rainbows, marching bubbles and floppy pipes –
some physics of liquid foams
bubbles immersed in a liquid form a simple system with complex properties,
displaying a large range of not-so-ordinary physical phenomena. A small
selection of these shall be presented here.
The minimisation of interfacial energy leads to interesting packing problems and pattern formation, allowing the patient experimentalist to discover a number of well-know structures from solid state physics or biology in ordinary dish-washing foams.
The confinement and transport of some of these structures in channel-networks provides fundamental ingredients to a “Discrete Microfluidics”, in which bubbles (or droplets) turn into micro-reactors which are generated, transported, manipulated, analysed and stored on a single “lab-on-a-chip”. In this, bubbles need to “aqua-plan” along channel walls, which presents a challenging non-Newtonian fluid dynamic problem, as the bubble shape adjusts to the stresses generated by the flow.
Similar mechanisms cause a stream of surfactant solution, which descends between two plates, to become floppy beyond a critical flow rate, thus displaying astonishingly regular, travelling meanders - which stubbornly resist theoretical explanation.
A promising application for dishwashing foams is also – to wash dishes!