Microfluidic Elasto-Magnetic Materials For Controlled Swimming And Pumping

Abstract

This thesis presents the results from investigating the behavior of different magnetically actuated systems capable of creating motion or activating fluid flow within a low Reynolds number regime. It demonstrates a number of designs where magnetic materials are integrated into elastic networks and the systems are activated by an oscillating magnetic field. The elasto-magnetic systems rely on magnetic and elastic interactions to generate their movement and are designed in such a way as to induce non-reciprocal motion that could be used to manipulate fluid. The thesis demonstrates the capability of these devices and investigates its potential application in microfluidic systems. The presented research is primarily focused on three elasto-magnetic designs. The first design was based on two interacting ferromagnetic particles with different properties connected by an elastic link. The devices produced were that of a 68 µm microswimmer and a membrane or network spanning 12 mm with feature sizes of the order of micrometers. Ellipsoidal CoNiP (major and minor diameters of 30 µm and 10 µm) and cylindrical Co (diameter 10 µm) particles were produced by electrodeposition. The electrodeposition parameters have been optimised on deposition of CoNiP films to produce an out-plane coercivity for CoNiP elements of 54 kA/m. A fabrication process was created using lithographic techniques to produce a highly structured network of a silicon-based organic polymer, Polydimethylsiloxane (PDMS), embedded with the described magnets. Although this design did not demonstrate active pumping, important conclusions were derived regarding the improvements required to make it more practically suited. The second design consisted of a series of magnetic discs connected to a frame via elastic axles. The paddles had the same magnetic properties but different elastic properties, due to the different widths of their connecting axles. These introduced a phase difference in the motion of the paddles when actuated. This phase difference was reminiscent of metachronal waves that commonly exist in cilia carpets. In order to understand this design further, a theoretical model was developed and various parameters were investigated, such as frequency and rotational stiffness of the paddles, which was implicative to the width of the axles. For the dimensionless parameters investigated, the maximum speed was generated when the center-to-center separation was 0.9, the frequency was 1.4, lp1= 0.130 and lp2 = 0.225, achieving maximum angles of 45° and 148° respectively. The third design comprised of only a single ferromagnetic component, which was at the `head' of the device, connected to an asymmetric `tail'. This 3-linked `tail' was inspired by Purcell's 3-link swimmer. The head of the device was actively driven while the elastic links followed. This introduced a phase difference in the generated motion of the device. The 3-linked structure was integrated within a microfluidic device to produce an enclosed pumping system. In this section, the properties of the path drawn by the head during its cycle and the flow rate generated by this device were researched for different fluid viscosities and driving frequencies. It was found that this system could provide a tuneable fluid flow with a flow rate of up to 700 µl/h. The fluid flow was able to be reversed by adjusting the driving frequency.