  Liquid marbles are microlitre droplets of liquid, encapsulated by
self-organised hydrophobic particles at the liquid/air interface. They offer an
efficient approach for manipulating liquid droplets and compartmentalising
reactions in droplets. Digital fluidic devices employing liquid marbles might
benefit from having embedded computing circuits without electronics and moving
mechanical parts (apart from the marbles). We present an experimental
implementation of a collision gate with liquid marbles. Mechanics of the gate
follows principles of Margolus' soft-sphere collision gate. Boolean values of
the inputs are given by the absence (FALSE) or presence (TRUE) of a liquid
marble. There are three outputs: two outputs are trajectories of undisturbed
marbles (they only report TRUE when just one marble is present at one of the
inputs), one output is represented by trajectories of colliding marbles (when
two marbles collide they lose their horizontal momentum and fall), this output
reports TRUE only when two marbles are present at inputs. Thus the gate
implements AND and AND-NOT logical functions. We speculate that by merging
trajectories representing AND-NOT output into a single channel one can produce
a one-bit half-adder. Potential design of a one-bit full-adder is discussed,
and the synthesis of both a pure nickel metal and hybrid nickel/polymer liquid
marble is reported.
