Creating a viscoelastic spectrum of aqueous polyacrylamide gel at varying concentrations

Current methods to obtain microrheological measurements inside liquid materials hold the problem of contaminating samples with an internalized foreign particle, especially problematic for intracellular microrheological measurements. Optical interferometry solves this problem by allowing for pico- and nanoscale rheological measurements without coming into contact with the interior system of a sample. Aqueous polyacrylamide (PAAm), a non-Newtonian gel can be used as a model of a cell, mirroring the experimental setup needed to study cellular systems. With a simple setup, a high-power green pump laser is incident on a PAAm droplet, inducing droplet height deformation via radiation pressure, then detected by interferometry using a low-power red probe laser. Firing a pump laser pulse deforms the gel droplet height between maxima and minima, from which the viscosity is calculated. Comparison of our experimental viscoelastic data of PAAm at a single pulse and varying frequencies to literature values confirms an accurate viscoelastic spectrum of PAAm. Based on our measurements and the noninvasiveness of this technique, mechanical perturbation of a gel surface demonstrates possibility of replication for cellular measurements.