Adaptive Microoptical Systems based on Ferrofluidic Actuators
Ferrofluids behave superparamagnetically and can be used as actuators in microfluidic systems, among others. In cooperation with the Institut für Mess- und Regelungstechnik and the Institut für Mikrotechnologie at Universität Hannover we are studying ferrofluidic actuators for the adaption of microoptical components. The example in the figure below explains the principle this concept is based on.
A ferrofluid plug is displaced in a microchannel by an external magnetic field. The displacement of the ferrofluid translates to the displacement of a second liquid, comprising a lens with an adaptable focal length. (a) and (b) refer to the lens without and with convex surface profile, respectively. 
Photographs of an adaptive liquid microlens for different displacements of the external magnet at (from bottom to top): initial position, 0.95 mm, 1.91 mm, 2.86 mm, 4.76 mm, 5.75 mm and 6.67 mm. The lens base is 2 mm in diameter and the cross sectional area of the channel is 0.17 mm2.
In contrast to the schematic representation in the figure the magnetic field will not be created by a permanent magnet, but by an array of microstructured coils, thereby constituting a compact actuator without moving parts.
The described actuation principle will also be useful for other applications not involving any optical functions, e.g. simply for microfluidic pumping. The coupling between the two liquids can either be hydraulic or pneumatic.
 W. Xiao and S. Hardt, An adaptive liquid microlens driven by a ferrofluidic transducer, J. Micromech. Microeng. 20 055032 (2010).
This research is support by the German Research Foundation (DFG) under grant no HA 2696/17-1.