Development and Characterization of Polymer Gel as Self-driven Actuators

Abstract

Actuators are objects that can transfer energy or signal to mechanical energy and are widely used in our daily life. Most of actuators are actuated by external energy suppliers that make the device big. Self-powered actuators, which are able to provide directional motion without external energy input, are extremely desirable for some special applications. In this study, a new type of self-powered and rechargeable actuators composed of polymer gel are developed and studied. These actuators can move at the water-air interface, liking some microorganisms that can move around without external energy supplying unit. To deepen the understanding of self-driven gel actuator, a systemic study of their characteristics was carried out. First of all, it is experimentally confirmed that the self-driven motions of gel actuators at the air-water interface is a universal phenomenon. When the solvent in gel is completely consumed (i.e., diffused to water), the gel actuators can be recharged by placing gel in solvent. Gel actuators with large mass result in larger mass loss and longer duration of motion. Both solvent diffusion and surface tension contribute to the observed self-driven motions. Solvent diffusion is the main driven force for the self-driven motion of gels, while surface tension makes a minor contribution. The diameter of beaker has an effect on the motion of gel actuators, while the water depth and volume of water has little effect on the motion. Next, the change in the gel’s characteristics during its motion at the air-water surface was characterized. Based on the experimental results, it is concluded that the mass of the gel decreases with time as solvent in gel diffusing into water. In addition, the gel actuators shrink after motion. The transparency of the gel decreases as motion at the air-water interface. Compared to the gels exposed to the air, gel actuators loss mass more, shrink faster and became less transparent after motions. The gel actuators undergo bending downward once placed at the air-water interface and transform to bending upward gradually during motion. Furthermore, a process to quantify the gel motion was established by taking the advantage of digit videos. Using this process, various variables/parameters about the motion of gel at the air-water surface: velocity, acceleration, kinetic energy and their time dependence are able to be obtained. Motion modes and motion variables of gel actuators can be clearly illustrated as function of time. It was found that the velocity of gel actuator decreases as it moves toward the wall of glass beaker and increases as it moves away from the wall of glass beaker. The motion in the first a few seconds is irregular, and the first few seconds are not used for motion mode study. Based on the experimental results, it is concluded that the shape/geometry of the gel actuators plays an important role on their motion mode. For example, gel in rectangle shapes with a higher length/width ratio have the tendency of spin motions; gel in circle and square shape have the tendency of orbital motions. Based on the geometry dependent results, gels built in 3D printed mold in a fan shape with releasing windows were designed and fabricated, which have controlled motion modes. Thanks to the digitalized results, friction force of gel motion at the air-water interface was studied and it is confirmed that the friction force obeys F = -αv2, where v is the velocity of the gel. Fourthly, energy conversion through the actuation process was studied. Self-driven gels convert stored chemical energy into mechanic motions. The consumption of the kinetic energy of a gel during the motion reflects the contribution of the friction force. The kinetic energy and work down by the friction force are obtained from experiment. The chemical energy change of gel actuators is linked with the mass loss of actuators originated by solvent diffusion. A new model was derived that considers one-dimensional diffusion into a semi-infinite bar. This model can help us understand the trend of mass loss as function of time. Based on the experimental results, mass loss as function of time has two segments of time, i.e., Period – I and Period – II. A set of equation was derived to describe the mass loss in the two time period. An empirical equation based on Eq. (5.8) can be used to predict mass loss as function of time in Period – I and a simplified equation from Eq. (5.28) can be used to predict mass loss as function of time in Period – II. This can be used for the future study to determine the velocity of solvent eject by the gel actuators, which is a step forward for the understanding of self-driven gels.