet oscillations amplify vapor-layer oscillations. Trampolining occurs when the ratio of the vapor-layer natural frequency to the Rayleigh frequency falls within instability zones of the corresponding Mathieu chart. This resonance-driven mechanism is observed across different liquids, initial volumes, and substrate temperatures, revealing its universal nature.

       Coalescence of interacting bubbles

The stability of the thin fluid film between bubbles, droplets, and solid surfaces governs whether these interfaces remain separate or coalesce. Film thinning and rupture lead to coalescence, making it essential to understand the mechanisms that control rupture and the parameters that stabilize or destabilize the film. Many applications require stable films—for example, in microfluidics, a persistent lubricating film prevents cells, drops, or bubbles from contacting channel walls. In contrast, rapid film rupture is desirable in heat-transfer systems to avoid heat-flux fluctuations. To study how these films evolve during bubble or droplet interactions, we solve film-evolution equations numerically and use color interferometry to understand  thin-film dynamics. 

Pool boiling on macroscale surfaces

Effective design of passive two-phase heat sinks requires predicting how boiling behavior varies along the height of millimetric fins in an array. Existing models typically assume that each fin behaves like an isolated flat surface with a uniform heat-transfer coefficient, but it is unclear when this assumption holds for real fin arrays. We conduct pool-boiling experiments on additively manufactured copper heat sinks featuring both rectangular and triangular fins, systematically varying fin spacing and height relative to the fluid capillary length. High-speed visualization reveals the onset of confinement, bubble dynamics between fins, and transitions between boiling regimes along the fin sidewalls. 

Role of extended surface on Quenching Boiling

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 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. 

Passive thermal desalination systems

Desalination promises to be sustainable choice to resolve fresh water shortages increasing day by day. We present 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. We create patterned condenser surfaces that enhance droplet removal and boost condensation efficiency. Further, the key practical challenge such as salt accumulation and limited scalability of passive thermal desalination are addressed. A siphon based evaporator is developed, which helped to create a scalable and salt resistant desalination system. The siphon desalination system with 15-stages produced a high water productivity flux of 6.23 Litres per square metre in an hour under 1000 W/m2 and 3.5 wt% saline water

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.