Microfluidic pycnometer for in situ analysis of fluids in microchannels.

This paper presents a new particle migration principle and demonstrates an analysis of solution density in a microfluidic channel. A particle situated in a microfluidic interface with different fluid density moves toward the lower-density fluid, driven by the asymmetric hydrostatic pressure acting on the submerged particle. Since the hydrostatic pressure is related to the diameter of a particle submerged in a solution and the density of a solution, we expected that the particle injected into the middle of three inlets would show larger lateral deflection as the particle size and density difference between two-injected solutions increase. In addition to hydrostatic pressure differences on the particle, we were concerned not only about fluid momentum difference caused by solution density and flow rate but also about rotational motion due to asymmetric buoyancy driven by the density gradient. The fluid momentum difference affects an initial position of the injected particles and its influence becomes enlarged according to the flow rate increases. The rotational motion of the particle had been evaluated using a computational tool. The experimental results and numerical expectation proved our theoretical estimation, and here this behavior explains a new principle termed pyklinophoresis (Greek; pyk-, density; -klino-, gradient; -phoresis, migration), which enables in situ analysis of microfluidic liquid samples. An analytical model for pyklinophoresis was provided as a proof-of-concept, and the analytical results of sucrose and volatile solutions were also demonstrated.