English: The injection of a dye in a viscous fluid sandwiched between two concentric cylinders (top panel). The core cylinder can be rotated to generate a shear forces resulting in the flow of the fluid and the dye getting mixed with the fluid (middle panel). The rotation then reversed bringing the cylinder to its original position. The dyes "unmix" to their initial position. The unmixed state is not perfect due to process diffusion of dye.