Spontaneous charging affects the motion of sliding drops

Abstract

Water drops moving on surfaces are not only an everyday phenomenon seen on windows but also form an essential part of many industrial processes. Previous understanding is that drop motion is dictated by viscous dissipation and activated dynamics at the contact line. Here we demonstrate that these two effects cannot fully explain the complex paths of sliding or impacting drops. To accurately determine the forces experienced by moving drops, we imaged their trajectory when sliding down a tilted surface, and applied the relevant equations of motion. We found that drop motion on low-permittivity substrates is substantially influenced by electrostatic forces. Our findings confirm that electrostatics must be taken into consideration for the description of the motion of water, aqueous electrolytes and ethylene glycol on hydrophobic surfaces. Our results are relevant for improving the control of drop motion in many applications, including printing, microfluidics, water management and triboelectric nanogenerators.

Dynamics of fog droplets on a harp wire

“…There are two points worth discussing regarding the model's requirement of dual numerical pre-factors, one for the viscous wedge (a) and one for the energy conversion efficiency of coalescence (b). First, it is unavoidable that analytical expressions for these terms must include numerical pre-factors to overlay the theoretical curve onto the experimental data, as evidenced by previous reports on viscous wedges [32][33][34][35][36] and coalescence-induced self-propulsion. 19,20,[23][24][25][26] This is because the precise geometry and velocity field of the viscous wedge, as well as the oscillatory hydrodynamics of a merging droplet, cannot be fully solved by analytical expressions.…”

“…The resulting viscous singularity in the receding wedge is typically far stronger than that of the bulk viscous stress across the droplet. [32][33][34][35][36] For a sliding droplet whose receding wedge is fully wetting the thin film on the wire, Tanner's law predicts a dynamic contact angle of y E (cmU d,i /g) 1/3 , where c E ln(L/e) is a numerical factor accounting for singular dissipation at the wedge, where L is the characteristic scale of the wedge and e is the film thickness on the wire. 37 From optical microscopy, we roughly estimate scales of L B 10 mm and e B 1 nm (i.e., the film is invisible to the microscope aside from its presence being inferred from the wedge curving flush to the wire).…”

Controllable and Gradient Wettability of Bilayer Two-Dimensional Materials Regulated by Interlayer Distance

“…Each solid atom of the sheet is treated to be neutral, and thus, the substrate can be seen as neutral materials as a whole. This work mainly focuses on the wettability variation induced by the interlayer distance without consideration of the electrical effect and spontaneous charging process as revealed recently . Moreover, as mentioned above, the existing works of the CA of droplets on graphene are within a wide range, implying that the wettability of monolayer graphene is still a controversial issue .…”

“…This work mainly focuses on the wettability variation induced by the interlayer distance without consideration of the electrical effect and spontaneous charging process as revealed recently. 43 Moreover, as mentioned above, the existing works of the CA of droplets on graphene are within a wide range, implying that the wettability of monolayer graphene is still a controversial issue. 32 Hence, to fully consider the uncertainty of wettability, we adopt three representative wettabilities from hydrophobicity to hydrophilicity in our simulations.…”

A review on control of droplet motion based on wettability modulation: principles, design strategies, recent progress, and applications

“…Figure 6(r) shows the control of droplet jump trajectory along an electric field on a superhydrophobic surface [ 123 , 124 ]. In controlling the charged droplet, both the coating surface and substrate permittivity also influence the droplet sliding behavior [ 125 ].…”