Electro Drop Friction Force Instrument*

Slide electrification¹ causes electrostatic force on drops². It is impractical to control drop speed and position on an inclined plane. As a result, drops reach velocity in the order of m/s. This velocity corresponds to high capillary numbers (Ca = μν/γ ≥ 10^-3; where μ is dynamic viscosity, γ is surface tension, and ν is speed) for a water drop. At such capillary numbers, viscous dissipation within the drop becomes significant. Thus, it is unclear how does charge separation influences the drop friction and what behavior it shows in the absence of hydrodynamic dissipation.  

Using in-house developed Electro-Drop Friction Force Instrument (eDoFFI), we realized measurements at low capillary numbers (Ca ≤ 10^-4). At low capillary numbers, viscous dissipation becomes negligible. The eDoFFI setup facilitates both drop friction and charge measurements simultaneously. For a 5 µL water drop sliding at 2mm/s over 40mm distance, we measure ≈ 3nC of charge on Trichloro(1H,1H,2H,2H-perfluorooctyl) silane (PFOTS) coated glass surface, and ≈ 1 nC on Trichloro Octyl Silane (OTS) coated glass surface. The measured charge does not depend on the drop speed (at mm/s). Next, we measure drop friction on 1 mm thick fused quartz, 1 mm thick glass, and 0.25 mm silicon wafer to investigate the influence of charge separation on drop friction. All the substrates are coated with PFOTS. The lowest friction ≈ 35µN is measured on silicon wafer followed by ≈ 50µN on glass, and ≈ 65µN on fused quartz. Hence, the drop friction is influenced by the substrate’s relative permittivity. Similar dependence of force on substrate’s permittivity is observed even when the charges are not extracted from the drop.

(1) Yatsuzuka, K.; et al., K. Electrification phenomena of pure water droplets dripping and sliding on a polymer surface. Journal of electrostatics 1994, 32 (2), 157-171.

(2) Li, X.; et al. Spontaneous charging affects the motion of sliding drops. Nat. Phys. 2022.