Electro-Stretching and Electro-Coalescence of Sessile Drops of Conducting Polymer Solutions with and without Surfactant and Dielectric Particles

The present study explores electrically driven stretching of individual conducting polymer drops and electro-coalescence of drop pairs on a dielectric surface under a strong electric field of 10 kV. Conducting PEDOT:PSS and PEDOT:PSS-PEO [poly(3,4-ethylenedioxythiophene):polystyrenesulfonate-poly(ethylene oxide)] drops were tested with and without a nonionic surfactant (Silwet L-77) and dispersed titanium dioxide (TiO₂) particles. The surfactant dramatically reduced the solution's surface tension from ∼70 to ∼20 mN/m, and PEO doping increased viscosity and imparted shear-thinning behavior. Under the applied field, drops stretched between the electrodes and spread much wider than without voltage. This pronounced stretching is driven by electrostatic Maxwell stress overcoming capillarity (the electric capillary number Ca_E ∼ 0.9-2.3). The surfactant further enhanced deformation by lowering surface tension, and polarizable TiO₂ particles introduced dielectrophoretic forces that also eased stretching. Furthermore, in surfactant-free cases, two initially separate drops underwent rapid electro-coalescence: upon field activation, finger-like protrusions formed within ∼2-5 ms from each drop to meet and create a narrow liquid bridge, which then expanded to merge the drops into one over a few seconds. However, drops containing surfactant (and TiO₂) failed to coalesce, as strong Marangoni flow from surfactant-induced surface tension gradients dominated the Maxwell stress-driven attraction. Such surfactant-laden drops instead developed dendrite-like patterns at their trailing edges, with only a brief (∼millisecond) "handshake" contact and no full merging. These findings clarify how solution composition and interfacial and electrohydrodynamic mechanisms govern drop deformation and merging, providing insights for controlling drop behavior in coating processes. Moreover, the present experiments with drops of solutions of the conducting polymer with a surfactant (a superspreader SILWET L-77) and particles added reveal a novel phenomenon─a competition of the concentration-gradient Marangoni flow with electro-coalescence driven by the electric Maxwell stresses, which causes a noncoalescence even at a very high applied voltage.