Non-coalescence of oppositely charged drops

TL;DR

This study demonstrates that oppositely charged drops can avoid coalescence and instead bounce off each other when subjected to electric fields exceeding a critical strength, revealing a new non-coalescence phenomenon.

Contribution

It identifies a critical electric field strength that prevents coalescence of oppositely charged drops, challenging the assumption of inevitable coalescence in such systems.

Findings

Drops coalesce at low electric fields

Above a critical field, drops repel and bounce

Transient meniscus bridge formation observed

Abstract

Oppositely charged drops have long been assumed to experience an attractive force that favors their coalescence. In this fluid dynamics video we demonstrate the existence of a critical field strength above which oppositely charged drops do not coalesce. We observe that appropriately positioned and oppositely charged drops migrate towards one another in an applied electric field; but whereas the drops coalesce as expected at low field strengths, they are repelled from one another after contact at higher field strengths. Qualitatively, the drops appear to `bounce' off one another. We directly image the transient formation of a meniscus bridge between the bouncing drops.