A newly revealed patent application from Nokia describes their “haptikos” technology for tactile touchscreens. According to Engadget:
Basically, the tech uses layers under the touchscreen to allow control
of surface roughness and friction. The patent describes a “plurality of
closely space voltage controllable protruberances” that can raise or
lower based on where the user is touching the screen, resulting in what
feels like resistance and tactile feedback. Filled with fluid, the
protruberances increase fiction and help users feel like they’re
actually touching something rather than poking at a plane of glass.