2.1.2. Energy Output
While the above investigation revealed the impact of the taper ratio of the bistable metal strip on the trigger and rest forces, it also revealed that the taper ratio impacted the energy that can be generated as a result of the snapping process. To further understand the relationship between snapping energy output and taper ratio, we performed energy output tests. Specifically, the benefit of using bistable metal strips for device propulsion is its fast and powerful snapping behavior, which hypothesized would lead to fast movement of the device developed here. Hence, we wanted to retain good snapping performance while minimizing the magnitude of the trigger and reset forces. To accomplish this, we used a regular ping pong ball that was hung over the top side of the bistable metal strip as shown in Figure S3. By triggering the tip of the bistable metal strip, the strip snapped, coiled, and struck the center of the ping pong ball, sending it into motion. The ping pong ball then moved in a circular path, like a pendulum, and eventually reached its highest point before swinging back. Assuming no energy loss during the process, the energy output of the bistable metal strip is directly proportional to the maximum height the ping pong ball reached. Analysis of the height the ping pong ball reached as a function of the bistable metal strip taper ratio allowed us to determine the relationship between energy generation and bistable metal strip taper ratio. As shown in Figure 7, the height the ping pong ball can reach (and the energy generated) greatly decreased with larger taper ratios; i.e., larger taper ratios yielded lower snapping energy output. In fact, the 7-0 taper ratio metal strip did not have enough energy to move the ping pong ball. Since snapping energy is essential to the performance of the swimming device, we decided to maximize the energy output by adopting a 4-3 taper ratio design for our future devices.