Content of SPECIAL TOPIC—Non-equilibrium phenomena in soft matters in our journal

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    Propulsive matrix of a helical flagellum
    Zhang He-Peng (张何朋), Liu Bin (刘斌), Bruce Rodenborn, Harry L. Swinney
    Chin. Phys. B, 2014, 23 (11): 114703.   DOI: 10.1088/1674-1056/23/11/114703
    Abstract693)      PDF (456KB)(396)      

    We study the propulsion matrix of bacterial flagella numerically using slender body theory and the regularized Stokeslet method in a biologically relevant parameter regime. All three independent elements of the matrix are measured by computing propulsive force and torque generated by a rotating flagellum, and the drag force on a translating flagellum. Numerical results are compared with the predictions of resistive force theory, which is often used to interpret micro-organism propulsion. Neglecting hydrodynamic interactions between different parts of a flagellum in resistive force theory leads to both qualitative and quantitative discrepancies between the theoretical prediction of resistive force theory and the numerical results. We improve the original theory by empirically incorporating the effects of hydrodynamic interactions and propose new expressions for propulsive matrix elements that are accurate over the parameter regime explored.

    Mechanical properties of jammed packings of frictionless spheres under an applied shear stress
    Liu Hao (刘浩), Tong Hua (童华), Xu Ning (徐宁)
    Chin. Phys. B, 2014, 23 (11): 116105.   DOI: 10.1088/1674-1056/23/11/116105
    Abstract1197)      PDF (416KB)(432)      
    By minimizing a thermodynamic-like potential, we unbiasedly sample the potential energy landscape of soft and frictionless spheres under a constant shear stress. We obtain zero-temperature jammed states under desired shear stresses and investigate their mechanical properties as a function of the shear stress. As a comparison, we also obtain the jammed states from the quasistatic-shear sampling in which the shear stress is not well-controlled. Although the yield stresses determined by both samplings show the same power-law scaling with the compression from the jamming transition point J at zero temperature and shear stress, for finite size systems the quasistatic-shear sampling leads to a lower yield stress and a higher critical volume fraction at point J. The shear modulus of the jammed solids decreases with increasing shear stress. However, the shear modulus does not decay to zero at yielding. This discontinuous change of the shear modulus implies the discontinuous nature of the unjamming transition under nonzero shear stress, which is further verified by the observation of a discontinuous jump in the pressure from the jammed solids to the shear flows. The pressure jump decreases upon decompression and approaches zero at the critical-like point J, in analogy with the well-known phase transitions under an external field. The analysis of the force networks in the jammed solids reveals that the force distribution is more sensitive to the increase of the shear stress near point J. The force network anisotropy increases with increasing shear stress. The weak particle contacts near the average force and under large shear stresses it exhibit an asymmetric angle distribution.
    Measurement of the friction coefficient of a fluctuating contact line using an AFM-based dual-mode mechanical resonator
    Guo Shuo (郭硕), Xiong Xiao-Min (熊小敏), Xu Zu-Li (徐祖力), Shen Ping (沈平), Tong Penger (童彭尔)
    Chin. Phys. B, 2014, 23 (11): 116802.   DOI: 10.1088/1674-1056/23/11/116802
    Abstract659)      PDF (622KB)(369)      
    A dual-mode mechanical resonator using an atomic force microscope (AFM) as a force sensor is developed. The resonator consists of a long vertical glass fiber with one end glued onto a rectangular cantilever beam and the other end immersed through a liquid-air interface. By measuring the resonant spectrum of the modified AFM cantilever, one is able to accurately determine the longitudinal friction coefficient ζv along the fiber axis associated with the vertical oscillation of the hanging fiber and the traversal friction coefficient ζh perpendicular to the fiber axis associated with the horizontal swing of the fiber around its joint with the cantilever. The technique is tested by measurement of the friction coefficient of a fluctuating (and slipping) contact line between the glass fiber and the liquid interface. The experiment verifies the theory and demonstrates its applications. The dual-mode mechanical resonator provides a powerful tool for the study of the contact line dynamics and the rheological property of anisotropic fluids.
    Near equilibrium dynamics and one-dimensional spatial-temporal structures of polar active liquid crystals
    Yang Xiao-Gang (杨小刚), M. Gregory Forest, Wang Qi (王奇)
    Chin. Phys. B, 2014, 23 (11): 118701.   DOI: 10.1088/1674-1056/23/11/118701
    Abstract552)      PDF (2136KB)(506)      

    We systematically explore near equilibrium, flow-driven, and flow-activity coupled dynamics of polar active liquid crystals using a continuum model. Firstly, we re-derive the hydrodynamic model to ensure the thermodynamic laws are obeyed and elastic stresses and forces are consistently accounted. We then carry out a linear stability analysis about constant steady states to study near equilibrium dynamics around the steady states, revealing long-wave instability inherent in this model system and how active parameters in the model affect the instability. We then study model predictions for onedimensional (1D) spatial-temporal structures of active liquid crystals in a channel subject to physical boundary conditions. We discuss the model prediction in two selected regimes, one is the viscous stress dominated regime, also known as the flow-driven regime, while the other is the full regime, in which all active mechanisms are included. In the viscous stress dominated regime, the polarity vector is driven by the prescribed flow field. Dynamics depend sensitively on the physical boundary condition and the type of the driven flow field. Bulk-dominated temporal periodic states and spatially homogeneous states are possible under weak anchoring conditions while spatially inhomogeneous states exist under strong anchoring conditions. In the full model, flow-orientation interaction generates a host of planar as well as out-of-plane spatial-temporal structures related to the spontaneous flows due to the molecular self-propelled motion. These results provide contact with the recent literature on active nematic suspensions. In addition, symmetry breaking patterns emerge as the additional active viscous stress due to the polarity vector is included in the force balance. The inertia effect is found to limit the long-time survival of spatial structures to those with small wave numbers, i.e., an asymptotic coarsening to long wave structures. A rich set of mechanisms for generating and limiting the flow structures as well as the spatial-temporal structures predicted by the model are displayed.

    IN SITU STM STUDY OF THERMAL ANNEALING OF Au THIN FILMS:AN INVESTIGATION ON DECAY OF NANOMETER Au CLUSTERS AND 2D ISLANDS
    LI NAN (李楠), D.ALLAN, LIU GANG-YU (刘刚玉)
    Acta Physica Sinica (Overseas Edition), 1997, 6 (7): 531-549.   DOI: 10.1088/1004-423X/6/7/009
    Abstract902)      PDF (939KB)(462)      
    An in situ, ultrahigh vacuum scanning tunneling micrmcopy(UHV STM) study of thermal annealing of gold thin films is presented in this paper. The gold thin films was heated and annealed in the UHV chamber in the temperature range From room temperature to maximum of 300℃, while a consecutive STM imaging was performed on the surface of the gold films during the heating and annealing. The STM results showed that the surface corrugation changes became more apparent after the temperature increased above 100℃, whereas much smoothened surface and large Au (111) crystalline terrace(>200nm) formed at temperature of 160℃ of above. Betides the surface morphology change, our images clearly revealed the melting of multilayer gold clusters and shrinking of monolayer gold islands in a nanometer scale. It was shown that the decay of the gold clusters and islands constitute the microscopic processes contributing to the thermal activated surface morphology change. A classical theory of mass flow kinetics was adopted in analyzing the decay processes. The results showed that surface diffusion is the dominate mechanism in the thermal annealing of the gold thin films. This study presents the first microscopic investigation of thermal annealing processes of metal thin films observed by in situ and real-time STM.
ISSN 1674-1056   CN 11-5639/O4

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