Research

All my research papers can be accessed on Google Scholar or ResearchGate by clicking on one of the following logos:

Selected Research Highlights

Using droplets to remove dirt from dirty surfaces

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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Visualising friction when droplets move on lubricated surfaces

On most surfaces, small droplets tend to stick, as we usually see on our windows on a rainy day. Droplets stick on these surfaces because they experience high friction. Sticky droplets can be inconvenient for many applications, making it desirable to develop surfaces that have low friction. One approach to reduce the friction is to lubricate the surface with an oil.

On lubricated surfaces, droplets move very easily, at least initially. But once in motion, the speed at which they move is rather unremarkable. When the lubricant is highly viscous (which is often a requirement to ensure the lubricant does not drain away), the droplet speed is not much faster than that of an adult snail (https://en.wikipedia.org/wiki/Land_snail). This lack of dynamism suggests that the droplets are experiencing a high kinetic friction (kinetic means friction experienced during motion).

The question we asked is: Where is the friction coming from?

To answer this question, we first had to develop a new computational algorithm that can accurately provide the fluid flow within the drop, lubricant, and the surrounding air. To ensure that the new algorithm was accurate, we compared the shape of the lubricant meniscus surrounding the drop to the corresponding shape that we observed in experiments. Once we were confident of the accuracy of our computational algorithm, we analysed the fluid flow in the drop and lubricant to calculate which regions dissipate the most energy and lead to friction. To visually identify where friction is localised, we created 'heatmaps', as shown in the figure above. These heatmaps show that the majority of energy is dissipated in the lubricant, with the drop contributing less than 30% of the total energy dissipated.

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