Directional fluid transport along artificial ciliary surfaces with base-layer actuation of counter-rotating orbital beating patterns

The generation of metachronal waves of beating cilia is a complex mechanism when reproduced in laboratory experiments. In addition, local manipulation of cilia in larger arrays becomes non-trivial when the beating patterns are not unidirectional. Herein, a more complex pattern of beating cilia is studied, where the cilia perform an orbital tip motion and rows of these cilia are counter-rotating in a traveling wave-like actuation. Interest in this type of potential fluid transport results from technical applications where localized streaks of directional flow need to be produced and the speed and direction of the fluid transport is open to be manipulated. A simple solution is found to generate the corresponding traveling wave and beating patterns using a base-layer actuation of a membrane covered with artificial cilia. A device is built where such a wave is generated mechanically using a ball chain positioned below the membrane. When the ball chain is moved, the elevation leads to an orbital beating pattern of the tips of the cilia on top of the membrane. This beating shows characteristics of non-symmetric motion in terms of fast and slow motion phases in the orbital cycle. When the ball chain is positioned centered between parallel rows of the cilia and moved parallel to the rows, a well-defined directional fluid transport is generated in the gap. Micro-PIV revealed that successive traveling waves generate a steady streaming transport along the rows of the cilia as a combination of the fluid squeezing and streaming components. Changing the direction and speed of the ball chain is possible and allows the localized fluid transport in speed and direction to be altered. 2D Arrays of piston-like actuators in the form of a grid of balls may give further full control of position and direction of transport pathways along the ciliary surface.