Directional Motion of Sodium Polyacrylate Gels Initiated by Ca²⁺-Induced Contraction is coupled to an NaCl Gradient

It is known that Na-polyacrylate gel threads contract in the presence of CaCl₂ but we have rather unexpectedly, initiate contraction at or very near the gap, rather than from the side immersed in CaCl₂ while the other side is immersed in NaCl. The main mechanism for ion transport initiates through diffusion; the lowest crosslinked gel allows for bulk diffusion while the higher crosslinked gels primarily propagate surface diffusion. Through surface diffusion, the binding mechanism propagates in a linear fashion, from the CaCl₂ towards the NaCl. The more interesting phenomena, the bulk diffusion, allows the ion gradients to meet within the middle of the gel. We propose this allows time for the chains to condense in the presence of NaCl, which allows for the entropically favored binding to Ca²⁺, immediately collapsing chains and allowing the facilitated binding down either end of the gel. Due to this, we are able to induce overall movement of the gel thread. While the phenomena is correlated to diffusion, the rates are mainly determined by polyelectrolyte dynamics. In all cases, original gel dimensions can be recovered upon treatment with chelators such as sodium triphosphate, an analog to ATP.