We discuss how surface roughness influences the adhesion between elastic solids. We introduce a Tabor number which depends on the length scale or magnification, and which gives information about the nature of the adhesion at different length scales. We consider two limiting cases relevant for (a) elastically hard solids with weak (or long ranged) adhesive interaction (DMT-limit) and (b) elastically soft solids with strong (or short ranged) adhesive interaction (JKR-limit). For the former cases we study the nature of the adhesion using different adhesive force laws (F ∼ u−n, n = 1.5–4, where u is the wall-wall separation). In general, adhesion may switch from DMT-like at short length scales to JKR-like at large (macroscopic) length scale. We compare the theory predictions to results of exact numerical simulations and find good agreement between theory and simulation results.

Significance: The technology for generating tactile feedback on a touchscreen via electroadhesion is already available—and straightforward to implement—but the knowledge on human skin contact mechanics is limited. To better understand the contact mechanism between the finger pad and touchscreen under electroadhesion, we investigated the sliding friction as a function of normal force and voltage using ( i ) a mean field theory based on multiscale contact mechanics, ( ii ) a full-scale computational contact mechanics study, and ( iii ) experiments performed on a custom-made tribometer. We show that the real contact area and the electroadhesion force depend strongly on the skin surface roughness and on the nature of the touchscreen coating. Thus, by reducing the effective thickness of the latter, the human tactile sensing can be drastically enhanced.