Surmounting the "Insurmountable": Self-organized Barrier Crossing in Biological Systems at Meso-scale
Wokyung Sung
Department of Physics, Pohang University of Science and Technology (POSTECH), South Korea

Due to structural connectivity and flexibility, bio-soft condensed matter, such as biopolymers, membranes and cells, manifest interesting cooperative dynamics under the barriers caused by external fields, confining and constraining environments. Its cooperative dynamics is important, no only in understanding how a biological system self-organizes by manipulating its flexible degrees of freedom, but also in a multitude of bio-technological applications. Nature utilizes the ambient fluctuations in such soft-condensed matter to facilitate crossing seemingly insurmountable barriers, typically assisted by shape changes and coupling of the collective modes of the fluctuations. After introducing the basic physical features of bio-soft matter, I will talk about the examples we studied, namely, biopolymer translocation through membranes and potential barriers, bubble formation in double-stranded DNA, membrane instability and fusion, blood flow in a narrow vessel.

Bio:
Professor Wokyung Sung received his Ph.D. in Physics from State Univ. of New York at Stony Brook. Currently, he is a professor at Department of Physics of POSTECH, and the director of POSTECH Center for Theoretical Physics. He is also an editor-in-chief of the Journal of Biological Physics, and the special advisor of Asia Pacific Center for Theoretical Physics (APCTP).
Professor Sung’s current research interests lie mainly in physical understanding of basic biological conformations and processes that emerge in mesoscopic (cellular) level. The methodology is theoretical statistical physics of soft matter (such as polymers, membranes) and stochastic phenomena (including barrier crossing, stochastic resonance and other noise-assisted cooperative dynamics). His group is currently working on a variety of (bio-)soft matter physics, such as DNA bubbles and mechanics, the polymer/colloid dynamics in fluctuating/confining environments and channels, investigating some physical principles behind the biological self-organizations. For more details, click Statistical and Biological Physics Laboratory.

Charge Density Fluctuation and Undulation Dynamics in a Rodlike DNA
Won Kyu Kim
Department of Physics, Pohang University of Science and Technology, South Korea

The double-stranded DNA is a strongly charged, semi-flexible polyelectrolyte of bending persistence length ~50nm. We study the interplay of charge and conformational degrees of freedom on dynamics of the DNA surrounded by an ionic solution. Using a mesoscopic model where the entropy and the screened Coulomb interaction associated with fluctuating charges are incorporated, we study charge correlation as functions of ionic concentration and counterion valency. Further incorporating local undulation in an otherwise rigid rod polyelectrolyte, we further study the effects of the charge fluctuation on the persistence length and undulation dynamics.

Moving contact line at chemically patterned surfaces: Molecular dynamics simulation and continuum calculation
Congmin Wu
Department of Mathematics, The Hong Kong University of Science & Technology, Hong Kong

In an earlier study based on our continuum hydrodynamic model of moving contact line, we predicted the stick-slip motion for moving contact line at chemically patterned surfaces [Wang et al., J. Fluid Mech. 605, 59 (2008)]. In this talk we show that the continuum prediction can be quantitatively reproduced by molecular dynamics (MD) simulations. The MD simulations were performed for two immiscible Lennard-Jones fluids confined by two planar solid walls in Poiseuille flow geometry. In particular, one solid surface is chemically patterned with alternating stripes. For comparison, the continuum model was numerically solved with material parameters directly measured in MD simulations. From the oscillatory fluid-fluid interface to the periodic stick-slip motion of the moving contact line, we have quantitative agreement between the MD simulation and continuum calculation results. This agreement is attributed to the accurate description by the generalized Navier boundary condition down to molecular scale in our continuum model.