Computational study of radiation torque on arbitrary shaped particles with MLFMA.

The surface integral equation (SIE) method is used for the computational study of radiation torque on arbitrarily shaped homogeneous particles. The Multilevel Fast Multipole Algorithm (MLFMA) is employed to reduce memory requirements and improve the capability of SIE. The resultant matrix equations are solved iteratively to obtain equivalent electric and magnetic currents. Then, radiation torque is computed using the vector flux of the pseudotensor over a spherical surface tightly enclosing the particle. We use, therefore, the analytical electromagnetic field expression for incident waves in the near region, instead of the far-field approximation. This avoids the error which may be caused when describing the incident beam. The numerical results of three kinds of non-spherical particles are presented to illustrate the validity and capability of the developed method. It is shown that our method can be applied to predict, in the rigorous sense, the torque from a beam of any shape on a particle of complex configuration with a size parameter as large as 650. The radiation torques on large ellipsoids are exemplified to show the performance of the method and to study the influence that different aspect ratios have on the results. Then, the code is used for the calculation of radiation torque on objects of complex shape including a biconcave cell-like particle and a motor with a non-smooth surface.

Electrochemical detection of single microbeads manipulated by optical tweezers in the vicinity of ultramicroelectrodes.

Latex micrometric beads are manipulated by optical tweezers in the vicinity of an ultramicroelectrode (UME). They are optically trapped in solution and approached the electrode surface. After the electrochemical measurement, they are optically removed from the surface. The residence time of the particle on the electrode is thus controlled by the optical tweezers. The detection is based on diffusional hindrance by the insulating objects which alters the fluxes of the redox Ru(NH3)6(3+) species toward the UME and thus its mass-transfer limited current. We have optically deposited successively 1, 2, and 3 beads of 3-µm radius on the UME surface, and we have recorded the variations of the current depending on their landing locations that were optically controlled. Finally we decreased the current by partially blocking the electroactive surface with a six-bead assembly. The variation of the steady-state current and the approach curves allow for the indirect electrochemical localization of the bead in the vicinity of the UME, not only when the bead is in contact but also when it is levitated at distances lower than the UME radius. These experiments show that single particles or more complex structures may be manipulated in situ in a contactless mode near the UME surface. From comparison with simulations, the electrochemical detection affords an indirect localization of the object in the UME environment. The developed approach offers a potential application for interrogating the electrochemical activity of single cells and nanoparticles.

Optical dynamometry to study phase transitions in lipid membranes.

The fluidity of the lipid matrix of cell membranes is crucial for the mobility of various inclusions like proteins. When the lipid bilayer undergoes phase transition from fluid-to-gel phase, the shear surface viscosity of the membrane diverges, thus hindering the motion of the membrane inclusions. On the other hand, the membrane bending stiffness drops down, and below the main phase transition, drastically increases with lowering the temperature. A tool to study the membrane properties when the lipid bilayer crosses the phase transition is provided by optical trapping and manipulation of microspheres attached to the membrane. Giant unilamellar vesicles are used, which allow for direct visualization of the membrane response, as model membranes. Following the motion of one or two particles attached to a vesicle, the microscope can provide evidence for the membrane elasticity and state of fluidity. As forces acting on the spheres, one can use gravity, thermal noise, or radiation pressure force.

Electrically induced interactions between colloidal particles in the vicinity of a conducting plane.

We address the problem of two-dimensional (2D) colloidal aggregation driven by an ac electrical field, by observing an aqueous dispersion of latex microspheres in contact with a conducting surface. Using micron-sized carboxylated polystyrene particles, we have systematically investigated the aggregation process, as a function of particle size and charge, and of the applied electric field amplitude and frequency. A low-density 2D phase is observed at high frequency (typically above 1 kHz), while at low frequency (below a "contact frequency" nu(c)) the collection of particles collapses into disconnected compact aggregates of crystalline (hexagonal) structure. We argue that this scenario is governed by the competition between an attractive force, of electrohydrodynamic nature, and a repulsive force, basically an electrical dipole-dipole interaction. Both contributions are revealed and analyzed in independent experiments on isolated particle pairs, using optical manipulation and dynamometry.

Size segregation in a fluid-like or gel-like suspension settling under gravity or in a centrifuge.

We investigate size segregation effects in a bidisperse concentrated suspension when slowly settling under gravity or when submitted to a centrifugal field. Experiments are carried out with PMMA spheres of two different mean diameters (190 and 25 microm) suspended in a hydrophobic index-matched fluid. Spatial repartitions of both small and large spheres and velocity fluctuations of particles are measured using fluorescently labeled PMMA spheres and a particle-image-velocimetry method. Large particles behave as hard spheres in purely hydrodynamic interactions, while small spheres interact through weakly attractive forces. For a small amount of small spheres among large ones, the suspension remains fluid during settling and the organization of the velocity field of particles into finite-sized structures also called "blobs" promotes size segregation. A larger proportion of weakly attractive small spheres in the bidisperse suspension causes a considerable slowdown of the settling process under gravity and the occurrence of a large-scale collective behavior together with a loss of size segregation. When centrifuging the gel-like bidisperse suspension, a shear-induced melting of the particle network induces a spectacular segregation of species. As a consequence, aging tests of soft yielding materials using centrifugation methods are not representative of the shelf-life stability of the products. A tentative model based on the competition between viscous stresses acting upon particles and adhesive stresses gives a correct estimate of the critical stationary acceleration for the destabilization of the particle network and the onset of size segregation in a gel-like suspension.

Drag of a Solid Particle Trapped in a Thin Film or at an Interface: Influence of Surface Viscosity and Elasticity.

We propose a theoretical model for the motion of a spherical particle entrapped in a thin liquid film or in a monolayer of insoluble surfactant at the air/water interface. Both surface shear and dilational viscosity, surface diffusion, and elasticity of the film are taken into consideration. The drag force acting on the particle is analytically calculated and asymptotic expressions of the problem are provided. The relevance of the model is discussed by comparing the calculated "viscoelastic" drag, gamma(vel), to the one predicted by Saffman's theory, gamma(S), for cylindrical inclusions in membranes. Numerical analyses are performed to evaluate the contributions of the surface viscosity and the diffusion coefficient of the layer on the hydrodynamical resistance experienced by the particle. Copyright 2000 Academic Press.