Title: High permeability and efficient relaxation of agar gel-an ideal substrate for bacterial swarming

Author (Talk): Jay Tang,

Abstract:

Agar, derived from the polysaccharide agarose, forms a highly permeable gel at remarkably low solid contents down to a fraction of one percent. An agar gel forms following the cooling of a heated agar solution, primarily due to an abundance of hydrogen bonds that crosslink the polymers and bundle multiple strains, yielding a highly porous network that contains over 99% water. We focus our study on a 0.5% agar gel, which has been shown to optimally support the surface swarming of bacteria. By measuring gravity-driven fluid flow through a gel formed inside a 30 or 60cc syringe, we determined its permeability coefficient to be ~10-14 m2, which is orders of magnitude larger than other well-known hydrogels, such as polyalginate (~10-16 m2) and polyacrylamide (~10-18 m2 at 5% with 1/200 bis-acrylamide as crosslinks). We also explored the relaxation of a 0.5% agar gel to mechanical deformation using an indentation device. The relaxation time we found was on the order of ten minutes. It was nearly independent of the indentation depth. The latter property suggests that the response of agar gel to deformation is dominated by viscoelastic relaxation, in sharp contrast to the poroelastic relaxation recently found for hydrogels of polyalginate and polyacrylamide. This simple mechanical measurement reveals a key difference in molecular origin: agar gels are formed by the more transient hydrogen bonds than chemical crosslinks present in other hydrogels.