(156f) Friction and Adhesion Forces of Spores of Bacillus Thuringiensis On Model Surfaces

Surface interactions of colloidal particles, including microorganisms, are ubiquitous in engineered and natural environments. Specifically, bacterial interactions are directly linked to human health and also have an economic impact. Considering that bacterial spores can be efficiently dispersed, understanding of their interaction forces with environmental surfaces can help us address prevention and control of bacterial contamination. Atomic force microscopy was employed to investigate the friction and adhesion forces of spores of Bacillus thuringiensis on planar surfaces including mica, glass, and gold. Experimental data show that these forces are influenced by the relative humidity (RH) and surface properties such as roughness, charge, and contact angle. As the RH increases, adsorbed water molecules lubricate the friction on the atomically flat surface. On the other hand, the friction increases on rough surfaces with increasing RH. The adhesion force does not response to an increase in RH as much as the friction force does. This observation indicates that the friction force is influenced by RH prior to a meniscus formation between the spore and the substrate surface. In addition, it directly shows a violation of Amontons’ Law at micro- and nano-scales. This law explains the relationship between the adhesion and friction forces at the macro-scale. The influence of the relative humidity is insignificant for hydrophobic surfaces. A comparison of the measured adhesion force with the theoretically calculated value shows the significance of surface contamination on the interaction forces, especially for substrates of high surface energy.