  Optical tweezers (OTs) can detect pico-Newton range forces operating on a
colloidal particle trapped in a medium and have been successfully utilized to
investigate complex systems with internal structures. Laponite clay particles
in an aqueous medium self-assemble to form microscopic networks over time as
electrostatic interactions between the particles gradually evolve in a physical
aging process. We investigate the forced movements of an optically trapped
micron-sized colloidal probe particle, suspended in an aging Laponite
suspension, as the underlying Laponite microstructures gradually develop. Our
OT-based oscillatory active microrheology experiments allow us to investigate
the mechanical responses of the evolving microstructures in aging aqueous clay
suspensions of concentrations ranging from 2.5% w/v to 3.0% w/v and at several
aging times between 90 and 150 minutes. We repeat such oscillatory measurements
for a range of colloidal probe particle diameters and investigate the effect of
probe size on the microrheology of the aging suspensions. Using cryogenic field
emission scanning electron microscopy (cryo-FESEM), we examine the average pore
areas of the Laponite suspension microstructures for various sample
concentrations and aging times. By combining our OT and cryo-FESEM data, we
report here for the first time to the best of our knowledge, an inverse
correlation between the crossover modulus and the average pore diameter of the
aging suspension microstructures for the different suspension concentrations
and probe particle sizes studied here.
