Remote sensing of microfluidic tracers by light scattering from microcrystals under magnetic fields

This paper presents a new magnetic method for microfluidic visualization using low-density, highly buoyant, strongly reflective and magnetically responsive microcrystals. Microcrystal suspensions were passed through a channel with or without exposure to a magnetic field using a permanent magnet. This approach enhanced the contrast of the flow patterns. We also observed the movements of microcrystals in a paramagnetic aqueous solution containing MnCl2. In the paramagnetic solution, guanine crystals showed migration both to and from the higher magnetic fields in the microfluidic path. In contrast, microcrystals with diamagnetic susceptibility caused a stream only towards lower magnetic fields. Flow pattern measurements were also carried out using microcrystalline cellulose and boron nitride crystals. The methods developed here are expected to be useful for advancing microfluidic mechanics through the use of magnetic field effects on microcrystals in liquids.