Cite this: RSC Advances, 2013, 3, 12735 Fluid propulsion using magnetically-actuated artificial cilia – experiments and simulations Received 3rd March 2013, Accepted 13th May 2013 DOI: 10.1039/c3ra42068j www.rsc.org/advances Syed Khaderi, a Jeanette Hussong, b Jerry Westerweel, b Jaap den Toonder c and Patrick Onck* d We conducted a combined modelling and experimental approach to explore the underlying physical mechanisms responsible for fluid flow caused by magnetically-actuated plate-like artificial cilia. After independently calibrating the elastic and magnetic properties of the cilia, the model predictions are observed to be in excellent agreement with the experimental results. We show that the fluid propelled is due to a combination of asymmetric motion and fluid inertia forces. The asymmetric motion of the cilia and inertial forces contribute equally to the total fluid flow. We have performed a parametric study and found the cilia thickness and magnetic field that should be applied in order to maximise the fluid transport. 1 Introduction One of the important components of a lab-on-a-chip (LOC) is the fluid propulsion system that transports the fluid to be analysed through the microchannels. Due to the large surface to volume ratios that occur at the small length scales of typical LOCs conventional methods no longer suffice, which has led to a search for new methods to propel fluids. This has resulted in magneto-hydrodynamic and electro-osmotic pumps and bio-inspired pumping mechanisms that mimic natural fluid propulsion systems at the micrometer length scale. The flows created by magneto-hydrodynamic pumps and electro-osmotic pumps depend strongly on the electro-kinetic properties of the fluid to be propelled. Moreover, the electric fields used in these devices can cause heating, bubble formation and interact with the biofluid to be analysed, thereby polluting the analyte. Recently, researchers have used the principles used by natural ciliates to design mechanical actuators (i.e. artificial cilia) that can create fluid transport in microchan- nels. The artificial cilia are actuated by various external triggers, including piezo actuation, 16 light 20 electric 3 and magnetic 2,5,6,8–12,17,18 fields. A review of various approaches to propel fluids using artificial cilia can be found elsewhere. 4 Some of the aforementioned actuation techniques have certain disadvantages. For instance, electric fields can interact with the biofluid to be analysed, the response time for light actuation can be very large and achieving asymmetric motion is difficult in the case of piezo actuation. The magnetically driven cilia systems do not suffer from these drawbacks, and hence, have potential for microfluidic fluid propulsion. In general, the fluid transport by artificial cilia can be due to (1) spatial asymmetry, and in the presence of fluid inertia, due to (2) temporal or (3) orientational asymmetry. 9 The spatial asymmetry is due to the non-reciprocal motion of the cilia during the effective and recovery strokes. The temporal asymmetry can be achieved when the velocities of the effective and recovery strokes are different. Orientational asymmetry will be present when the mean position of the cilia is inclined at an angle to the axis of the microchannel. In the Stokes regime, the fluid transport by magnetic artificial cilia is governed by the competition between the elastic forces of the cilia and the viscous and magnetic forces acting on it. Using numerical simulations, 10 it has been shown that the amount of spatial asymmetry exhibited by super- paramagnetic cilia, and hence the fluid transport, is increased when the magnetic forces are high and when the viscous forces are low. The spatial asymmetry was achieved due to the interaction of a rotating magnetic field with the super- paramagnetic cilia. In contrast to natural cilia, the recovery stroke was observed to be faster compared to the effective stroke (temporal asymmetry). The flow transported by the cilia also depends on geometric parameters such as the cilia spacing and the channel height. 11 When the width of the channel is large in comparison to the height, the fluid propelled increases when the height of the channel is increased, and also when the cilia spacing is decreased. Natural cilia also beat in a coordinated manner so that neighbouring cilia beat out-of-phase, thereby generating metachronal waves. The effect of such an out-of-phase motion of magnetically actuated cilia has also been investigated. 7,12,13 a Department of Engineering, Cambridge University, Cambridge, UK b Laboratory for Aero and Hydrodynamics, Delft University of Technology, Delft, The Netherlands c Eindhoven University of Technology, Eindhoven, The Netherlands d Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands. E-mail: p.r.onck@rug.nl RSC Advances PAPER This journal is ß The Royal Society of Chemistry 2013 RSC Adv., 2013, 3, 12735–12742 | 12735