Research
All my research papers can be accessed on Google Scholar or ResearchGate by clicking on one of the following logos:
Selected Research Highlights
When and how do drops capture and remove particles from dirty surfaces?
Water drop moving slowly at 50 µm/s successfully captures the particle. The snapshots in (a) show a bottom-view perspective of a drop (blue) removing a particle (red). The size of the particle is around the thickness of human hair. The graph in (b) shows the force required to move the drop on the surface. Initially, the force is zero because the drop starts at rest.
Water drop moving fails to capture the particle when moving fast (500 µm/s). In this case, the particle enters and exits the drop instead of remaining attached to it. The maximum force that the drop can exert on the particle is insufficient to move the particle across the surface.
Over time, dust particles accumulate on surfaces, such as windows, leaves, and solar panels. When it rains, some of these particles may be removed by raindrops. But how and when does a drop remove a particle?
To investigate the mechanism behind how a drop removes a particle from a surface, we designed an experimental setup to image collisions between water drops and tiny dust particles on flat surfaces. With this setup, we also quantified the force that the drop needs to exert to move the particle along the surface. We observed that a drop is more likely to capture a particle when the drop is moving slowly, at 50 µm/s (see figure). Above a threshold speed, the particle can no longer be captured because the force required to move the particle at such high speeds is greater than the maximum force that the drop can exert on the particle.
A particle can be removed when the maximum surface tension force that the drop can exert on the particle is greater than the frictional forces that must be overcome to move the particle. These forces depend on the material that the particle is made of, its shape, and on how it moves (does it roll or slide?).
Interestingly, particle removal on flat surfaces is fundamentally different from particle removal on self-cleaning superhydrophobic surfaces, such as the lotus leaf. On a flat surface, a water drop pulls a glass particle horizontally along the surface. In contrast, on superhydrophobic surfaces, particles are typically lifted off the surface. Despite this difference, a unifying principle to enhance removal efficiency is to minimize the adhesion and friction force between the particle and the surface.
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