| Nature has shown that surface properties of contacting surfaces, such as adhesion, compliance, and friction can be controlled by structuring the near-surface architecture. For example, the feet of many insects and lizards use variations on the theme of thin-film terminated fibrillar structures. This has recently inspired much work on the development of bio-inspired fibrillar structures. It is important to understand the underlying adhesion mechanics of such architectures to establish structure-property relations. We have studied two types of structured surfaces: (a) A fibrillar array with a thin terminal film that shows significant enhancement in adhesion, compliance, and static friction, and (b) Rippled surfaces with mechanically tunable adhesion.;To study the properties of these surfaces using indentation, we developed a model-independent method to quantitatively extract the work of adhesion of structured surfaces from such experiments that does not require a theory such as that of Johnson, Kendall, and Roberts, which is used widely for flat surfaces but is not applicable to structured surfaces.;Using the indentation method we have studied a basic question related to the behavior of the film-terminated bio-inspired fibrillar architecture: what is the coupling between the crack-trapping mechanism that causes adhesion enhancement due to architecture and the intrinsic work of adhesion, which can be modified by surface chemistry? We have studied this in two ways, (a) by examining the rate dependence of adhesion, and (b) by measuring adhesion under water. From this study, we have established that the crack-trapping mechanism couples multiplicatively to the intrinsic work of adhesion of the interface. Also, we have conducted preliminary tests to study the effect of roughness on the adhesion of these samples. These samples showed better tolerance for roughness than flat control samples.;The indentation method has also been used to study how modulation of the amplitude of a rippled surface by applied strain leads to mechanically tunable adhesion. We have explained this quantitatively by developing a simple contact mechanics model for indentation of rippled surfaces. The effect of ripple amplitude on adhesion is studied in different samples: (1) Original wrinkled samples with a stiff, residually-stressed, silicaceous surface layer, (2) Rippled samples obtained by replicating the rippled surface structure of original samples on fresh stress-free surfaces. These surfaces showed a significant enhancement of adhesion when complete interfacial contact is achieved, but lose adhesion when contact is partial. A more detailed contact mechanics based model has been formulated to understand the mechanics of adhesion in these surfaces in the regimes of complete and partial interfacial contact.;Using complementary rippled surfaces, we have demonstrated high adhesion selectivity, which is studied theoretically as well as through finite element analysis in this work.;Behavior of surfaces in contact with other surfaces is highly influenced by their frictional response. We studied a phenomenon, called Schallamach waves, that is commonly observed while sliding a soft rubbery surface against a rigid surface. This involves folding of the soft surface that leads to formation of tunnels of air at the interface traveling in the direction opposite to sliding. We prepared a finite element model to study the conditions for formation of these detachment waves. |
