Reversibility of microflows

For small Reynolds numbers a flow caused by the boundary movement of the fluid boundaries is time-reversible. It means that when a direction of the motion of the boundary is reversed, the flow direction is also reversed. On the basis of the Taylor's movie "Low Reynolds Number Flow", a similar experiment was preformed in our laboratory. The experimental setup was composed of two coaxial cylinders. The inner cylinder was movable, the other one static, and the space between the cylinders was filled with glycerol. During the experiment a small droplet of dye (rhodamine B) dissolved in glycerol was placed between the cylinders. Afterwards, the small cylinder was rotated several times in one direction. Next, the same number of turns in the reverse direction was performed to achieve exactly the initial position of the cylinder. We noticed that the coloured droplet approximaty returned to the initial position. Such behaviour is typical for the flows with small Reynolds number (Re << 1). We observed a different motion of a small drop of coloured water which mixed with the clear water.

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