Micro−Nanofluidic Interfaces: A Wonderland for the Study of Soft Matter
Chia-Fu Chou
Institute of Physics and Genomics Research Center and Research Center for Applied Sciences, Academia Sinica, Taiwan

In the past decade, fluidic interfaces between micro- to nanoscale environments and the extreme confinement effects provided by nanofluidic channels have offered unique platforms for the study of molecular or cellular biophysics. Not only new physics emerges, but also practical applications have been derived from these experimentally tailored fluidic environments. We will review recent developments in this field and discuss a few cases of using micro-nanofluidic interfaces for particle transport and for molecular and cellular analysis: (1) DNA/protein capture and preconcentration using nanoscale molecular traps, (2) enhanced transport and sensing kinetics using nanoslits, (3) confinement-induced entropic recoiling of single DNA molecules, and (4) perspectives of cell-cell interactions and cancer cell dynamics at micro-nanofluidic interfaces.  

Bio:
Professor Chia-Fu Chou earned his B.S. in physics from National Tsing Hua University (Hsinchu) in 1986, and Ph.D. in physics from State University of New York at Buffalo in 1996. From 1997-2000, he was a NIH postdoctoral fellow at Princeton University (in physics and molecular biology). In 2000, he joined the Solid State Research Center of Motorola Labs in Tempe, AZ, as a Lead Scientist, and later promoted to Principal Staff Scientist in 2001. In late 2002, he co-founded the interdisciplinary Center for Applied Nanobioscience in Biodesign Institute at Arizona State University, and served as an Associate Professor and Principal Investigator. Since Spring 2006, he has been a Research Fellow (Professor) at Institute of Physics with affiliation at both Genomics Research Center and Research Center for Applied Sciences at Academia Sinica, Taipei, Taiwan. His current research interests include single molecule and cellular biophysics, nanobioscience, and super-resolution imaging. 

Dynamic Pattern Switching in Bacterial Oscillating System under Micro/Nano-fluidic Constrictions
Jie-Pan Shen
Institute of Physics, Academia Sinica, Taiwan

Successful binary fission in Escherichia coli (E. coli) relies on remarkable oscillatory behavior of the MinCDE protein system to determine the exact division site. Disability of such a biological oscillator in E. coli by genetic deletion of minCDE locus has been reported to perturb septum positioning, and cause mini-celling with void genetic materials inside, in addition to another large daughter strand with two chromosomes [1]. The most favorable models to explain this fascinating spatiotemporal regulation on dynamic pattern formation of MinDE proteins in cells are based on the physical scheme of reaction-diffusion, that is, protein-membrane and protein-protein interactions, and following proteins diffusion between cell poles [2, 3]. Although not fully understood, geometric factors, such as membrane curvature and varying scale of cell boundary, caused by bacterial morphology play a crucial role in pattern dynamics of membrane-associated MinD proteins. In the present study, bacteria were cultured, confined and reshaped in various types of pre-defined PDMS microfluidic chambers or fused silica nanoslits, to mimic either negative curvature of cell poles or planar membrane surface of in vitro synthetic systems [4, 5]. In vivo time-lapsed fluorescence imaging was utilized to detail the dynamics of mode transitions between multiple pattern formations caused by collective MinDE interactions. Here, we characterized the dynamic patterns of orchestrated MinDE proteins due to the curvature effect and the varying boundary conditions imposed by micro/nanofluidic constrictions. The understanding of the physics underlying multiple pattern formations in both in vivo and in vitro experiments are further complemented through in silico modeling via Monte Carlo method. The study synergizes the join merits of in vivo, in vitro and in silico approaches, to grasp the insight of stochastic dynamics inherited from the noisy mesoscopic biology of bacteria, and also elaborates sub-cellular biophysics in prokaryotic cell division through micro/nano-fluidic manipulations, together with in vivo imaging techniques.

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2. Y.-L. Shih, T. Le, and L. I. Rothfield, PNAS, 2003, 100, 7865-7870
3. K. C. Huang, Y. Meir Y, N. S. Wingreen, PNAS, 2003, 100,12724–12728
4. M. Loose, E. Fischer-Friedrich, J. Ries, K. Kruse, P. Schwille, Science 2008 320, 789–792
5. V. Ivanov and K. Mizuuchi, PNAS, 2010, 107, 8071-8078

Digital Flow Control of Electroosmotic Pump:
Onsager Coefficients and Interfacial Parameters Determination
Zuli Xu
Department of Physics, The Hong Kong University of Science and Technology

Electroosmosis and streaming potential are two complementary electrokinetic processes related by the Onsager relation. In particular, electroosmotic pump (EOP) is potentially useful for a variety of engineering and bio-related applications. By fabricating samples consisting of dry-etched cylindrical pores (50 μm in length and 3.5 μm in diameter) on silicon wafers, we demonstrate that the use of digital control via voltage pulses can resolve the flow regulation and stability issues associated with the EOP, so that the intrinsic characteristics of the porous sample medium may be revealed. Through the consistency of the measured electroosmosis (EO) and the streaming potential (SP) coefficients as required by the Onsager relation, we deduce the zeta potential and the surface conductivity, both physical parameters pertaining to the liquid-solid interface.