It's all in the mixing

May 12, 2022

A group of scientists led by Nate Cira of Harvard and Cornell University and Stefan Karpitschka of the Max Planck Institute for Dynamics and Self-Organization has discovered that some liquid droplets first spread out on surfaces and then contract again on their own. This boomerang effect depends on the composition of the droplets. Since these leave virtually no traces when they contract, unlike conventional drying, this opens up new possibilities for cleaning and removing particles from sensitive surfaces such as microchips.

While brooms and sponges are the means of choice to fight contamination in everyday life, mechanical cleaning of sensitive surfaces such as electronic components is not possible. Yet, impurities created during their production must be removed to ensure technical function. Known methods based on evaporation often leave behind small particles on the surface.

Through their work on the dynamics of liquid mixtures, the scientists have developed a new approach to solving this problem. They used the so-called Marangoni effect, the physical principle behind the Boomerang motion.

A droplet of water, alcohol and propylene glycol first expands and then contracts on its own, picking up and removing soluble particles.

A well-known phenomenon among wine experts: The Marangoni effect

Wine enthusiasts may have noticed that mixtures of liquids expand or contract and form patterns. After pouring the wine, the mixture of alcohol and water forms various patterns on the surface of the glass. The reasons for this are the different surface tensions and evaporation rates of water and alcohol.

This is also the basis for the observed boomerang effect, but using mixtures of three liquids, water, alcohol and propylene glycol. The latter also has wide industrial applications, including in paints, coatings and cosmetics. Professor Nate Cira describes the results: "Depending on the composition, a droplet of the three components either spreads out or contracts when you put it on a glass surface. However, at certain mixing ratios, we observe spreading first, followed by contraction a few seconds later." In addition to the composition of the droplets, humidity and temperature also play a role, the researchers noted in their study.

New possibilities for particle removal

But how does this unusual behavior of the mixture come about? Dr. Stefan Karpitschka used simulations to get to the bottom of this effect: "We observe competing Marangoni effects here: The alcohol at the edge of the drop evaporates first, surface tension changes and generates an outward force. The result: the drop spreads out. Once the alcohol at the edge has gone, the effect is reversed and the drop contracts again. Since water evaporates from the edge as well, the drop takes solutes with it instead of leaving residues behind."

Based on this discovery, a completely new method for cleaning sensitive surfaces and materials could be established. Impurities and particles deposited during manufacturing could be removed cleaner and by less solvent than in traditional approaches.

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