1. Introduction A substance offering no resistance to shear deformation is a fluid. A liquid is an incompressible fluid. Following the overall theme of this special issue ‘Liquid brains, solid brains’, co-edited by Ricard Solé, Melanie Moses and Stephanie Forrest, I focus on computing devices where a liquid plays a key role, either as a material substance for signal representation and transmission or as a substrate where computation takes place. A liquid plays a key role in the functioning and ...