(104i) Bioinspired Multiphase Humidity- and Rate-Dependent Capillary Adhesives

It is well-known that viscous dissipation plays an important role in many natural and synthetic adhesives. Many liquid biological adhesive systems utilize capillary forces, which can be either static or hydrodynamic, including fluids on spider silk and on insect legs. In such cases, it is important to maintain viscosity within a defined range to preserve the adhesive characteristics, such as the liquid spreading rate and the magnitude and rate-dependence of the adhesive forces. Thus, natural liquid adhesives must employ methods to counteract changes in physical properties due to water loss or uptake associated with variations in humidity. Here, we report on the ability of an important natural capillary adhesive, pollenkitt (the liquid coating found on pollen particles), to stabilize viscosity relative to humidity changes. Pollenkitt consists of an aqueous phase and an oily phase, but the contributions of these phases to adhesion was previously unknown. In this study, we measured the wet adhesive force required to separate a pollen particle from a pollenkitt drop as a function of separation rate and relative humidity (%RH). To investigate the function of the oily phase coat on wet adhesion, pollenkitt drops without the oily phase coat are prepared as the control group. A remarkable rate-dependent adhesive forces is observed, and shown to be attributed to strong capillary hydrodynamics above a critical separation rate. At lower rates, the adhesion is due to static capillary forces. We find that the aqueous phase is the main contributor to this rate-dependent adhesion. The oil phase forms a thin coating around the aqueous phase droplet. Interestingly, the oily phase contributes very little to the observed capillary adhesion, but rather it serves to prevent excessive water absorption under elevated humidity (or drying under reduced humidity). This action stabilizes the viscosity of the aqueous phase during humidity changes, thereby stabilizing the capillary static and hydrodynamic adhesion contributed by the aqueous phase. This result provides inspiration for future development of novel humidity-stabilized and rate-dependent adhesive materials.