Impact of Surface Topography on Colloidal and Bacterial Adhesion
Master of Science
Although the importance of substrate surface topography in colloidal and bacterial adhesion is widely recognized, how it affects the adhesion process has been a controversial topic. In this study, the impact of surface topography on adhesion of biological (i.e., bacteria) and non-biological colloids was investigated using natural and engineered surfaces with well-defined surface topographic patterns. Adhesion experiments using carboxylate modified latex (CML) microspheres of 4μm in diameter and Psudomonas Aeruginosa on the taro leaf of Colocasia esculenta, a plant known for its self-cleaning property similar to that of the lotus leaf, in a 100 mM NaCl solution at pH 4 under submerged conditions showed that nanoscale surface structures on the papilla of the Colocasia esculenta leaf surface resisted adhesion by both CML and P. Aeruginosa. This resistance to adhesion was found to be independent of the wetting condition of the surface, suggesting that the surface superhydrophobicity was not the reason for the observed lack of adhesion. Interfacial force mapping by atomic force microscopy (AFM) revealed markedly lower adhesion forces over the surface area covered by these nano-structures where adhesion resistance was observed. Adhesion experiments were also performed using 6 μm CML particles on engineered micro-patterns fabricated on silicon wafers. The micro-patterned surfaces consisted cuboid pillars or pits of a wide range of sizes arranged at various spacings. Adhesion of CML particles on all micro-patterned surfaces was significantly less than on the smooth control surface. In general, adhesion decreased with decreasing pillar or pit size and spacing between the features. Adhesion was minimum on the micro-patterned surface when the dimension (pillar size) of patterns is close to/smaller than the size of the colloid when spacing between pillars was fixed to a size a bit smaller than the particle size; while the adhesion on patterns with fixed pillar size (a bit smaller than the particle size) was low for a wide range of spacings. Analysis of the spatial distribution of adhered particles on the pillar-patterned surfaces showed that more than 98% of the particles adhered on the edge of the pillars (between the pillars) when the spacing between pillars was smaller than the particle diameter; the particles adhered in the valley close to the pillars when the spacing was larger than the particle diameter. The characteristic adhesion distribution of the colloidal particles on the micro-patterned surfaces was also validated by the AFM adhesion force mapping: when spacing between pillars was smaller than the particle size, adhesion force was larger on the edge of the pillars; when spacing between pillars was larger than the particle size, adhesion force was larger on the valley. However, the AFM results could not explain the reduced adhesion on the patterned surface compared to the smooth surface.
Applied sciences; Environmental engineering