CiteULike: di dchen Zhang

Effective potential between two spheres in a suspension of adhesive rods

Chengyu Zhang — 2008-06-11T14:49:19-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 75, No. 4. (2007)

Analytic treatment and Monte Carlo simulations are used to study the effective potential between two spheres in a suspension of rods whose ends can adhere to the surface of the spheres. When only one end of each rod is adhesive the effective potential changes from being attractive to repulsive with enhancing the adherence, but when both ends of each rod are adhesive the effective potential is not a monotonic function of the distance between the two spheres for strong adherence. As the adhesive strength is fixed, the range of the effective potential will increase with increasing the length of the rods. When the adhesive range is much smaller than the diameter of the spheres, its influence on single end adhesion is approximately linear and on two end adhesion is about quadratic. Our results are qualitatively consistent with a recent experimental work.

Microscopic wrinkles on supported surfactant monolayers

Quan Zhang — 2008-06-10T19:23:07-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 76, No. 4. (2007)

We discuss mechanical buckling instabilities of a rigid film under compression interacting repulsively with a substrate through a thin fluid layer. The buckling occurs at a characteristic wavelength that increases as the one-fourth power of the bending stiffness, such as the gravitational instability studied previously by Milner et al. However, the potential can affect the characteristic buckling wavelength strongly, as predicted by Huang and Suo. If the potential changes sufficiently sharply with thickness, this instability is continuous, with an amplitude varying as the square root of overpressure. We discuss three forms of interaction important for the case of Langmuir monolayers transferred to a substrate: Casimir–van der Waals interaction, screened charged double-layer interaction, and the Sharma potential. We verify these predictions numerically in the van der Waals case.

Anomalous behavior of a single particle falling through a funnel

Yuan Fang — 2008-06-08T23:48:50-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 77, No. 4. (2008)

We show several surprising phenomena that occur in an extremely simple system of a single frictionless, inelastic, spherical particle falling under gravity through a symmetric funnel. One might naively expect that particles would fall through funnels with steeper sides more quickly, exert a smaller total impulse on the funnel walls, and lose less energy. However, we show that there are special ranges of angles of the funnel walls for which exactly the opposite occurs. Typically, the particle will experience a sequence of collisions that is highly sensitive to the location at which it enters the funnel and nearby particle trajectories become widely dispersed. However, in the special angular ranges this is not the case and the particle can experience sequences of collisions that have a highly coherent structure. We provide a theoretical analysis that can predict and explain this surprising behavior.

Solvent-Induced DNA Conformational Transition

B Gu — 2008-05-07T23:28:21-00:00

Physical Review Letters, Vol. 100, No. 8. (2008)

Modified water models with scaled charges are used to investigate solvent polarity effects on DNA structure. Several intensive molecular dynamics simulations of the DNA EcoRI dodecamer d(CGCGAATTCGCG) in different model solvents are performed. When the polarity of the solvent molecule decreases, from overpolarized to less polarized, DNA experiences the conformational transitions of constrainedB form(A-B)mixA form. We demonstrate that one important cause of these structure changes is the competition between hydration and direct cation coupling to the free oxygen atoms in the phosphate groups on DNA backbones.

Time-Resolved Shadowgraphs of Material Ejection in Intense Femtosecond Laser Ablation of Aluminum

Nan Zhang — 2008-05-03T23:46:49-00:00

Physical Review Letters, Vol. 99, No. 16. (2007)

The dynamic process of intense 50 fs laser ablation of aluminum is investigated by ultrafast time-resolved microscopy. A stripe pattern preceding phase explosion is clearly seen in the shadowgraph of 1 ns time delay. Intermittent material ejections are observed within the ejected plume after 2.5 and 7 ns time delay, respectively, which may be attributed to the material response to the generation of an extremely strong thermoelastic wave. Similar processes are also recorded in the ablation of silicon and glass samples, except for the glass samples, the intermittent material ejections are not found.

Speckle Evolution of Diffusive and Localized Waves

Sheng Zhang — 2008-05-03T19:57:29-00:00

Physical Review Letters, Vol. 99, No. 6. (2007)

We show that while the statistics of static speckle patterns are generic, fluctuations in the change within speckle patterns are greatly enhanced in the localization transition. The probability distributions of the displacement of phase singularities and the standard deviations of the changes of phase and intensity with frequency shift of incident microwave radiation are given in terms of the same expression which describes the probability distribution of total transmission. This function depends only upon a single parameter, the variance of the corresponding variable. The changing statistics in the localization transition reflects the number of underlying electromagnetic modes with which the incident wave interacts.

Shake-gels: shear-induced gelation of laponite-PEO mixtures

J Zebrowski — 2008-04-24T22:24:38-00:00

Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 213, No. 2-3. (12 February 2003), pp. 189-197.

Suspensions of clay particles (laponite), mixed with poly(ethylene oxide) (PEO) undergo a dramatic shear thickening when subjected to vigorous shaking, which transforms them from a low viscosity fluid into a [`]shake-gel', a solid with elasticity sufficient enough to support its own weight. The shake-gel is reversible, relaxing back to a fluid with a relaxation time that is strongly dependent on PEO concentration. Shake-gels are observed for PEO concentrations slightly below the threshold for complete saturation of the laponite particles by the polymer. Light scattering measurements confirm that the PEO is adsorbed on the surface of the laponite particles, and suggests that shear induces a bridging between the colloidal particles, resulting in a gel network which spans the system. Desorption of the polymer reduces the bridging and thus relaxes the network.

Characterization of atomic motion governing grain boundary migration

Hao Zhang — 2008-04-15T20:22:51-00:00

Physical Review B (Condensed Matter and Materials Physics), Vol. 74, No. 11. (2006)

Molecular dynamics simulations were employed to study atomic motion within stationary and migrating asymmetric tilt grain boundaries. We employ several measures of the "complexity" of the atomic trajectories, including the van Hove correlation function, the non-Gaussian parameter, and dynamic entropy. There are two key types of dynamical events within the grain boundaries (i) a stringlike cooperative motions parallel to the tilt axis and occurring on a characteristic time scale of 25 ps and (ii) atomic motion across the grain boundary plane occurring on a characteristic time scale of 150 ps. The characteristic times associated with each type of event decreases with increasing driving force for boundary migration. We present evidence as to how the driving force biases these types of events, leading to boundary migration. While the stringlike atomic motion is an intrinsic feature of grain boundary dynamics and is important for grain boundary migration, it is the second type of event that controls grain boundary migration rates.