Electric Field Driven Reversible Spinodal Dewetting of Thin Liquid Films on Slippery Surfaces

Abstract

The stability of thin liquid films on a surface depends on the excess free energy of the system involving various short‐range and long‐range interactions. When forced to wet an unfavorable surface, thin liquid films dewet into multiple small‐sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using external stimuli, for example, electric field, temperature, or light, one can control the stability of thin liquid films on demand. Here the electric‐field induced reversible dewetting and rewetting of thin liquid films underneath aqueous drops on slippery surfaces is studied. Upon applying voltage, hundreds of nanometer thick stable liquid films dewet in a manner identical to the spinodal dewetting of few nanometer‐thick liquid films following the linear stability analysis. Upon removing the applied voltage, the dewetted droplets spread, coalesce with neighboring ones, and form a uniform film again, albeit taking significantly longer times. The characteristic features defined via spatial and temporal evolution of the dewetting and rewetting processes are present over multiple cycles suggesting the complete reversibility of the process.

Frequency-Dependent Dewetting of Thin Liquid Films Using External ac Electric Field

“…Before the voltage is applied, only thermal fluctuations are present at the film–drop interface, and the film is stabilized due to the long-range Lifshitz–van der Waals interaction. After applying the voltage, surface capillary waves with wavelengths larger than the critical ones appear, which tend to destabilize the thin PDMS films . As a result, the uniform PDMS films finally dewet into multiple smaller dewetted droplets.…”

“…After applying the voltage, surface capillary waves with wavelengths larger than the critical ones appear, which tend to destabilize the thin PDMS films. 31 As a result, the uniform PDMS films finally dewet into multiple smaller dewetted droplets. Figure 1(b) schematically shows different dewetting morphologies of PDMS films for different frequencies of the applied ac electric field of 30 V with the corresponding fluorescent optical micrographs shown in Figure 2.…”

“…S > 0 and A DLS > 0 correspond to the stable configuration of the thin liquid films. − Sharma et al also reported the stability of thin liquid films on a solid substrate depending on the sign of the polar ( S P ) and dispersive ( S LW ) components of the spreading coefficient. , Although thin liquid films should be stable for S > 0, various research works have shown that even for S > 0, thin poly­(dimethylsiloxane) (PDMS) films on PDMS grafted substrates under various bounding medium were found unstable. − Therefore, it is necessary to investigate the role of interfacial and intermolecular forces in predicting the stability of thin films. It has been shown experimentally and theoretically that for top aqueous fluids, thin lubricating films of oils stay stable on hydrophobic surfaces but dewet on hydrophilic ones. ,,, The stability of thin lubricating films underneath aqueous drops on slippery surfaces can also be understood in terms of their total excess free energy, which subsequently depends on the intermolecular interactions between different media. , For most of the slippery surfaces, the thin lubricating fluids are dielectric in nature; hence, their stability can further be controlled using an external electric field. − …”

“…It has been shown experimentally and theoretically that for top aqueous fluids, thin lubricating films of oils stay stable on hydrophobic surfaces but dewet on hydrophilic ones. 20,21,23,31 The stability of thin lubricating films underneath aqueous drops on slippery surfaces can also be understood in terms of their total excess free energy, which subsequently depends on the intermolecular interactions between different media. 24,26 For most of the slippery surfaces, the thin lubricating fluids are dielectric in nature; hence, their stability can further be controlled using an external electric field.…”

“…However, the frequency dependence of the electric field is not captured in the linear stability analysis. 31,33 The experimental system consists of two conducting layers, an aqueous drop and substrate, and a dielectric film sandwiched between them; the whole arrangement is similar to a parallel plate capacitor filled with a dielectric. After applying an ac electric field, dielectrics get polarized while showing different relaxation dynamics such as α-, β-, and γ-relaxations, which subsequently depend on the applied ac frequency.…”