Micropipette Deflection and Constrained Blister Measurements of Agar-Glass Adhesion
We have studied the adhesion between agar hydrogels and glass substrates using micropipette deflection and constrained blister experiments. This work was motivated by observations of the twitching motility of P. aeruginosa bacterial colonies at an agar-glass interface, characterized by transitions in the average bacterial colony edge speed and the average width of fingers of the advancing colonies, and the local agar concentration near the agar-solid surface, as the agar concentration was increased above 1.6% w/v. We observed a similar transition in the peak shear stress required to break the agar-glass interface in the micropipette deflection experiment. We determined the dependence of the agar-glass adhesion on agar concentration by designing a constrained blister experiment, which more closely approximated the delamination caused by expanding bacterial colonies. In the constrained blister experiment, the delamination of thin agar layers was produced by creating and growing a blister at the agar-glass interface through the application of a pressure, with the deformation of the agar layer limited to 500 nm. We analyzed image sequences of blister growth to measure the equivalent circular radius for both the delamination front and the constrained region, of blisters that were formed at constant pressures ranging from 0.59 kPa to 1.93 kPa for agar concentrations between 1.0% w/v and 1.9% w/v. In our experiment’s. The blister radius increased linearly with time for each applied pressure at each agar concentration. Additionally, at each agar concentration, blister growth rate increased linearly with increased pressure. We extrapolated the pressure dependence of the blister growth rate to the minimum pressure required to observe blister growth, for each agar concentration, which allowed us to determine the energies of adhesion. These measurements revealed an agar concentration dependence of that was consistent with transitions near 1.6% w/v agar concentration observed in the bacterial twitching and micropipette deflection experiments. Taken together, the micropipette deflection and constrained blister experiments allow a general strategy for characterization of hydrogel adhesion on a variety of substrates.