Capillary force in adhesive contact between hydrogel microspheres

The paper reports an experimental study of the adhesive contact between liquid-bridged hydrogel microparticles. A novel nanomechanical tester has been developed to measure both the force-approach curves at sub-micro resolutions as well as the side-view images between two agarose spheres during pull-off. Meanwhile, the JKR theory has been extended to quantify the work of adhesion at the solid-liquid-vapor interface and Young's modulus of the particles based on the measured parameters. Our research findings show the work of adhesion is independent of the separation speed and, by contrast, Young's modulus exhibits a linear increase. Our study also demonstrates that JKR reconciles with the generalized Hertz theory, which takes capillary force into account for soft microspheres in a relationship that the work of adhesion is equal to twice the surface tension of water. These new findings are essential for developing techniques to quantitatively characterize capillary adhesion of soft particulate materials and potentially to improve the material performance in their applications.