Biomedical subjects

Pik-Yin Lai

Publications and source records attributed to Pik-Yin Lai.

4 recordsLinked to original sources

PubMed2002

Adsorption of a random copolymer at a lipid bilayer membrane.

We examine the conformational properties of a random copolymer, containing a disordered hydrophobic/hydrophilic sequence of monomers, in the presence of a hydrophobic potential well. The model can be used to understand the structural properties of the adsorption of a protein molecule at a lipid-bilayer membrane, and the properties of a random copolymer at the surfaces of a microphase-separated layered polymer structure. Using a trial-potential treatment we demonstrate that a mainly hydrophilic chain may localize on the surface of the bilayer and that a mainly hydrophobic chain may have two typical conformations: localization on the surface or complete adsorption inside the two surfaces.

Adsorption↗

PubMed2002

Dynamics of polymer knots at equilibrium.

The relaxation and diffusion dynamics of knotted polymers at equilibrium under good solvent conditions are investigated by dynamic Monte Carlo simulations. Prime knots of chain lengths up to N=240 monomers and knots up to 20 essential crossings are studied. The relaxation dynamics of the prime knots at equilibrium do not display the classification into group as in the case of the nonequilibrium relaxation of cut knots [Phys. Rev. E 58, R1222 (1998); Phys. Rev. Lett. 87, 175503 (2001)]. Furthermore, the time autocorrelation functions for the radius of gyration of the nontrivial knots can be fitted by a sum of two exponential decays of long and short characteristic relaxation times. These two relaxation times decrease with the number of essential crossings C. The faster relaxation follows the Rouse behavior and scales as N1+2nu and its dependence on C is consistent with the scaling analysis using the blob picture. The mean-square displacement of the center of mass of the knots obeys the free diffusion behavior compatible with the Rouse dynamics. The diffusion coefficients of the knots, D approximately 1/N for large N, but D decreases for knots with increasing C. These results are analyzed using scaling theories and discussed in terms of topological interactions in the knots.

Journal Article↗

PubMed2002

Turbulent drag reduction and degradation of DNA.

Turbulent drag reduction induced by lambda-DNA is studied. The double-stranded DNA is found to be a good drag reducer when compared with the other normal linear polymers. However, this drag reducing power disappears when the DNA denatures to form two single-strand molecules. Mechanical degradation of DNA is also different from that of the normal linear-chain polymers: DNA is always cut in half by the turbulence. Our results suggest that the mechanism for turbulent degradation of DNA is different from that of the normal flexible long-chain polymers.

Bacteriophage lambda↗

PubMed2002

Jamming pattern in a two-dimensional hopper.

We perform granular flow experiments using metal disks falling through a two-dimensional hopper. When the opening of the hopper d is small, jamming occurs due to formation of an arch at the hopper opening. We study the statistical properties of the horizontal component X and the vertical component Y of the arch vector that is defined as the displacement vector from the center of the first disk to the center of the last disk in the arch. As d increases, the distribution function of X changes from a steplike function to a smooth function while that of Y remains symmetrical and peaked at Y=0. When the arch vectors are classified according to the number of disk n in the arch, the mean value is found to increase with d. In addition, the horizontal component X(n) and the absolute value of the vertical component /Y(n)/ in each class have mean values increasing with n. Regarding the arch as a trajectory of a restricted random walker, we derive an expression for the probability density function a(n)(X) of forming an n-disk arch. The statistics ( , , and the fraction g(d)(n) of n-disk arches) of the arches generated by a(n)(X) agree with those found in the experiment.

Journal Article↗