Solvent Driven Motion of Lithographically Fabricated Gels

Abstract

We investigated the solvent driven motion of lithographically structured poly- N-isopropylacrylamide (PNIPAm) gels. The gels were soaked in ethanol and then transferred to water, where they moved spontaneously. This movement was driven by the expulsion of the ethanol from the gel and subsequent ethanol spreading at the air-water interface. We utilized lithographic patterning of the gels at the micron-millimeter length scales to investigate the effect of size, shape and symmetry. Lithographic patterning allowed the structures to be fabricated in an identical manner, thereby allowing a single variable (such as shape, size, or symmetry) to be altered while minimizing change in other variables such as thickness, roughness and swelling characteristics. The diverse range of motions including translation, precession and rotation could be controlled and were recorded using videography. Gels were lithographically patterned with features less than 100 microm, and exhibited remarkably high linear and rotational velocities of up to 31 cm/s and 3529 rpm over time spans of seconds to minutes. We observed a reciprocal dependence of maximum rotational velocity on linear dimension. The linear velocity for all types of motion, along a line or curve, was analyzed and found to be similar across different shapes and sizes. This velocity was in the range of 17-39 cm/s even though our sizes and shapes varied across orders of magnitude. We postulate that this velocity is related to the velocity of spreading of ethanol on water, which is approximately 53 cm/s. Additionally, since this solvent powered motion is a clean, quiet and reusable source of motive power, with no need for on-board wiring or batteries, we explored applications in moving lithographically integrated metallic payloads on top of the gels and utilized the gels to move larger floating objects.