Direct Measurement of Unsteady Microscale Stokes Flow Using Optically Driven Microspheres

A growing body of work on the dynamics of eukaryotic flagella has noted that their oscillation frequencies are sufficiently high that the viscous penetration depth of unsteady Stokes flow is comparable to the scales over which flagella synchronize. Incorporating these effects into theories of synchronization requires an understanding of the global unsteady flows around oscillating bodies. Yet, there has been no precise experimental test on the microscale of the most basic aspects of such unsteady Stokes flow: the orbits of passive tracers and the position-dependent phase lag between the oscillating response of the fluid at a distant point and that of the driving particle. Here, we report the first such direct Lagrangian measurement of this unsteady flow. The method uses an array of 30 submicron tracer particles positioned by a time-shared optical trap at a range of distances and angular positions with respect to a larger, central particle, which is then driven by an oscillating optical trap at frequencies up to 400 Hz. In this microscale regime, the tracer dynamics is considerably simplified by the smallness of both inertial effects on particle motion and finite-frequency corrections to the Stokes drag law. The tracers are found to display elliptical Lissajous figures whose orientation and geometry are in agreement with a low-frequency expansion of the underlying dynamics, and the experimental phase shift between motion parallel and orthogonal to the oscillation axis exhibits a predicted scaling form in distance and angle. Possible implications of these results for synchronization dynamics are discussed.

Realizing laminar-like flow in charged bunches with density evolution equations

In the ultra-fast electron microscopy community, electron bunches with much smaller longitudinal widths than transverse widths are emitted from the cathode surface. The community has believed that these bunches evolve to a uniform ellipsoid, but recent simulations by our group and others suggest that if the bunch has an initially transverse Gaussian profile, a ring-like density “shock” emerges at the median of the bunch during evolution. To explain these results, we generalized Reed’s 1D fluid model of charged bunch expansion to cylindrical and spherical geometries demonstrating such a shock emerges analytically under these symmetric geometries. Mathematically, the shock in these models occurs when particles more toward the middle “catch-up” to outer particles, and eventually the trajectory of the more central particle crosses-over the outer particle’s trajectory. This cross-over marks the transition from the laminar to nonlaminar regime. However, this theory has been developed for cold-bunches, i.e. bunches of electrons with zero initial momentum. Here, we briefly review this new theory and extend it to the cylindrically- and spherically-symmetric cases that have nonzero initial momentum. This formulation elucidates how charge-dominated bunches may be manipulated to maintain laminar conditions even through focussing of the bunch.

Computer Simulation of Three Particles Sedimentation in a Narrow Channel

The settling of three particles in a narrow channel is simulated via the lattice Boltzmann direct-forcing/fictitious domain (LB-DF/FD) method for the Reynolds number ranging from 5 to 200. The effects of the wall and the Reynolds number are studied. It is interesting to find that at certain Reynolds numbers the left (right) particle is settling at 0.175 (0.825) of the channel width irrespective of its initial position or the channel width. Moreover, numerical results have shown that the lateral particles lead at small Reynolds numbers, while the central particle leads at large Reynolds numbers due to the combined effects of particle-particle and particle-wall interactions. The central particle will leave the lateral ones behind when the Reynolds number is large enough. Finally the effect of the Reynolds number on the trajectory of the lateral particles is presented.

Measurement and modelling of reactive transport in geological barriers for nuclear waste containment

Unit cell illustrating potential diffusion paths (bonds, yellow and red) in the neighbourhood of central particle (green); these join neighbouring cell faces and show where elongated pores may be assigned to the experimental pore system information.

Proteasomes and Proteasomal Gene Polymorphism in Association with Inflammation and Various Diseases

A proteasome, a multicatalytic protein complex, is a central particle of the ubiquitinproteasome proteolytic pathway in all eukaryotic cells. Through the degradation of most intracellular proteins, proteasomes play a significant role in cell processes, such as cell cycle and division, posttranslational protein quality control, cell signaling, and apoptosis. Therefore, the ubiquitinproteasome system is necessary to ensure the normal functioning of cells and an organism. The associations between alterations in the ubiquitin-proteasome pathway and the development of various autoimmune, neurodegenerative, inflammatory and other diseases in humans have been established. Moreover, the findings of some studies suggest that proteasomes may participate in the pathogenesis of asthma through the regulation of the nuclear factor kappa B signaling pathway. Recently, much attention has been given to the associations between genes encoding the proteasome and their polymorphism, and various diseases. Associations between some proteasomal genes and myocardial infarction, type 2 diabetes mellitus, and other diseases have already been established. However, the results are inconclusive or conflicting and need further clarification.