Biomimetic Micro/nano-Structured Surfaces: A Potential Tool for Tuning of Adhesion and Friction

Abstract

Effects of biomimetic micro-patterning of polymeric materials on their interfacial properties were studied experimentally. Micropillars of PDMS and SU-8 epoxy were fabricated through soft lithography and UV lithography techniques, respectively. PDMS pillars were topped by thin terminal films of the same material through dipping method with different thicknesses and viscosities. Adhesion and frictional properties of biomimetic microstructures were examined in two modes of contact, i.e. laid and conformal contact. In the first mode of contact, i.e. laid contact, the contact between adhesive and adherent is laid on top of the micro-protrusions or is in contact with side wall of micropillars. Adhesion properties of the smooth and patterned PDMS were characterized through micro-indentation test. Moreover, the friction properties of the smooth PDMS sample and PDMS micropillars with different aspect ratios were examined in unidirectional friction testing. JKR theory of continuum contact mechanics was utilized to interpret the obtained data. To study the effect of second mode of contact, peeling behaviour of a conformal contact between solidified liquid PDMS and SU-8 micropillars was monitored. Kendall’s model of elastic peeling was used to interpret the peeling data. It was found that patterning of the materials would decrease the real area of contact and accordingly adhesion and friction to the mating surface. Termination of the micropillars with a thin layer of the same material result in increment of adhesion as reduction of the real contact area could be compensated and the compliance of the near surface increases. Elastic energy dissipation as a result of enhanced compliance and crack trapping and crack propagation instabilities are the main reasons behind increment of adhesion of thin film terminated structures. Viscoelasticity of the terminal thin film remarkably increased the adhesion as a result of coupling mentioned mechanisms and viscoelastic loss on the surface. Decline of the overall friction could be tailored through use of different aspect ratios. Higher aspect ratios pillars show higher friction comparing to lower aspect ratio pillars. 550 folds enhancement of adhesion was observed for peeling of the PDMS tape from rigid micropillars with aspect ratio ranging from 0 to 6. It is concluded that for the lower aspect ratio micropillars, the elastic energy dissipation is playing the key role in adhesion enhancement. This role shifts toward side-wall friction during separation by increase in aspect ratio. These all give in hand a versatile tool to control and fine tune the interfacial properties of materials, whether they are concerned with adhesion or friction.