Weak adhesion at the mesoscale: particles at an interface

Adhesion of colloidal particles (synthetic or biological) to one another or to other surfaces, typically immersed in a fluid medium, is frequently a mesoscale phenomenon, intermediate between microscopic and macroscopic scales. Similar to other mesoscale phenomena, gaining a useful understanding based on ideas from either continuum or atomistic concepts alone is limited. Combination of experimental techniques like optical trapping and scanning probe techniques with simultaneous imaging provides direct insight into the space-and time-dependent dynamics of the adhesion process in the relevant scales. The strength of adhesion typically evolves with time; the details of the processes depend on the overall strength of attractive interaction among the adhering objects as well as its spatial fluctuations governed by the randomness of the surfaces involved and their own time-dependent reconstruction due to varying local stresses. This review focuses on situations where the adhesive inter-object interactions are weak compared to the cohesive intra-object interactions whereby all processes occur, to a good approximation, in the interfacial region. In these situations, the dynamics of adhesion is affected by both the quenched spatial disorder of the surfaces as well as the thermal fluctuations. A local microrheological characterization of the stress-coupling in the interfacial region provides a heuristic description of the adhesion process. The discrete, thermally assisted punctuated descent of the effective adhering system down a rough energy landscape through a hierarchy of finite metastable minima reflects the mesoscopic nature of adhesion. We review these experimental observations of particle adhesion in the weak limit and explore possible common mechanisms underlying apparently disparate phenomena and processes.