Abstract: In some aspects, a device comprises a plurality of walls defining one or more channels; wherein the one or more channels have a cross-section in a plane perpendicular to a vertical axis of the device that changes along the vertical axis; and one or more floats sized to allow movement of the one or more floats within said one or more channels, wherein the one or more floats have a surface characteristic that is different from the surface characteristic of the walls such that, upon contact with a fluid, said walls and said floats form different contact angles and induce a repulsive capillary force between the walls and the one or more floats at a surface of the fluid.

Abstract: In some aspects, a device comprises a plurality of walls defining a slot and one or more channels wherein the one or more channels are intersected by the slot; wherein the one or more channels have a cross-section in a plane perpendicular to a vertical axis of the device that changes along the vertical axis; and one or more floats sized to allow movement of the one or more floats within said one or more channels, wherein the one or more floats have a surface characteristic that is different from the surface characteristic of the walls such that, upon contact with a fluid, said walls and said floats form different contact angles and induce a repulsive capillary force between the walls and the one or more floats at a surface of the fluid; and wherein the slot has a width that is smaller than the width of the one or more channels such that the one or more floats are prevented from moving between the one or more channels.