Since Matthewson (1987) proposed the well-established model for viscous force of spheres by viscous liquid film, the influence of viscosity, initial film thickness, and separation distance on the wet adhesion of simple geometries have been well studied. However, most of current knowledge of the wet adhesive mechanism with a viscous liquid bridge is still under the limit of simple geometries without considering organized fine-scale surface features. We investigated and discussed how the fine-scale morphology on a surface could affect the wet adhesive mechanism of the viscous liquid bridge at various separation rates and various film thicknesses. To investigate the surface morphology effect on the wet adhesive mechanism with a viscous liquid bridge, the wet adhesive mechanism of pollen grains on pollenkitt liquid bridge was studied qualitatively and quantitatively. Pollens are well-known microparticle with organized fine-scale surface morphology, and pollenkitt is the common adhesive material coating the surface of pollen pollinated by animals. Two different pollens (ragweed and sunflower) were used to examine the effect of surface morphology on the wet adhesion mechanism.
The pollenkitt on the solid substrate (Si) enhanced the adhesion of ragweed pollen by two orders magnitudes from dry adhesion, and the attractive force was dominated by capillary and viscous forces. The pollenkitt was viscous enough to achieve strong hydrodynamic response with retraction rate of 5000 nm/s and above. The surface morphology difference between ragweed and sunflower pollens causes the different shapes of approach (delayed approach of ragweed pollen) and retraction (multiple steps of sunflower pollen) curves. Moreover, the different surface morphology of both pollens caused the different magnitude and sensitivity of hydrodynamic response with different thickness of pollenkitt films. The ragweed pollen had very stronger capillary and viscous forces with thicker liquid film (1 ~ 2 μm) than sunflower pollen, but the sunflower pollen had stronger wet adhesion and more sensitive hydrodynamic response with thinner liquid film (100 ~ 500 nm). The reversal of the wet adhesion magnitude of both pollens is affected by the length and shape of spikes on the surface of both pollens.