Unravelling the secrets of droplet motion

How do drops of water move on surfaces? It may sound simple, but the truth is that scientists still do not have a full grasp of the forces acting on them. Recent efforts to generate electricity from moving drops have made closing this knowledge gap more vital.

Now, research supported in part by the EU-funded DynaMo project has revealed that a drop’s motion is not only affected by surface energy and viscous friction – friction between the individual water molecules within the drop – as previously thought. It appears that electrostatics also play a key role. The study was published in the journal ‘Nature Physics’.

“Until now, it was assumed that the surface coating was responsible for how the droplet moves on a surface—that is, the first few molecular layers,” explains study senior author Prof. Hans-Jürgen Butt of the Max Planck Institute for Polymer Research, Germany, in a news item posted on ‘Phys.org’. This, together with the viscous friction happening inside the drop of water when it moves, was known to influence the drop’s movement.However, a simple experiment conducted by the researchers showed that the motion of drops cannot be predicted with any precision based on these forces alone. The first piece of evidence was that they observed different average velocities on surfaces with identical surface chemistry, but different substrate conductivities and substrate thicknesses. Water drops were found to move faster on gold surfaces coated with a monolayer of perfluorodecanethiol or Teflon films than on silicon dioxide (SiO2) surfaces coated with perfluorooctadecyltrichlorosilane (PFOTS). The second piece of evidence was that the sliding speeds of a series of drops on a particular surface become dependent on the drop number, and therefore on surface history. For example, the 50th drop slides faster down a PFOTS-coated SiO2 plate than the first drop.

So what could be the missing force? “I filmed the drops on different substrates, extracted velocity and acceleration profiles from their motion, calculated out the forces that were already known to calculate the force that we had not yet had a look at,” states study first author Xiaomei Li, who is a PhD student at the Max Planck Institute for Polymer Research.

Based on their observations, the team concluded that the missing force must be electrostatics. The calculated force tallies with an electrostatic force they had described in an earlier model. “By comparing the experimental results with this numerical model, we can explain previously confusing droplet trajectories,” notes Prof. Stefan Weber, also from the same institute.

When previously neutral droplets slide over an insulator, they can become electrically charged, but on an electrically conductive substrate the drop of water immediately releases its charge back to the substrate. “The electrostatic force, which no one had previously considered, therefore has a major influence: it must be taken into account for water, aqueous electrolytes and ethylene glycol on all hydrophobic surfaces tested,” Prof. Weber concludes.

The results of the study supported by DynaMo (Dynamic charging at moving contact lines) could help improve the control of drop motion in a wide range of applications, such as printing, microfluidics, water management and triboelectric nanogenerators.

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DynaMo project