Droplets
Droplet impact dynamics
Droplet impact is ubiquitous in many natural phenomenon and industrial applications. Due to the intricate interplay among impact inertia, surface tension, and viscous effects, the impact can results in a plethora of interesting phenomena such as bouncing, merging, and splashing. In our lab, we use high-speed imaging and various optical diagnostic techniques to characterize the behavior of the impact and unveil the controlling physics through theoretical and numerical analysis. The deepened understanding will provide design rules for industrial applications.
Application areas: 3D printing, bioprinting, spray coating, spray cooling, material synthesis
Water droplet bouncing from water pool
Water droplet merging with water pool
Regime map for impact outcomes
The bouncing and merging impact outcomes are categorized in a non-dimensional space of Weber number which is the ratio between inertia and surface tension effect and pool thickness normalized by droplet radius. The transition boundaries are quantified with scaling analysis. The prediction can serve as design rules for industry to optimize the impact process.
Interferometric measurement of micron-scale gas layer
During the bouncing process, there is a gas layer trapped between the droplet and pool. The gas layer is of micron thick, thus is hard to observe directly. Here we utilized white light interferometry to measure the gas layer thickness through the interference fringes formed by the light reflected from the droplet and pool surface. The measure gas layer profile provides insight in how the gas layer affects the bouncing behavior.
Laser Induced Fluorescence measurement of the mixing process after merging
Mixing dynamics
After merging, the liquid in the droplet mixes with the liquid in the pool through complex vortex dynamics. The vortex structure greatly increases the contact area between the liquids from the droplet and the pool, which is of critical interest when the liquid react with each other to form comple morphologies of the final product. We use Laser Induced Fluorescence to characterize the mixing dynamics, which lays the foundation for further manipulating the reaction product for material synthesis.
Droplet evaporation
When suspension droplet evaporates, the particles accumulate at the edge, which is the so-called “coffee ring” effect. In many applications, a uniform particle distribution after evaporation is more desirable. In our lab, we investigate the controlling mechanisms for the particle distribution during droplet evaporation and explore ways to form targeted patterns. The formed patterns can also be utilized to back track the particle properties and evaporation conditions for forensic study and disease diagnosis.
Application areas: 3D printing, bioprinting, coating, forensic study, disease diagnosis
Control particle distribution during drop evaporation
Particles
Particle delivery into deadend pores
How to deliver particles into a deadend pore? Convection is blocked by the deadend and diffusion of particles is extremely slow. Here we utilize a phenomenon called diffusiophoresis where particles migrate under solute gradients. Thus the particles can detect and migrate towards the target. Chemical reactions can be further utilized to establish the solute gradient.
Application areas: drug delivery, oil recovery
Particle patterning
In many applications, there are spots with heterogeneous surface charge. It can be a contaminated spot on a surface, an oil drop trapped somewhere in the reservoir, or a cancerous cell that has different surface charge than normal cells. Their location may not be known and we want to detect them or deliver something to that target. How do we do it? Let’s think about a positively charged surface with a negative charged region somewhere on the surface, but we don’t know its location. Can we utilize colloidal particles to detect it? Or in other words, would a particle migrate in response to a surface charge heterogeneity? It will be very useful if we can achieve that, because for any pre-existing surface chemistry heterogeneity at unknown locations, we can utilize particles to sense the change in the surface charge, migrate towards the target, assemble there, and so we can find it! Our goal is to achieve this and develop customized strategies for different applications.
Application areas: sensors, self-assembly, pattern formation
