Microfluidic Device Using Viscoelastic Flow

Abstract

Multiple-line particle focusing has been identified in a straight channel, while single-line particle focusing has been only reported in previous works. By controlling the force balance between elastic force and inertial force induced by geometric features of a channel, a new focusing mechanism has been suggested. Also, another hypothesis has been emerged that each normal stress component is acting on a particle traveling on the viscoelastic flow. Applications of the multiple-line particle focusing have been focused on microfluidic logic operations by developing a microfluidic logic device and a particle valve system. <br/> In Chapter II, we observed the transition between single-line and multiple-line particle focusing in a microfluidic device. The elastic and inertial forces acting on suspended particles were manipulated by tuning the concentration of dilute polymer solution and the flow rate of a fluid. The finding showed that the confinement effects determined by the channel aspect ratio and the inlet geometry lead to the multiple-line focusing of particles in the microfluidic channel due to the fluid elasticity and hydrodynamic behavior of the fluid. A microfluidic channel with high channel aspect ratio possesses broad minimal region of the elastic force across the channel, which generates a wide particle focusing band rather than a single particle focusing at the center. The multiple-line particle focusing occurs as the inertial force outweighs the elastic force, resulting in the particle migration towards the channel sidewalls.<br/> In Chapter III, we investigated a microfluidic logic device that is built based on the particle dynamics in viscoelastic fluid. A logic gate system employing a Boolean function was implemented by utilizing multiple line particle focusing behavior in the microfluidic channel. The device was designed and fabricated to hydrodynamically control the logic operations of XOR, OR, AND, Buffer, and NOT under the fixed flow condition (e.g., flow rate, particle size, and fluid elasticity~1.483). In addition, numerical simulation was carried out to understand the fundamental physics of the particle and fluid behavior in viscoelastic flow. Clear multiple particle focusing lines with high separation resolution (R_ij~6.19) were observed and the particle extraction at the outlets were analyzed by image-processing. <br/> In Chapter IV, we proposed an unique valve system for smart particle control by employing a transparent shape memory polymer (SMP) as a constituent material of the microchannel. The core of this strategy was to use SMP instead of poly(dimethylsiloxane) (PDMS) to give the channel shape-programmable function. The magnitude of the hydraulic resistance formed in the SMP microchannel could be selectively controlled according to the deformation and recovery of the channel. Due to the pressure difference between the two outlets, the path of the focused particles was determined depending on the channel-shape. The pressure distribution and shape memory-recovery behavior of the SMP microchannel were theoretically investigated by the numerical calculations. The operation as a particle valve and its repeatability were confirmed by experimental observation.<br/> In Chapter V, the lateral particle migration was investigated in a hydroxypropyl cellulose (HPC) viscoelastic fluid with a negative first normal-stress difference. Unlike common viscoelastic fluids with positive normal stress differences, double-line particle focusing was identified in a microfluidic channel, which was caused by the negative first normal stress difference. More importantly, unique particle migration with different sized particles in a microchannel was observed in which bigger particles were double-line focused along the channel walls while smaller particles were single-line focused at the center. A new dimensionless parameter, the ratio of the normal force to the viscous drag force, was defined to demonstrate this unique double line focusing behavior of particles in the viscoelastic fluids.<br/>