Real Area of Contact in a Soft Transparent Interface by Particle Exclusion Microscopy

Soft matter mechanics are characterized by high strains and time-dependent elastic properties, which complicate contact mechanics for emerging applications in biomedical surfaces and flexible electronics. In addition, hydrated soft matter precludes using interferometry to observe real areas of contact. In this work, we present a method for measuring the real area of contact in a soft, hydrated, and transparent interface by excluding colloidal particles from the contact region. We confirm the technique by presenting a Hertz-like quasi-static indentation (loading time<1.4 hrs) by a polyacrylamide probe into a stiff flat surface in a submerged environment. The real contact area and width were calculated from in situ images of the interface processed to reduce image noise and thresholded to define the perimeter of contact. This simple technique of in situ particle exclusion microscopy (PEM) may be widely applicable for determining real areas of contact of soft, transparent interfaces.