Instability-Induced Mixing of Ferrofluids in Uniform Magnetic Fields

The advantages of ferrofluids in microfluidic lab-on-a-chip applications include remote control of the fluid flow within the chips, e.g., mixing of the species using an external uniform magnetic field. Hence, three-stream flow systems consisting of a ferrofluid core clad by two streams of diamagnetic silicone oil were studied. The instability of the ferrofluid, subjected to an external uniform magnetic field, was also studied. When the strength of this magnetic field was increased to a critical value, the ferrofluid was spread toward the silicone oil and a transient instability developed at the ferrofluid-silicone oil interface. Further increasing magnetic field resulted in periodic instability structures and permanent instability. The effect of magnetic field strength, flow rate, and flow rate ratio were determined. With a higher flow rate ratio, the permanent instability was observed only at the larger magnetic field strength. Our modeling results were consistent with these experimental results. Our work shows that an external uniform magnetic field of only a few millitesla can lead to instability and mixing, thus it is relevant to mixing in practical microfluidic devices.