(471g) New Discoveries of Solid-Liquid Phenomena in Biological Adhesive Microparticles

Nature provides remarkable examples of adhesive bioparticles that function in a wide range of environmental and dynamic conditions. This talk will review recent discoveries of the roles of solid-liquid interfaces in governing adhesion of plant pollens, as well as the application of these ideas in biomimicry for advanced adhesive design.  Pollen microparticles provide robust examples of nature’s solutions to adhesion in air under a wide range of humidities, transport conditions (wind vs. animal vectors) and on surfaces with a wide variety of structures and chemistries. While the adhesion mechanisms have previously not been well understood, these lessons could upon new opportunities in designing tunable wet-dry adhesives for composites, sensors, drug-delivery, wound closure, coatings, and adsorbents.

Two separate pollen adhesion mechanisms have been identified recently by the author’s group: a dry adhesion mechanism (determined by pollen surface geometry) and a capillary liquid-solid mechanism (determined by presence of a nanoscale liquid pollenkitt coating).  These two mechanisms work together to give pollen the ability to ‘tune’ adhesion to different transport mechanisms (wind vs. insect pollenation) and to different surface chemistries. We will outline how the adhesive forces can be predicted by combining a capillarity model with the Hamaker adhesion model. Because of the small size scale of the pollen spine tips (~50 nm), capillary models of pollen adhesion must include a ‘LaPlace’ term that captures the large curvature of the liquid bridge. We have also discovered that the combination of solid spines or reticulate structures with the liquid pollenkitt coating also gives pollen a load-sensitive adhesion on rough or patterned surfaces.  A third discovery that will be presented is the unique humidity sensitivity of the pollenkitt liquid coating, which allows for an enhanced adhesion to hydrophobic substrates under conditions of high humidity, contrary to intuition.  This result derives from the changes in the dynamic characteristics of the pollenkitt, which derive from the fact that pollenkitt is a water-in-oil emulsion.  Finally, several engineering applications as biomimicry-enabled sensors are reviewed.