Jeong Yong Lee

Fast dynamics and relaxation of colloidal drops during the drying process using multispeckle diffusing wave spectroscopy.

The fast dynamics generated by the Brownian motion of particles in colloidal drops, and the related relaxation during drying, which play key roles in suspension systems, were investigated incorporating multispeckle diffusing wave spectroscopy (MSDWS). MSDWS equipment was implemented to analyze the relaxation properties of suspensions under a nonergodic and nonstationary drying process, which cannot be elucidated by conventional light scattering methods, such as dynamic light scattering and diffusing wave spectroscopy. Rapid particle movement can be identified by the characteristic relaxation time, which is closely related to the Brownian motion due to thermal fluctuations of the particles. In the compacting stage of the drying process, the characteristic relaxation time increased gradually with the drying time because the particles in the colloidal drop were constrained by themselves. Moreover, variations of the initial concentration and particle size considerably affected the complete drying time and characteristic relaxation time, producing a shorter relaxation time for a low concentrated suspension with small particles.

Conformational dynamics of sub-micron sized wormlike polyelectrolyte polymer in flow fields

AbstractConformational dynamics of a single chain of wormlike polyelectrolyte xanthan polymer has been investigated in the external flow fields by employing a well-suited coarse-grained Brownian dynamics simulation. This goes beyond other simulations, which do not consider the hydrodynamic interaction between pairs of beads in polyelectrolyte polysaccharide and the long-range electrostatic screening effect. Conformational properties, such as the radius of gyration and the static structure factor, were unchanged with the flow strength parameter (i.e., Weissenberg number) in the uniform flow. However, influences by flow strength as well as flow type were evident in both simple shear and extensional-like flows with non-zero velocity gradients in flow regimes, commonly exhibiting a sigmoidal transition in the radius of gyration. Transition to a higher plateau, and independence of long-range electrostatic screening on chain conformation, can be encountered earlier with increasing flow strength, as a special feature of a polyelectrolyte in extensional-like flow. The translational self-diffusion coefficient increases when increasing either the flow strength or the electrostatic screening effect in uniform and simple shear flows. Scaling behavior of the static structure factor is quite well-correlated with respect to each flow field, where the Flory-Edwards exponent (ν) decreases with higher values of flow strength and flow type parameters, but for lower screening effect. Present results on the mesoscopic scale devoted to the bulk space can readily serve as the basis for further scrutiny of the behavior of wormlike polyelectrolytes within various flow fields in confined spaces.

Rotational relaxation time of polyelectrolyte xanthan chain via single molecule tracking method

Effect of solvent viscosity on the longest rotational relaxation time of xanthan molecule has been examined using a single molecule tracking method. Incorporating inverted epi-fluorescence microscope and chargedcoupled device (CCD) camera, various features of xanthan (i.e., radius of gyration, orientation angle, etc.) were interpreted by image processing algorithm from the captured real xanthan images. From the best-fit of the autocorrelation function on the orientation angle, the longest rotational relaxation time was effectively determined. Rotational relaxation time increases with the medium solvent viscosity due to the slow movement of xanthan molecule. It is confirmed that there is a good agreement between experiments and Brownian dynamics simulations on the relaxation patterns of xanthan chain.

Transient Solutions of Nonlinear Dynamics in Film Blowing Accompanied by Flow‐induced Crystallization

Using the newly developed real space vortex-lattice based theory of superconductivity, we study the maximum superconducting transition temperature (T c ) in the iron-based superconductors. We argue that the c-axis lattice constant plays a key role in raising the T c of the superconductors. It is found that all the reported FeAs superconductors can be divided into two basic classes (c/a ≈ 3 and c/a ≈ 5/2) depending on the lattice constants, where a is the Fe-Fe distance in the xy-plane and c is the Fe-Fe layer distance along the z-axis. Our results suggest that the former class has a maximum T c < 60 K, while the latter class has a lower T c ≤ 40 K. Our investigations further indicate that, in order to enhance T c in this family of compounds, new class of superconductors with a larger ratio of c/a should be synthesized. It is likely that their T c values could be raised into the liquid nitrogen range (77 K) and 100 K, supposing the new analogues with c/a ≈ 5 (approximately c > 13 Å, if a = 2.750 Å) and c/a ≈ 11 (c > 31 Å) can be experimentally achieved, respectively. For the new FeSe series, our mechanism predicts that their T c is impossible to exceed 30 K due to a relatively shorter c-axis lattice constant (c/a ≈ 2). Finally, based on the new experimental results (arXiv:0811.0094 and arXiv:0811.2205), the possible ways to raise the Tc of the iron-based superconductors into 70 K are also suggested.

Dynamics and Conformation of Single Polymer Chain in a Slot Coating Flow

To guarantee good mechanical and optical properties of coating products, it is important to control molecular orientation of polymer chains in liquids to be coated on the web or substrate. In this study, Brownian dynamics (BD) and the conventional computational flow dynamics (CFD) simulations have been incorporated to elucidate dynamics and conformation of a polymer molecule in coating flow regime. Flexible polymer chain, e.g., λ‐DNA molecule here, changes its conformation, relying on its flow strength (Weissenberg number, Wi) and flow type parameter (α). Interestingly, in slot coating flow, the chain can be more extended by the extension‐like flow field near the downstream free surface curvature. Due to the complex flow field within the slot coating bead region, the extensibility of polymer chain moving along the streamline has also been investigated by correlating with the effective Wieff and process conditions such as coating gap, web speed, and so on.