Indentation adhesion of hydrogels over a wide range of length and time scales

Abstract

Hydrogels are both important engineering materials and living components. In both native and engineering settings, hydrogels often interface with other materials. Quantifying the adhesion properties and understanding the adhesion mechanisms of hydrogels are important. Among various measuring techniques, indentation is the most convenient method to be applied on hydrogels with the minimum requirement for sample preparation. It is also easy to be applied across a wide range of lengths scales. In this work, we use polystyrene spheres to probe polyacrylamide hydrogel surfaces. Using Atomic Force Microscope (AFM) and microindenter, we could carry out indentation measurements across a wide range of length scales with the contact radius ranging from 1.94 μm to 75.5 μm. As the indenter approaches and is pressed into the hydrogel, no long-range interaction is observed. The force–displacement curve during the loading period follows the Hertzian solution. The indenter is then held in a certain depth of indentation for a different amount of time and retracted from the surface until full separation. During the holding time, the hydrogel relaxes due to poroelasticity. Adhesion only plays a role during the retraction process. The pull-off force and energy of separation are observed increasing over contact time, and the time scale is independent of the poroelastic relaxation time scale of the hydrogel. The pull-off force and energy of separation are also found increasing with contact size and reaching plateaus at large contact sizes. This phenomenon cannot be modeled by the conventional adhesion theories, such as the JKR theory or the Maugis–Dugdale theory. In order to model adhesion hysteresis and the size-dependence and to extract intrinsic adhesion properties of the interfaces, we develop an analytical model. As the indenter is pulled up, a ring of cohesive zone is developed. As the indenter is pulled further, the cohesive zone size increases, but the apparent contact radius is kept constant until the energy release rate reaches the adhesion energy. Based on this picture, analytical solutions are developed, and the theoretical results fit well with experimental data. This systematic study provides more precise descriptions of the adhesive contact of hydrogels and provoke a deeper understanding of the adhesion mechanisms of hydrogels.