Adhesion of patterned polymer interfaces
Nature has demonstrated that a powerful strategy for tuning adhesion lies in the development of patterns at an interface. Inspired by the amazing attachment abilities of geckos, we demonstrate that similar design approaches can tune the adhesion of polymer interfaces. In this research, we investigate the role of patterned surfaces in the control of polymer adhesion. Specifically, we demonstrate that patterned surfaces control adhesion by enhancement of the total contact line, or perimeter of the interface, as opposed to increasing the total contact area. This insight is very powerful as it provides new strategies for designing patterned adhesives.
To demonstrate that the pattern control of adhesion is associated with the enhancement in contact line, we explore two unique types of patterned interfaces and their control in adhesion. First, we begin our investigation of patterned adhesion (Chapter 2) by understanding how surface-chemical patterns - i.e. a pattern of periodic variation in surface chemistry, can tune the adhesion of a commercial silicone elastomer. This type of patterned adhesive is unique as almost all previous patterned adhesives are based on topographic patterns. We find a surface-chemical pattern can enhance the adhesion of the elastomer and significant increases are observed for specific pattern geometries. More importantly, the mechanism of enhancement is linked to the changes in the contact line which is controlled by the periodic variation in surface chemistry.
Our results on the adhesion of surface-chemical patterns open new opportunities for developing alternative patterned interfaces for adhesion control. Since the mechanism of control is associated with the enhancement in contact line, we explore an alternative patterning strategy without the use of lithography. In Chapter 3, we explore the concept of surface wrinkling to pattern polymers. We use a combination of osmotic stress, coupled with lateral confinement to generate wrinkles on polymer surfaces. We show control of both the orientation and the length-scale the wrinkle patterns. The wrinkles form as a result of the development of a compressive stress within the polymer film due to the balance between osmotic stress and lateral confinement. There are two main contributions from this work. First, we demonstrate the control of the degree of lateral confinement determines the wrinkling morphology. More importantly, this control leads to the discovery of two new wrinkling morphologies that have not been observed previously. Second, our approach provides direct applications of these structured materials as functional devices. Specifically, we illustrate that our wrinkled polymers can directly be used as an optical array or patterned adhesive (Chapter 4).
In Chapter 4, we combine the lessons learned in the adhesion of surface-chemical patterns (Chapter 2) along with the patterning approach based on surface wrinkling (Chapter 3) to generate a “self-patterned” wrinkled “smart” adhesive. We demonstrate that the adhesive properties are connected with the wavelength of the surfaces wrinkles. With the understanding of the mechanism of contact line enhancement, wrinkled patterns are developed that enhance adhesion by increasing the total contact line during separation. Based on the mechanism of contact line splitting, we develop a scaling relationship that explains how the contact line enhancement is controlled by the wavelength of the wrinkles.