Harvard University has created the HAMR robot which can move across water or sink itself and walking beneath the surface on command.
The robot was first announced in 2013, which was the original version of the tether powered/controlled HAMR (Harvard Ambulatory MicroRobot), which measures in at just 4.4cm in length and can move along the ground at a maximum speed of 8.4 body lengths per second. The idea of the robot was inspired by pop-up books and involved sandwiching together 23 microscopic layers of material and then laser-cutting them into the desired patterns.
The latest version of the HAMR features foot pads; which provides surface tension-induced buoyancy when the robot enters the water, keeping it from sinking. When it is needed, HAMR can apply an electrical current to the water through the underside of the pads. This will result in a process known as electrowetting, in which the application of voltage reduces the contact angle between an object and the surface of the water.
When it is time to get out of the water for the micro-robot, the tiny robot needs some help breaking back through the same surface that previously kept it afloat. To that end, the new amphibious version of HAMR has a stiffer transmission along with soft pads in its front legs, which together serve to increase its payload capacity and redistribute friction when it is climbing.
The latest version of HAMR can be seen in the action in the video below.