You’ve probably experienced the frustration of preparing for a nice dinner party only to realize that all of your clean glasses have dirty-looking streaks or white spots on them. They’re not actually dirty – the spots are just watermarks, which occur when droplets evaporate from a solid surface. They sure don’t look nice, though, which is why so many dish soap brands promise “streak-free glassware!” That’s often not the case, unfortunately – the way that watermarks appear on surfaces is uncontrollable because the shape and location of a droplet as it evaporates is unpredictable.
This is problematic when it comes to more than just glassware. In inkjet printing, a stray ink droplet can leave a distorted shape on a piece of paper, and in micro-engineering, watermarks can affect the performance of delicate microstructures.
But now, for the first time, researchers at Northumbria University’s Smart Materials and Surfaces Laboratory and The Open University’s School of Mathematics and Statistics have figured out a way to control the shape and location of a drying droplet. Typically, when a droplet evaporates from a solid surface, its edge “pins” and “depins” in an uncontrolled manner, due to the microscopic roughness of the solid surface. But in this case, the researchers were able to control the way the droplets dried through a combination of wavy solid geometry and an ultra-smooth surface treatment. They call the controlled manner of evaporation “snap evaporation.”
“An egg-box is an example of a wavy solid: it has repeating peaks and valleys that form a wavy pattern,” said Dr. Gary Wells, Senior Lecturer at Northumbria University. “We 3D printed such a wavy pattern and covered its rough surface with a thin lubricant layer. The resulting composite surface keeps the wavy shape, but becomes ‘ultra-smooth’. When we left water droplets to evaporate on these wavy surfaces, they initially retracted from the solid in a smooth way, as one would expect for a perfectly smooth solid.However, the wavy surface makes the droplets ‘snap’ at specific points, changing their position and shape. This is a new mode of evaporation, which we have named ‘snap evaporation’. Remarkably, this process is highly reproducible, and we have found that the actual design of the wavy pattern can control the position and shape of the droplet.”
The reason for the snap behavior can be explained by bifurcation theory, a branch of mathematics that studies how a system responds to a change in a control parameter. In this case, the system is the droplet, and the change in the control parameter is the reduction of mass due to evaporation.
“The main idea behind our theory is that the configuration that a droplet takes on a wavy solid pattern is not unique,” said Dr. Marc Pradas, Lecturer at The Open University. “There are different shapes and positions that the same amount of liquid can occupy on a given wavy pattern. “During evaporation, the mass of a droplet changes, and it turns out that what was a stable drop shape and position becomes unstable. At this point, which is known as a bifurcation, the droplet must change its shape and position. The wavy surface acts as a steering wheel, guiding the droplet to the next stable configuration after a snap has occurred.”
The researchers also took inspiration from the pitcher plant, which has a super-smooth surface that allows insects to slide in and become trapped. They created a smooth, lubricated surface that kept the droplets from “pinning” as they would to an ordinary rough surface. You can read more about the details of the process here.
The research could have an impact on many applications. The automotive industry, for example, could treat car surfaces differently to minimize watermarks, and the computer and smartphone industries could improve the efficiency of micro-heatpipes, which remove heat from microprocessors.
“The implications of our study can have an impact in many everyday applications, and we are currently working with industrial partners that can benefit from our research,” said Dr. Rodrigo Ledesma-Aguilar, Associate Professor at Northumbria University. “For example, we are working with Jaguar Land Rover to develop new strategies that minimise watermarks on the surfaces of cars. Another example is our collaboration with Sustainable Energy Systems, who can benefit from our results by improving the efficiency of heat-removal systems used in micro-processors such as CPUs and GPUs.”
The research was documented in a paper entitled “Snap evaporation of droplets on smooth topographies,” which you can read here. Authors include Gary G. Wells, Élfego Ruiz-Gutiérrez, Youen Le Lirzin, Anthony Nourry, Bethany V. Orme, Marc Pradas and Rodrigo Ledesma-Aguilar.
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