Differing effects of AC fields on charged droplet morphologies

Active, non-contact manipulation of a droplet’s shape has important applications in fields such as mass spectrometry and nanomaterial synthesis. This can be done through charging the droplet, making the electrohydrodynamics of charged droplets an emerging critical area of research.

However, droplet dynamics under direct current (DC) fields are much better explored compared to those under alternating current (AC) fields. As a result, insights into electrohydrodynamics could be missing because instability dynamics under DC and AC fields differ significantly, the latter producing droplets blooming into distinct halberd-like and lotus-like morphologies.

To understand differences between morphological transitions of charged droplets, Wang et al. used high-speed imaging to characterize methanol droplet dynamics in a highly viscous medium under a non-uniform AC field.

“Practically, our research offers a new methodology for actively and precisely regulating multi-scale fluid interface morphologies,” said author Daorui Wang. “This capability is expected to advance technologies requiring precise droplet manipulation, such as micro-encapsulation, electro-spinning for nanofiber production, and the synthesis of complex functional materials in chemical engineering.”

Notably, their results showed that when the AC field strength exceeds a critical threshold, the shapes of the droplets transition from disordered to highly ordered, non-equilibrium polygonal contours — the electric field has shifted from being disruptive to constraining. Subsequent quantitative analysis revealed a nonlinear saturation mechanism that explains the observed phenomenon.

The authors believe that interaction between hydrodynamic forces, electric stresses, and complex rheology will be the next frontier of nonlinear electrohydrodynamics.

“Our future work will likely explore the coupling effects of electric fields with non-Newtonian fluid properties, which are commonly found in biological and industrial applications,” said Wang.

Source: “Morphological evolution of electrified droplet: From fingering instability to Coulomb explosion,” by Daorui Wang, Junfeng Wang, Dongbao Wang, Hang Yang, Jian Gao, and Rui Yuan, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0304599 .