  Utilizing colloidal probe, lateral force microscopy and simultaneous confocal
microscopy, combined with finite element analysis, we investigate the mechanism
of static friction for a microparticle on a soft, adhesive surface. We find
that the surface can form a self-contacting fold at the leading front, which
results from a buildup of compressive stress. A sufficiently high lateral
resistance is required for folding to occur, although the folds themselves do
not increase the peak force. Experimentally, folds are observed on substrates
that exhibit both high and low normal adhesion, motivating the use of
simulations to consider the role of adhesion energy and interfacial strength.
Our simulations illustrate that the interfacial strength plays a dominating
role in the formation of folds, rather than the overall adhesion energy. These
results reveal that adhesion energy alone is not sufficient to predict the
nucleation of folds or the lateral force, but instead the specific parameters
that define the adhesion energy must be considered.
