Geometric and Energetic Properties of Defects at Complementary Soft Material Interfaces

Surface architecture can influence mechanical properties, such as adhesion and friction, in many natural systems. The careful study of these systems elucidates understanding of the many biological roles these properties serve and the mechanisms by which they occur. One means of controlling surface mechanical properties is through shape complementarity. Predicated on some natural systems' surface architectural design, shape complementarity can be used to enhance selectively between synthetic elastomeric surfaces. Complementary arrays of surface structures, such as 1D ridges or fibrils arranged in a 2D lattice, can inter-digitate to achieve adhesion enhancement controlled by shape recognition. It has been shown that relative misorientation (twist) is accommodated by defects that are mesoscale screw dislocations. The arrangement of such dislocations plays a critical role in determining the mechanical properties of the interface. The objective of our work is to increase the understanding of adhesive and frictional enhancement mechanisms through the study of complementary surface pattern interactions of precisely designed soft elastomeric materials. Here we study the geometric properties of one-dimensional (ridge/channel) and two-dimensional (arrays of pillars) shape-complementary interfaces in the presence of relative misorientation and difference in lattice spacing. Relative misorientation without difference in lattice period spacing is accommodated by arrays of screw dislocations. Differences in lattice spacing without misorientation is accommodated by arrays of edge dislocations. In general, we observe arrays of dislocations with mixed screw and edge character. The spacing, orientation, and potential mechanical properties of these arrays can be predicted using the geometry of Moiré patterns. More broadly, we show that soft materials with shape-complementary patterns can be used to generate dislocations of arbitrary edge and screw character at the mesolength scale of tens of microns. Because these dislocations are easily observed and occur periodically, Moiré pattern information is used to study relationships between dislocations, parameter selection, and surface mechanical properties.We extend the studies further by examining and taking advantage of the translational symmetry of fibrillar lattice structures for our experiments. We use typical 2D Bravais lattice structures patterned by microfibrils on the surfaces of a soft material and attempt to understand the roles of periodicity, fibril size, and density on surface mechanical properties. We develop a means of interpreting these results systematically based on pattern misorientation that is applicable to soft materials, thereby bridging crystallography with 2D soft material interactions. The increased understanding and interdisciplinary impact of these surface interfacial interactions can be used in many fields of study including soft material substrate tissue and graft engineering, mechanical engineering and mechanics scale up studies, and various industrial applications.