Electric-Field-Controlled Surface Instabilities in Soft Elastic Films

Understanding morphological instabilities at soft interfaces is important in a variety of diverse problems, such as adhesion/debonding in polymer films and biological cells, friction, cavitation, and patterning. Non-specific interactions of electromagnetic origin play an important role in these instabilities. Spontaneous surface patterns appear in soft solid films that are in contact proximity (< 50 nm) with a rigid contactor (or another film) due to a competition between destabilizing van der Waals attractive force (between the film and the contactor) and the stabilizing elastic-strain energy. It is known that the wavelength of the pattern that appears depends only on the film thickness. These instabilities, while profoundly influencing the adhesion/debonding characteristics, also provide an attractive route to patterning and morphological control of soft thin films by external interactions. However, intermolecular interactions, being material properties, cannot be easily modulated. Moreover, these interactions are short-ranged and thus even nanometer-scale surface roughness and defects affect the robustness of these patterns. Here, we show the effect of electric fields on the generation and movement of patterns on solid surfaces. Previously, external fields have been used only to control surface instabilities and patterns in thin liquid films where, unlike the solid films, the instability is dominated by a long-wavelength mode and is strongly dependent on the field strength and its decay characteristics. Unlike the control of liquid movement and wetting/dewetting by electric fields, the movement, adhesion/debonding, and pattern formation in soft solids by electric-field-induced elastic deformations are completely unexplored phenomena. The application of an electric field modulates the morphology, dimensionality, and the adhesive strength of surface patterns—elements that are useful in the design of smart adhesives. On application of an electric field, distinct morphological changes to the patterns are observed. These are “edge straightening”, “finger elongation”, and “pillar formation”. This ability of the electric field to control these adhesive and morphological properties of a soft interface can also be employed to engineer surface structures for microfluidics and soft-lithography related applications. Our experimental setup consisted of a wedge-shaped geometry schematically shown in Figure 1. A contactor is placed on top of a soft elastic film with one end on the film and the other on top of a spacer. The contact between the cover slip and the film engenders a zone of complete adhesive contact terminating in a finger pattern and an air gap (d) between the cover slip and the film ahead of the finger patterns. The air gap in this wedge geometry increases along the y-direction away from the contact zone. The finger patterns cease to exist in the absence of the top contactor. Small-amplitude, well-defined finger patterns (Fig. 2a), formed due to the adhesive van der Waals interactions were ensured before turning on the voltage. Upon introduction of the voltage the patterns were modified. The response to the electric field (EF) can be classified based on the appearance of three distinct surface morphologies: “edge straightening” (Fig. 2b), “finger elongation” (Fig. 2c), and “pillar formation” (Fig. 2d). The manner in which the patterns evolve depended on the film parameters such as the shear modulus (l) and the thickness (h). These emerging morphologies can be understood in terms of the stiffness parameter (l/h) (the ratio of the shear modulus to that of the thickness of the elastic film). “Edge straightening”, “finger elongation” and “pillar formation” processes were observed for high, intermediate, and low stiffness (l/h) films, respectively. For highly stiff films (10 lm 6 MPa), the electric field appreciably decreases the amplitude (a) of the initial finger patterns beyond a critical voltage ( cric). It is observed that the amplitude decrease is not accompanied by any discernable change in the wavelength of the finger pattern. The straightening of the finger pattern occurs asymmetrically, with a decreasing with respect to a stationary tip of the finger pattern, leading to complete intimate contact (Fig. 2b, inset). A complete straightening of the fingers happens, i.e., a→ 0, leading to a straight edge at a higher voltage (Fig. 2b). Further increase in the voltage results in a gradual displacement of this linear edge towards regions of lower electrostatic energy, thereby increasing the adhesive contact area. The other features that are observed are: i) cric does not depend significantly on the ramping rate or on the polarity of the voltage. The emergent morphology is dependent on the electrostatic C O M M U N IC A TI O N S