Sungyon Lee’s research while affiliated with University of Minnesota, Duluth and other places

Fluid-fluid interfaces laden with discrete particles behave curiously like continuous elastic sheets, leading to their applications in emulsion and foam stabilization. Although existing continuum models can qualitatively capture the elastic buckling of these particle-laden interfaces -- often referred to as particle rafts -- under compression, they fail to link their macroscopic collective properties to the microscopic behaviors of individual particles. Thus, phenomena such as particle expulsion from the compressed rafts remain unexplained. Here, by combining systematic experiments with first-principle modeling, we reveal how the macroscopic mechanical properties of particle rafts emerge from particle-scale interactions. We construct a phase diagram that delineates the conditions under which a particle raft collapses via collective folding versus single-particle expulsion. Guided by this theoretical framework, we demonstrate control over the raft's failure mode by tuning the physicochemical properties of individual particles. Our study highlights the previously overlooked dual nature of particle rafts and exemplifies how collective dynamics can arise from discrete components with simple interactions.

Free surface flows driven by boundary undulations are observed in many biological phenomena, including the feeding and locomotion of water snails. To simulate the feeding strategy of apple snails, we develop a centimetric robotic undulator that drives a thin viscous film of liquid with the wave speed $V_w$ . Our experimental results demonstrate that the behaviour of the net fluid flux Q strongly depends on the Reynolds number Re . Specifically, in the limit of vanishing Re , we observe that Q varies non-monotonically with $V_w$ , which has been successfully rationalised by Pandey et al. ( Nat. Commun. , vol. 14, no. 1, 2023, p. 7735) with the lubrication model. By contrast, in the regime of finite inertia ( ${Re} \sim O(1)$ ), the fluid flux continues to increase with $V_w$ and completely deviates from the prediction of lubrication theory. To explain the inertia-enhanced pumping rate, we build a thin-film, two-dimensional model via the asymptotic expansion in which we linearise the effects of inertia. Our model results match the experimental data with no fitting parameters and also show the connection to the corresponding free surface shapes $h_2$ . Going beyond the experimental data, we derive analytical expressions of Q and $h_2$ , which allow us to decouple the effects of inertia, gravity, viscosity and surface tension on free surface pumping over a wide range of parameter space.

Examples of fluid flows driven by undulating boundaries are found in nature across many different length scales. Even though different driving mechanisms have evolved in distinct environments, they perform essentially the same function: directional transport of liquid. Nature-inspired strategies have been adopted in engineered devices to manipulate and direct flow. Here, we demonstrate how an undulating boundary generates large-scale pumping of a thin liquid near the liquid-air interface. Two dimensional traveling waves on the undulator, a canonical strategy to transport fluid at low Reynolds numbers, surprisingly lead to flow rates that depend non-monotonically on the wave speed. Through an asymptotic analysis of the thin-film equations that account for gravity and surface tension, we predict the observed optimal speed that maximizes pumping. Our findings reveal how proximity to free surfaces, which ensure lower energy dissipation, can be leveraged to achieve directional transport of liquids.

Examples of fluid flows driven by undulating boundaries are found in nature across many different length scales. Even though different driving mechanisms have evolved in distinct environments, they perform essentially the same function: directional transport of liquid. Nature-inspired strategies have been adopted in engineered devices to manipulate and direct flow. Here, we demonstrate how an undulating boundary generates large-scale pumping of a thin liquid near the liquid-air interface. Two dimensional traveling waves on the undulator, a canonical strategy to transport fluid at low Reynolds numbers, surprisingly lead to flow rates that depend non-monotonically on the wave speed. Through an asymptotic analysis of the thin-film equations that account for gravity and surface tension, we predict the observed optimal speed that maximizes pumping. Our findings reveal a novel mode of pumping with less energy dissipation near a free surface compared to a rigid boundary.

A granular raft—a two-dimensional (2D) particle layer floating at a fluid-fluid interface—collapses when losing its stability under compression. Although granular rafts are frequently encountered in various natural and engineering settings, how a raft fails under compression is still an open question. Here, by combining experiments with theoretical modeling, we examine the failure modes of granular rafts without free periphery under quasistatic biaxial compression. Different from granular rafts with open periphery, granular rafts in our study remain stable under finite compressive stresses. More surprisingly, under large compression, granular rafts made of small particles sink gradually by expelling individual particles, whereas rafts of large particles collapse catastrophically by forming large-scale creases. The collective creasing is enhanced by the 2D particle density and is suppressed by the density difference of the two fluids. We develop a one-dimensional continuum model for the shape of the granular rafts and the concentration of particles along the fluid-fluid interface, which provides quantitative explanations of our experimental findings.

Partially wetting droplets under an airflow can exhibit complex behaviours that arise from the coupling of surface tension, inertia of the external flow and contact-line dynamics. Recent experiments by Hooshanginejad et al. ( J. Fluid Mech. , vol. 901, 2020) revealed that a millimetric partially wetting water droplet under an impinging jet can oscillate in place, split or depin away from the jet, depending on the magnitude (i.e. 5\unicode{x2013}20\ {\rm m}\ {\rm s}^{-1} ) and position of the jet. To rationalise the experimental observations, we develop a two-dimensional lubrication model of the droplet that incorporates the external pressure of the impinging high-Reynolds-number jet, in addition to the capillary and hydrostatic pressures of the droplet. Distinct from the previous model by Hooshanginejad et al. ( J. Fluid Mech. , vol. 901, 2020), we simulate the motion of the contact line using precursor film and disjoining pressure, which allows us to capture a wider range of droplet behaviours, including the droplet dislodging to one side. Our simulations exhibit a comparable time-scale of droplet deformations and similar outcomes as the experimental observations. We also obtain the analytical steady-state solutions of the droplet shapes and construct the minimum criteria for splitting and depinning.

We experimentally investigate a millimetric armored droplet of a water-isopropyl alcohol solution sedimenting through oil and approaching a water layer at the bottom of the container. Upon reaching the oil-water interface, the droplet is shown to rupture and coalesce with the water either for low droplet densities (floating rupture) or for low oil viscosities (sinking rupture). By contrast, for sufficiently large drop density or oil viscosity, the oil covering the armored drop pinches off in the underlying water, as the armored drop continues to sink. This leads to the double encapsulation of an aqueous solution in water, which can be utilized to transport desired ingredients within a wet environment. We show that a simplified quasistatic model of a rigid sphere successfully captures the limit of the floating rupture behavior. We also rationalize the transition from the sinking rupture to oil pinch-off, by comparing the timescales of the film drainage versus sinking. Our results demonstrate that an effective Bond number and an effective Ohnesorge number are the two key dimensionless parameters that characterize the pinch-off threshold in good agreement with the experiments.

Partially wetting drops are ubiquitous in nature and industry and are often subject to a combined forcing by wind and gravity. In particular, the stability of water drops under the combination of wind and other external forces is relevant in numerous applications that include aircraft de-icing, heat exchangers, and fuel cells. In this paper, we investigate the onset of droplet depinning from a solid substrate when a partially wetting water droplet is simultaneously exposed to high-Reynolds-number airflow and to gravity. We first develop simple scaling arguments which explain that the critical flow velocity for depinning, Ucr, and the droplet volume, V, scale as Ucr∼V−1/6 in the absence of gravitational effects, in good agreement with the existing experimental data. We then develop a two-dimensional model for a unidirectional flow over a slender drop under two scenarios: aligned with (downwind) or opposing the gravitational body force (upwind). Our results show a clear deviation in Ucr between the upwind and downwind cases as the droplet size is increased. The differences between these two regimes are further manifested in the distinct droplet shapes at the critical onset of depinning. Finally, we investigate the role of flow separation from the leeward side of the drop by systematically increasing the slope of the droplet surface at the separation point. Our results point to more pronounced effects of the flow separation in the upwind regime.

When a mixture of viscous oil and non-colloidal particles displaces air between two parallel plates, the shear-induced migration of particles leads to the gradual accumulation of particles on the advancing oil–air interface. This particle accumulation results in the fingering of an otherwise stable fluid–fluid interface. While previous works have focused on the resultant instability, one unexplored yet striking feature of the experiments is the self-similarity in the concentration profile of the accumulating particles. In this paper, we rationalise this self-similar behaviour by deriving a depth-averaged particle transport equation based on the suspension balance model, following the theoretical framework of Ramachandran ( J. Fluid Mech. , vol. 734, 2013, pp. 219–252). The solutions to the particle transport equation are shown to be self-similar with slight deviations, and in excellent agreement with experimental observations. Our results demonstrate that the combination of the shear-induced migration, the advancing fluid–fluid interface and Taylor dispersion yield the self-similar and gradual accumulation of particles.

In nature, high-speed raindrops often impact and spread on particulate surfaces (e.g., soil, plant leaves with spores or pollen). We study the dynamics of droplet impact on a loosely packed monolayer of particles by combining experimental and mathematical approaches. We find that the presence of mobile particles lowers the critical impact velocity at which the droplet exhibits corona splashing, as the particle area fraction is systematically increased. We rationalize this experimental observation by considering the jamming of frictional particles at the spreading rim. Elucidating the splashing transition of the drop on a particulate bed can lead to a better understanding of soil loss and erosion from falling raindrops.