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We investigate the effective friction encountered by an intruder movingthrough a sedimented medium which consists of transparent granular hydrogelsimmersed in water, and the resulting motion of the medium. We show that theeffective friction $\mu_e$ on a spherical intruder is captured by the inertialnumber $I$ given by the ratio of the time scale over which the intruder movesand the inertial time scale of the granular medium set by the overburdenpressure. Further, $\mu_e$ is described by the function $\mu_e(I) = \mu_s +\alpha I^\beta$, where $\mu_s$ is the static friction, and $\alpha$ and $\beta$are material dependent constants which are independent of intruder depth andsize. By measuring the mean flow of the granular component around the intruder,we find significant slip between the intruder and the granular medium. Themotion of the medium is strongly confined near the intruder compared with aviscous Newtonian fluid and is of the order of the intruder size. The returnflow of the medium occurs closer to the intruder as its depth is increased.Further, we study the reversible and irreversible displacement of the medium bynot only following the medium as the intruder moves down but also whilereturning the intruder back up to its original depth. We find that the flowremains largely reversible in the quasi-static regime, as well as when $\mu_e$increases rapidly over the range of $I$ probed.

Micromechanics of intruder motion in wet granular medium