EXPERIMENTS AND THEORY ON A BIO-INSPIRED FILM-TERMINATED MICRO-FIBER ADHESIVE
The structure considered in this thesis is a simple array of micro-pillars terminated by a continuous thin film. The normal adhesive properties of this structure are studied in chapters 2 and 3. In chapter 2, we present experimental data which shows that adhesion hysteresis, measured in cyclic indentation experiments, can be nearly five times the absolute work of adhesion of a flat control sample. To demonstrate how a purely elastic system can be dissipative, we present a model which shows that adhesion hysteresis develops due to trapping of the interface crack by fibril edges. Since a compliant interface promotes good contact, we present a contact mechanics model to predict the contact compliance of fibrillar structures. In chapter 3, we studied the effect of geometry on the crack trapping mechanism using a two dimensional finite element model. Several experimental observations are explained by this model: (1) the work to separate a unit area of the interface (W+) is larger than the work of adhesion (Wad ) of the flat control sample; (2) the work to heal a unit area of the interface ( ) is smaller than the work of adhesion of the flat control sample; (3) W+ increases with fibril spacing; (4) W+ decreases with film thickness; (5) W+ decreases with fibril height. The response of our film-terminated micro-fiber array under shear is studied in chapter 4. These friction experiments are carried out by dragging the samples indented by a spherical glass indenter with a constant normal force. Our experiments shows that the force requires to initiate sliding between the indenter and the sample, its static friction, is strongly enhanced compared to a flat unstructured control sample. This enhancement is due to the crack trapping mechanism and increases with inter-fibril spacings. Our experiment shows that the transition from static to dynamic friction is due to a mechanical instability. A surprising result is that the dynamic friction is independent of the fibril spacing. Furthermore, the dynamic friction is practically the same as that of control sample. A preliminary explanation of these observations is given. The friction response of our fibrillar samples is studied using different indenters in chapter 5. We investigate the effect of displacement rates on friction properties. We study in detail the mechanism governing both the static and dynamic friction. We demonstrated that the static friction and adhesion are correlated and can be attributed to the crack trapping mechanism. During steady sliding of the indenter, we observed micro stick-slip in the contact region. The contact shear stress as well the nominal contact area are found to be independent of fibrillar spacing and loading displacement rate. Our data supports a simple model which states that the dynamic friction force is a constant shear stress multiplied by the nominal contact area.
Royal Society of Chemistry (RSC)
Fibrillar interface; adhesion; Friction; FEM
Previously Published As
L. Shen, N. Glassmaker, A. Jagota, C-Y. Hui, Soft Matter, 4 (2008) 618-625,
dissertation or thesis