In lab-on-chip devices, on which complete (bio-)chemical analysis laboratories are miniaturized and integrated, it is essential to manipulate fluids in sub-millimetre channels and sub-microlitre chambers. A special challenge in these small micro-fluidic systems is to create good mixing flows, since it is almost impossible to generate turbulence. We propose an active micro-fluidic mixing concept inspired by nature, namely by micro-organisms that swim through a liquid by oscillating microscopic hairs, cilia, that cover their surface. We have fabricated artificial cilia consisting of electro-statically actuated polymer structures, and have integrated these in a micro-fluidic channel. Flow visualization experiments show that the cilia can generate substantial fluid velocities, up to 0.6 mm s(-1). In addition, very efficient mixing is obtained using specially designed geometrical cilia configurations in a micro-channel. Since the artificial cilia can be actively controlled using electrical signals, they have exciting applications in micro-fluidic devices.

In this work we mimic the efficient propulsion mechanism of natural cilia by magnetically actuating thin films in a cyclic but non-reciprocating manner. By simultaneously solving the elastodynamic, magnetostatic, and fluid mechanics equations, we show that the amount of fluid propelled is proportional to the area swept by the cilia. By using the intricate interplay between film magnetization and applied field we are able to generate a pronounced asymmetry and associated flow. We delineate the functional response of the system in terms of three dimensionless parameters that capture the relative contribution of elastic, inertial, viscous, and magnetic forces.

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Micro-sized hair-like structures, such as cilia, are abundant in nature and have various functionalities. Many efforts have been made to mimic the fluid pumping function of cilia, but most of the fabrication processes for these "artificial cilia" are tedious and expensive, hindering their practical application. In this paper a cost-effective in situ fabrication technique for artificial cilia is demonstrated. The cilia are constructed by self-assembly of micron sized magnetic beads and encapsulated with soft polymer coatings. Actuation of the cilia induces an effective fluid flow, and the cilia lengths and distribution can be adjusted by varying the magnetic bead concentration and fabrication parameters.

The spreading of a liquid droplet on a smooth solid surface in the partially wetting regime is studied using a diffuse-interface model based on the Cahn--Hilliard theory. The model is extended to include non-90$^{\circ}$ contact angles. The diffuse-interface model considers the ambient fluid displaced by the droplet while spreading as a liquid. The governing equations of the model for the axisymmetric case are solved numerically using a finite-spectral-element method. The viscosity of the ambient fluid is found to affect the time scale of spreading, but the general spreading behaviour remains unchanged. The wettability expressed in terms of the equilibrium contact angle is seen to influence the spreading kinetics from the early stages of spreading. The results show agreement with the experimental data reported in the literature.

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SynopsisThe breakup of confined drops in shear flow between parallel plates is investigated as a function of viscosity ratio and confinement ratio. Using a boundary-integral method for numerical simulations and a counter-rotating experimental device, critical capillary numbers in shear flow are obtained. It is observed that different viscosity ratios yield different trends with increasing confinement ratio: a low viscosity ratio drop shows an increase in critical capillary number, at a viscosity ratio of unity no major trend is seen, and the critical capillary number for a high viscosity ratio drop decreases significantly. A generalized explanation for all viscosity ratios is that confinement affects the orientation of the drop with respect to the direction of the local strain field. At moderate confinement ratios, the drop orients more toward the strain direction, where it experiences a stronger flow and hence, the critical capillary number is decreased. As the drop gets more confined, it aligns more in the flow direction. Hence, the drop experiences a weaker flow and thus, additionally stabilized by wall effects, it breaks at a higher critical capillary number. In principle, this behavior is the same for all viscosity ratios, but transitions occur at different confinement ratios. Most of the breakup is of a binary nature, but ternary breakup can occur if the drop length is larger than 6 undeformed drop radii, consistent with arguments based on the Rayleigh-Plateau instability.