The University of Colorado Boulder engineers have shown that the same physical process that helps remove dirt from laundry could play a role in how tropical fish get their colourful stripes and spots. The movement of molecules during diffusiophoresis (the migration of a colloidal particle in a solution in response to the macroscopic concentration gradient of a molecular solute) always follows a clear trajectory and gives rise to patterns with sharp outlines. To see if it may play a role in giving animals their vivid patterns, the researchers ran a simulation of the purple and black hexagonal pattern seen on the ornate boxfish skin using only the Turing equations. The computer produced a picture of blurry purple dots with a faint black outline. Then the team modified the equations to incorporate diffusiophoresis. The result turned out to be much more similar to the bright and sharp bi-colour hexagonal pattern seen on the fish. The team’s theory suggests that when chemical agents diffuse through tissue as Turing described, they also drag pigment-producing cells with them through diffusiophoresis — just like soap pulls dirt out of laundry. These pigment cells form spots and stripes with a much sharper outline.