Effects of direct current electric fields on internal flow in sessile water droplets

The effect of a uniform direct current (DC) electric field on the internal flow field of sessile de-ionized water droplets has been investigated experimentally. Herein, a parallel-plate electrode configuration was employed, which generates a uniform electric field that causes negligible droplet deformation. The results show that under an electric field of 80 V/mm, the flow field within the droplet transforms from an initially symmetric double-vortex structure to a markedly asymmetric one, with a vorticity difference between the two sides reaching up to 40%. Vector field analysis indicates that variations in the flow field arise from a tangential flow near the droplet surface induced by the electric field. Further experiments and simulations indicate that this surface flow is independent of the particles within the droplet or the electrothermal effects at the droplet surface. Instead, it arises from the combined effects of the tangential Maxwell stresses and the migration of H3O+ ions enriched at the droplet surface. The asymmetric flow field subsequently results in an orderly “semi-ring” deposition, exhibiting a capability for improved uniformity of particles. This work provides a better understanding of the role of a uniform DC electric field in the internal flow behavior within liquid water, offering a potential basis for developing a low-powered technology to suppress the “coffee-ring” phenomenon.