Particle Transport Phenomena in Non-Newtonian Microfluidics

In the past two decades, microfluidic devices have become attractive platforms for many chemical and biomedical applications due to their enhanced efficiency and accuracy at a reduced cost. Many of the fluids encountered in these applications exhibit non-Newtonian behaviors. However, the majority of current particle transport studies have been limited in Newtonian fluids only. Very little work has been done on particle transport in non-Newtonian fluids. This dissertation presents experimental and numerical studies of particle transport phenomena in both electric field- and pressure-driven flows in non-Newtonian fluids through microchannels. In the first part, electrokinetic transport phenomena are investigated in viscoelastic polymer solutions though a constricted microchannel. The first experimental study of particle electrophoresis shows an oscillatory particle motion in the constriction region. This oscillatory motion is affected by the electric field magnitude, particle size and fluid elasticity (i.e., polymer concentration). Then the viscoelastic effect on electrokinetic particle focusing is presented via the study of particle charge effect. The particle focusing trend observed is opposite to that in a Newtonian fluid when the electric field varies. Particle aggregation phenomena are also found at high electric fields. These phenomena are speculated to be a consequence of the fluid viscoelasticity effects. Inspired by the interesting electrokinetic particle transport phenomena, the flow visualization study in the viscoelastic fluid is conducted by using small fluorescent particles as trackers. It is showed that the small particle trajectories, which represent the electroosmotic flow streamlines, are significantly different from those in the Newtonian fluid at the upstream of the microchannel constriction due to the viscoelastic instability. The 2D numerical result of Oldroyd-B model obtains a smaller flow rate than the Newtonian one, but fails to predict the deflected particle trajectories via Lagrangian particle tracking method. In the second part, comprehensive studies are performed for particle transport in pressure driven flows through straight rectangular microchannels. A continuous size-based separation is achieved via elasto-inertial pinched flow fractionation (eiPFF). The separation is found to be affected by the flow rate, polymer concentration and channel aspect ratio significantly. Then elasto-inertial particle focusing is studied, which also demonstrates a sheath-free particle separation. An interesting trend has been observed that the particle size (blockage ratio) plays a less significant role on the particle equilibrium position with the increase of channel aspect ratio. Shear-thinning effect is studied in Polyvinylpyrrolidone (PAA) solutions of varied glycerol concentrations in a near-slit channel, which has been demonstrated to inhibit the elastic lift and deflect particles towards the walls. The 2D numerical studies of the particle motion via Oldroyd-B and Giesekus models are qualitatively consistent with our experimental observations of the viscoelastic and shear thinning effects on the elasto-inertial particle focusing. Moreover, shape-based particle separations are demonstrated via both eiPFF and the elasto-inertial lift in sheath-free flows. The rotational motion of non-spherical particles in the viscoelastic fluid is speculated to affect the elasto-inertial lift and lead to different migrations of particles with varied shapes

Cinematic Representation of Ethnic Minorities in PRC and Postcolonialism

This paper explores the notions of “Sinophone” and “Chinese-language cinema” under the rubrics of postcolonialism in Chinese film studies both in China and elsewhere around the world. The paper argues that these postcolonial-inspired notions misconstrue Chinese national identity building as imperialist/colonialist endeavours, and dichotomize Han and Chinese ethnic minorities. The paper offers its counterargument by examining cinematic practices of people’s cinema, minority nationality films and native-language films in the PRC

A probable Milli-Parsec Supermassive Binary Black Hole in the Nearest Quasar Mrk 231

Supermassive binary black holes (BBHs) are unavoidable products of galaxy mergers and are expected to exist in the cores of many quasars. Great effort has been made during the past several decades to search for BBHs among quasars; however, observational evidence for BBHs remains elusive and ambiguous, which is difficult to reconcile with theoretical expectations. In this paper, we show that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and an orbital period of ~1.2 year. The flat optical and UV continua are mainly emitted from a circumbinary disk and a mini-disk around the secondary black hole (BH), respectively; and the observed sharp drop off and flux deficit at wavelength lambda ~ 4000-2500 Angstrom is due to a gap (or hole) opened by the secondary BH migrating within the circumbinary disk. If confirmed by future observations, this BBH will provide a unique laboratory to study the interplay between BBHs and accretion flows onto them. Our result also demonstrates a new method to find sub-parsec scale BBHs by searching for deficits in the optical-to-UV continuum among the spectra of quasars.Comment: a typo in equation (2) and also in equation (5) of the Appendix is fixed; 9 pages, 7 figure