Hydrogels hold promise in agriculture as reservoirs of water in dry soil, potentially alleviating the burden of irrigation. However, confinement in soil can markedly reduce the ability of hydrogels to absorb water and swell, limiting their widespread adoption. Unfortunately, the underlying reason remains unknown. By directly visualizing the swelling of hydrogels confined in three-dimensional granular media, we demonstrate that the extent of hydrogel swelling is determined by the competition between the force exerted by the hydrogel due to osmotic swelling and the confining force transmitted by the surrounding grains. Furthermore, the medium can itself be restructured by hydrogel swelling, as set by the balance between the osmotic swelling force, the confining force, and intergrain friction. Together, our results provide quantitative principles to predict how hydrogels behave in confinement, potentially improving their use in agriculture as well as informing other applications such as oil recovery, construction, mechanobiology, and filtration.
The goal of this study is to engineer 3D-microswimmers containing a bubble that can be stimulated and guided with acoustic waves emitted by transducers. By using 3D-microfabrication techniques, we designed 20⇥20⇥26 µm swimmers with a trapped air bubble pointing towards the substrate, thus mimicking an hovercraft. We then remotely applied acoustic vibrations to the bubble, which generates a strong steady flow (0.1-2 mm/s), an e↵ect referred as acoustic streaming, resulting in a jet below the hovercraft. We found that the motion of the swimmer relies on two parameters, namely the frequency and amplitude of the acoustic wave. We measured the swimmer velocities and observed a very wide distribution: from 0.05 to 350 mm/s or 17500 body lengths. Such a high velocity in terms of body length makes this swimmer one of the fastest among the di↵erent microswimmers reported in the literature.[25] The motion of the swimmer is found to be a combination of two forces orientated in di↵erent directions: the streaming force and the radiation force. While the first one is reducing adhesion, the second one is helping the motion. Using di↵erent transducers orientated towards di↵erent directions, we were able to navigate the swimmer into di↵erent directions as well.
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