Electrostatics

Electrostatics Influences Droplet Movement on Surfaces Water droplets moving across surfaces is generally considered a simple and straightforward phenomenon. However, there remain many unanswered questions about the forces acting on sliding droplets. A research team from the Max Planck Institute for Polymer Research, in collaboration with colleagues from TU Darmstadt, made a discovery. According to their findings, in addition to surface energy and viscous friction within the droplet, electrostatics plays an important role. The results were recently published in Nature Physics. When raindrops hit a car windshield, wind pushes them sideways. Even today, exactly how droplets move across a windshield is not fully understood. Yet such understanding is important in many fields, including autonomous driving. For example, cameras mounted on windshields must monitor the road and traffic conditions; for this reason, windshield surfaces must be designed so that droplets are completely blown away by air currents and the image remains clear even in rain. The opposite principle applies to other examples, such as spray paint or insecticide applications, where droplets need to adhere to surfaces. Prof. Hans-Jürgen Butt, Director of Interface Physics at the Max Planck Institute for Polymer Research, explained, "Until now, it was assumed that surface coating was responsible for how droplets move across a surface – that is, in the first few molecular layers." For instance, whether a droplet forms a spherical or flat shape depends on the surface. If the surface attracts the droplet, it flattens itself to provide as much contact as possible. If the surface repels it, as in the well-known lotus effect, it curls up. Additionally, when a droplet moves, viscous friction occurs within the droplet (for example, friction between individual water molecules), which also affects its movement.

Electrostatics Causes Speed Differences

The research team at MPI for Polymer Research discovered that neither capillary nor viscoelastic forces could explain the differences in how quickly droplets move across different surfaces. In particular, the fact that droplets flow at different speeds on different substrates – even when those substrates have identical surface coatings – raised questions, since no difference would be expected. For this reason, researchers first introduced a mysterious "extra force." To track it down, Xiaomei Li, a doctoral student in Hans-Jürgen Butt's department, conducted a droplet race. Li explained, "I filmed droplets on different surfaces, extracted velocity and acceleration profiles from their movements, calculated the forces we already knew, and calculated the force we hadn't yet considered." A surprising result emerged: the calculated force was in the same direction as an electrostatic force that the researchers had first described in a model several years earlier. Jun.-Prof. Stefan Weber, group leader in Butt's department, stated, "By comparing the experimental results with this numerical model, we can now explain previously puzzling droplet trajectories." When previously neutral droplets slide on an insulator, they can become electrically charged: thus electrostatics plays an important role here. On the other hand, on an electrically conductive substrate, the droplet immediately releases its charge back to the substrate. Weber summarized, "The electrostatic force, which no one had previously considered, therefore has a major impact; it must be taken into account for water, aqueous electrolytes, and ethylene glycol on all tested hydrophobic surfaces." The research team has now published the results in Nature Physics. These findings will improve control of droplet movement in many applications, from printing to microfluidics, and from water management to power generation via droplet-based micro-generators. Source https://phys.org/news/2022-04-electrostatics-movement-surfaces.html More information: Xiaomei Li et al, Spontaneous charging affects the motion of sliding drops, Nature Physics (2022). DOI: 10.1038/s41567-022-01563-6 Journal information: Nature Physics Provided by Max Planck Society