Slide Electrification: Charging Surfaces with Sliding Water Droplets

When water droplets slide down hydrophobic surfaces, they charge the substrate in a process known as slide electrification. This effect is well documented, but the underlying mechanism of charge separation and charge dissipation remain unclear. We study this effect using water droplets sliding down a tilted glass plate coated in polydimethylsiloxane (PDMS) in a sealed container with fixed relative humidity. We varied the humidity from 14-90%. 

After each droplet passed underneath an electrostatic voltmeter, we observed a voltage (V) as large as -200 V, which decayed over several days. We developed an empirical function V(t) that fits data across varying humidities, initial voltages, and number of droplets:

V(t) = A1·exp(-t/τ₁) + A2·exp((-t/τ₂)^β)+A3[(1+t/λ)^μ+C].  

The first two terms decay most rapidly, with (τ₁, τ₂ ≈ 100s). 

We interpret these terms as fast capacitor-like discharge followed by a stretched exponential with β ≈ 0.5, which has been reported in transport in disordered dielectric relaxation (known as the Kohlrausch effect). The last term is a far slower component with λ ≈ 10⁷ s, possibly due to slow migration of ions or exchange with the atmosphere.

Our current work attempts to conclusively link each decay term to a physical mechanism, and test these interpretations experimentally. Slide electrification is observed naturally in rainfall, storm clouds, and maybe even in insect pollination, suggesting the mechanisms we study here could help explain previously unknown electrostatic phenomena in nature.