In nature, cockroaches can survive underwater for up to 30 minutes. A robotic cockroach was developed that can walk on land, swim on the surface of water, and walk underwater for as long as necessary.
HAMR (Harvard's Ambulatory Micro-Robot) uses multifunctional foot pads that rely on surface tension and surface-tension-induced buoyancy when it needs to swim, but can also apply a voltage to break the water surface when it needs to sink. This process is called electrowetting, which is the reduction of the contact angle between a material and the water surface under an applied voltage. This change of contact angle makes it easier for objects to break the water surface.
Moving on the surface of water allows a microrobot to evade submerged obstacles and reduces drag. Using four pairs of asymmetric flaps and custom-designed swimming gaits, HAMR robo-paddles on the water surface to swim. Exploiting the unsteady interaction between the robot's passive flaps and the surrounding water, the robot generates swimming gaits similar to that of a diving beetle. This allows the robot to effectively swim forward and turn.
HAMR weighs 1.65 grams (about as much as a large paper clip), can carry 1.44 grams of additional payload without sinking, and can paddle its legs with a frequency up to 10 Hz. It is coated in Parylene to keep it from shorting underwater.
Once below the surface of the water, HAMR uses the same gait to walk as it does on dry land and is just as mobile. To return to dry land, HAMR faces enormous challenge from the water's hold. A water surface tension force that is twice the robot's weight pushes down on the robot and in addition, the induced torque causes a dramatic increase of friction on the robot's hind legs. The robot's transmission was stiffened and soft pads were installed on the robot's front legs to increase payload capacity and redistribute friction during climbing. Finally, walking up a modest incline, the robot is able to break out of the water's hold.
Next, the researchers hope to further improve HAMR's locomotion and find a way to return to land without a ramp, perhaps incorporating gecko-inspired adhesives or impulsive jumping mechanisms.