Something as simple as a drop of water on the skin is really understandable – some people think. Of course, there are many unanswered questions about the forces that form a sliding droplet. A team of researchers from the Max Planck Institute for Polymer Research in collaboration with colleagues from TU Darmstadt found: In addition to the surface energy and viscous friction in the droplet, electrostatics play a big role. The results were published in a journal Nature Physics.
The rain hits the car window and the wind pushes the drops to the side. Even today, no one can accurately predict the movement of drops on a windmill. However, this information is important in many areas, such as autonomous driving: For example, cameras fitted with windshield wipers are supposed to be aware of the road and traffic conditions – for this reason, the skin of the windshield must be prepared in this way. a way for the drops to be blown down by the stream so that the rain could be seen clearly. Other features with a different texture are applications where the drops need to be applied to the skin, such as spray paint or pesticides.
“Until now, it has been thought that the skin layer is responsible for how the droplet moves through the skin – that is, the first molecular layers,” says Prof. Hans-Jürgen Butt is the director of the “Physics of Interfaces” department at the Max Planck Institute for Polymer Research. For example, it affects the skin if it has a spherical shape or a flat drop. If the drop wants to touch the skin, press it firmly against it as much as possible. If she does not like the skin, as in the case of the famous lotus effect, curls. It is also clear that when a droplet moves, the viscous pressure – that is, the friction between each water molecule – is inside the droplet, which stimulates its movement.
Electrostatics change speed
The team of researchers at MPI for Polymer Research found that capillary or viscoelastic particles cannot explain the differences in the speed at which the particles move in different directions. Questions were raised because the droplets travel at different speeds on different substrates – even if these substrates have the same surface coating, the differences are unlikely. So the researchers first put in a “super strength.” To follow him, Xiaomei Li, a Ph.D. student in the office of Hans-Jürgen Butt, organizing the fall race. “I took the falls on different substrates, subtracted the speed and velocity from their movement, calculated the previously known forces to calculate the strength that we didn’t know,” he said. he explained.
The surprising result: the calculated strength agrees with the electrostatic force previously described by the researchers in a model a few years ago. “By comparing the experimental results with this statistical model, we can describe the first mixed droplet pathways,” said Jun.-Prof. Stefan Weber is a team leader in the Butt office.
If non -permanent droplets slip on an insulator, they can become electrical: So electrostatics play an important role there. On the other hand, the droplet immediately releases its charge on the substrate. “The electrostatic force, which was never thought of before, is therefore a great power: it has to be considered for water, water electrolytes and ethylene glycol in all hydrophobic areas tested,” Weber said. The research team published the results in a journal Nature Physics. These results will improve the control of droplet movement in many applications from printing microfluidics or water dispersing to power generation through droplet-based mini-generators.
The multi-function electrostatic droplet tweezer is remote control
Xiaomei Li et al, Voluntary influx contributes to the movement of slippery falls, Nature Physics (2022). DOI: 10.1038 / s41567-022-01563-6
Presented by Max Planck Society
Directions: Electrostatics influences the movement of droplets on surfaces (2022, April 15) Retrieved 15 April 2022 from https://phys.org/news/2022-04-electrostatics-movement-surfaces .html
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