  An aqueous sodium chloride solution was injected at a controlled rate into a
granular bed in a quasi-two-dimensional cell. The granular bed was made of
dried, highly water-absorbent gel particles whose swelling rate was controlled
by the salinity of the injected fluid. At a high salinity level (low swelling
rate), high injection rate, and short timescale, the injected fluid percolated
between the gel particles in an isotropic manner. Meanwhile, at a low salinity
level (high swelling rate), low injection rate, and long timescale, the gel
particles clogged the flow path, resulting in anisotropic branch-like
structures of the injected fluid front. The transition of the injection pattern
could be understood based on the ratio of the characteristic timescales of
swelling and injection. Moreover, the clogged pattern showed an oscillatory
pressure drop whose amplitude was increased with higher salinity. Such an
oscillatory behavior observed in an injection process in a swelling gel
particle may be relevant in geological situation; i.e., such as fluid migration
underground.
