Physicists have found that the sliding of a drop on a surface depends on electrostatics

A team of physicists from the Max Institute for Polymer Research and the Darmstadt Technical University

has found that neither capillary nor viscoelastic forces canExplain the differences in the speed at which droplets move across different surfaces.As scientists note, questions were raised, in particular, by the fact that the drops are drippingat different speeds across multiple substrates—even if those substrates haveIdentical surface coating.

To explain the deviation of the trajectory of the actual motion, the drops of calculated physics introduced an additional force.The authors of the work organized a kind of "race": they filmed a large number of droplets moving on different surfaces.For each movement, the researchers determined velocity and acceleration profiles, calculating forces that were already known to determine the additional force. 

The analysis showed that the value of this new force was consistent with the electrostatic interaction that the researchers first described in the model several years ago.

“By comparing the experimental results with this numerical model, we can explain previously confusing droplet trajectories,” says Stefan Weber, one of the study participants.

As the scientists note, if previously neutral droplets slide across the insulator, they canOn the other hand, on an electrically conductive surface, the droplet immediately gives offits charge back to the substrate. 

"Electrostatic force, which no one has previously taken into account, therefore has a big impact on the movement: it must be taken into account for water, aqueous electrolytes and ethylene glycol on all hydrophobic surfaces tested," addsWeber. 

According to physicists, the results of the study will helpimprove droplet control in many applications, including printing, microfluidics, and even power generation with mini-drop generators.

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