Microfluidic flow actuation using magnetoactive suspensions

The rheological behavior of magnetotactic bacterial suspensions is analyzed using a continuum kinetic theory. In both unbounded and confined geometries, the response of these suspensions under simple external flows can be controlled by applying a magnetic field and hinges in a subtle way on the interplay of magnetic alignment, rotation under shear, and wall-induced accumulation under confinement. By tuning magnetic field strength and direction, the apparent viscosity can either be enhanced or reduced, and the mechanisms for these trends are elucidated. In the absence of any applied flow, we further demonstrate the ability of magnetoactive suspensions to internally drive steady unidirectional flows upon application of a magnetic field, thus suggesting novel avenues for the design of microfluidic pumps and flow actuation devices. Copyright c ⃝ EPLA, 2018 Introduction. – Suspensions of active particles, such as motile microorganisms, synthetic microswimmers, or externally actuated colloids, exhibit unusual rheological properties that are unlike those of classical complex fluids [1]. While the additional viscous dissipation incurred by flow around suspended particles typically enhances viscosity in passive systems [2], such is not the case in active suspensions, where mechanical stresses generated on the microscale as a result of activity can have the opposite effect of reducing flow resistance [3]. This curious trend has been characterized in detail in the case of swimming bacteria [4,5], where the coupling of particle reorientations by the applied flow and of dipolar stresses exerted during self-propulsion indeed causes a decrease in viscosity in weak flows [6]. In sufficiently concentrated systems, the apparent viscosity can in fact reach zero [5,7], indicating a transition to a superfluid-like state where internal activity exactly compensates viscous dissipation. A dramatic manifestation of this transition is the emergence of spontaneous directed motions in confined systems [8,9], which has been explained as a linear instability driven by active stresses [10]. The ability to harness these flows for applications, however, remains limited due to the lack of external control on particle configurations, which instead emerge spontaneously from internal mechanical couplings. (a)E-mail: [email protected] Tunable rheological properties are typically achieved in passive systems by applying external electric or magnetic fields [11,12], which drive particle rearrangements or reorientations and thus affect resistance to flow. The viscous properties of passive magnetic fluids have been extensively studied, with experiments [13] showing an increase in the viscosity of ferromagnetic fluids when a constant magnetic field is applied. This effect is well understood theoretically [14–18] as a consequence of the magnetic torque acting on the particles, which hinders their rotation by the applied vorticity and results in an additional stress contribution. In this work, we investigate the use of magnetic fields as a means to control the effective rheology and internallydriven flows of active suspensions in microfluidic channels. The system of choice for this problem is magnetotactic bacteria, which are motile prokaryotes mostly present in marine habitats that synthesize intracellular magnetic membrane-bound crystals known as magnetosomes. These bacteria, which swim by similar mechanisms as other flagellated bacteria, behave as self-propelled permanent magnetic dipoles that orient and migrate along the local magnetic field lines [19]. Magnetotactic suspensions thus behave as magnetic fluids with additional complexities arising from self-propulsion, which causes particle accumulation at walls in confined systems [20,21], and from active stresses, which modify the rheology [1,6,7]. Although the