Protoplasmic streaming in plant cells is directly visible in the cases of Chara corallina and Nitella flexilis, and this streaming is understood to play a role in the transport of biological materials. For this reason, related studies have focused on molecular transportation from a fluid mechanics viewpoint. However, the experimentally observed distribution of the velocity along the flow direction x, which exhibits two peaks at Vx = 0 and at a finite Vx(≠0), remains to be studied. In this paper, we numerically study whether this behavior of the flow field can be simulated by a 2D stochastic Navier-Stokes (NS) equation for Couette flow in which a random Brownian force is assumed. We present the first numerical evidence that these peaks are reproduced by the stochastic NS equation, which implies that the Brownian motion of the fluid particles plays an essential role in the emergence of these peaks in the velocity distribution. We also find that the position of the peak at Vx(≠0) moves with the variation in the strength D of the random Brownian force, which also changes depending on physical parameters such as the kinematic viscosity, boundary velocity, and diameter of the plant cells.

-We report experimental observation of the shear thickening oscillation, i.e. the spontaneous macroscopic oscillation in the shear flow of severe shear thickening fluid. Using density-matched starch-water mixture, in the cylindrical shear flow of a few centimeters flow width, we observed that well-marked vibrations of the frequency around 20 Hz appear via a Hopf bifurcation upon increasing externally applied shear stress. The parameter range and the frequency of the vibration are consistent with those expected by a simple phenomenological model of the dilatant fluid.Introduction. -Dense colloids and dense granulefluid mixtures are often called dilatant fluids and known to show severe shear thickening, i.e. its viscosity changes discontinuously by orders of magnitude upon increasing the shear rate [1][2][3][4]. This is a source of a variety of unintuitive behaviors of the media [5][6][7][8], and might be used for interesting application like a body armor [9], but often causes problems in industrial situations, such as the damage of mixer motors due to overloading [1].Although the severe shear thickening is a property that can be demonstrated easily with common material like starch and water, physicists have not reached a general agreement on its microscopic mechanism. Originally, the order-disorder transition of the dispersed particles was proposed [1,[10][11][12], but later the hydrodynamics cluster formation by the lubrication force is considered to be more consistent with experimental observations and numerical simulations [13][14][15][16]. More recently, the shear thickening transition is discussed in connection with the jamming and/or the compaction of granules [17][18][19][20].Recently, the present authors constructed a phenomenological model that describes macroscopic flowing properties of a shear thickening fluid [21,22]. By analyzing the model, they pointed out that the discontinuous thickening upon increasing the shear rate signifies that the shear thickening fluid is actually shear stress thickening, not shear rate thickening because only the shear stress thickening can produce the S-shaped flow curve with an unstable

In the present study, we investigate the migration of DNA molecules through a microchannel using a series of electric traps controlled by an ac electric field. We describe the motion of DNA based on Brownian dynamics simulations of a bead-spring chain. The DNA chain captured by an electric field escapes due to thermal fluctuation. The mobility of the DNA chain was determined to depend on the chain length, the mobility of which sharply increases when the length of the chain exceeds a critical value that is strongly affected by the amplitude of the applied ac field. Thus we can optimize the separation selectivity of the channel for DNA molecules that is to be separated, without changing the structure of the channel. In addition, we present a phenomenological description for the relationship between the critical chain length and the strength of binding electric field.