Motion Of Microparticles Research Articles

The Structure and Dynamics of Microparticles at Pickering Emulsion Interfaces

We have employed a laser scanning confocal microscope (LSCM) to study the structure and dynamics of microparticles at Pickering emulsion interfaces. The microparticles can have rich morphology at the emulsion interfaces, ranging from an aggregated structure to colloidal lattices, with a possibility of involving heterogeneous particles. With a specific interest in colloidal lattices, we find that although the enhanced electrostatic repulsion explains the formation of colloidal lattices by sulfate-treated polystyrene (S-PS) particles, it fails to interpret the unsuccessfulness of assembling lattices containing single-species carboxylate-treated polystyrene (C-PS) particles. A small percentage of C-PS particles in the colloidal mixture does not disturb the formation of lattices made of S-PS particles. The LSCM also provides a meaningful way to probe dynamic information. The diffusion of single particles at the emulsion interfaces depends strongly on the oil phase viscosity, particle size, and particle wettability. A highly curved emulsion interface slows the motion of microparticles at oil-water interfaces but the interface curvature effect decreases with increasing oil phase viscosity. Although the confocal microscope was originally used as an imaging tool, we find that the thermodynamic equilibrium of colloidal lattices can be disturbed and even destroyed when increasing the output laser intensity.

Differences in magnetically induced motion of diamagnetic, paramagnetic, and superparamagnetic microparticles detected by cell tracking velocimetry

Magnetic separation in biomedical applications is based on differential magnetophoretic mobility (MM) of microparticulate matter in viscous media. Typically, the difference in MM is obtained by selectively labeling the target cells with superparamagnetic iron oxide nanoparticles (SPIONs). We have measured the MM of monodisperse, polystyrene microspheres (PSMs), with and without attached SPIONs as a model of cell motion induced by nanoparticle magnetization, using variable H field and cell tracking velocimetry (CTV). As a model of paramagnetic microparticle motion, the MM measurements were performed on the same PSMs in paramagnetic gadolinium solutions, and on spores of a prokaryotic organism, Bacillus globigii (shown to contain paramagnetic manganese). The CTV analysis was sensitive to the type of the microparticle magnetization, producing a value of MM independent of the applied H field for the paramagnetic species, and a decreasing MM value with an increasing field for superparamagnetic species, as predicted from theory. The SPION-labeled PSMs exhibited a saturation magnetization above H approximately = 64,000 A m(-1) (or 0.08 tesla). Based on those data, the average saturation magnetizations of the SPIONs was calculated and shown to vary between different commercial sources. The results demonstrate sensitivity of the CTV analysis to different magnetization mechanisms of the microparticles.

Dynamics in Ultrathin Films: Particle Tracking Microrheology of Langmuir Monolayers

Particle tracking has been shown to be a powerful technique for measuring bulk and interfacial rheology of flu- ids. The Brownian motion of microparticles trapped at interfaces is very sensitive to the viscosity of the subphase, and to the contact angle of the particles. The Stokes-Einstein relation is fulfilled if the friction factor is properly taken into ac- count. The diffusion coefficient of the latex microparticles spread on surfactant monolayers allows one to calculate the shear viscosity of the monolayer using Danov's theory. Good agreement was found with previous results for monolayers of pentadecanoic acid. The method has also been used to study monolayers of n-dodecanol. Moreover, the shear viscosity of a polymer monolayer has been calculated by particle tracking, and the results show good agreement with data obtained by canal viscosimetry. The temperature dependence of the shear viscosity shows the existence of a glass transition for monolayers of poly(4-hydroxystyrene).

On correctness of canonical formulation of the problem of motion of submicron particles in the Earth’s plasmasphere

The general conditions of applicability are formulated for the canonical formulation of the problem of motion of micro-particles with variable electric charge in the near-Earth space. The validity of these condi- tions is demonstrated for particles of sub-micron dimension executing orbital motion in the Earth's plasmas- phere.

Constrained Drying of an Aqueous Yttria-Stabilized Zirconia Slurry on a Substrate II: Binary Particle Slurry

Binary-sized particle slurries (binary slurries) prepared by mixing micro-particles and electrostatically stabilized nano-particle slurries (nano-slurries) were cast onto metal substrates to form wet coatings and the drying process was studied. The highly charged nano-particles were found to deposit on the surface of the micro-particles, resulting in a decrease in the viscosity of the binary slurry. The drying kinetics were classified as an initial constant rate period (CRP), followed by a falling rate period. In the early stage of CRP, the movement of micro-particles induced coating shrinkage and tensile stresses developed due to the constraint of the substrate. Thereafter, the nano-slurry continued to dry and shrink around the static micro-particles without further coating shrinkage, but with an increase in the stress. The addition of micro-particles reduced the level of the maximum stress and prevented crack formation during drying. The microstructure of green coatings shows the presence of nano-particles between micro-particles that cement the micro-particles together.

Orbital motion of spherical microparticles trapped in diffraction patterns of circularly polarized light

We investigate arrays and rotation of spherical microparticles trapped by focusing a circularly polarized Gaussian beam just above the top coverslip-water interface. Particles are trapped in various patterns due to a Fresnel diffraction, such as rings and close-packed structures. We find that rings of particles orbit around the beam axis, whereas close-packed arrays spin in the opposite sense on the beam axis. The sense of the orbiting, as well as that of the spinning, is determined by the input circular polarization handedness. The results are interpreted on the assumption that the spin angular momentum carried by a circularly polarized beam is converted into the orbital angular momentum in the optical process by which the focused Gaussian beam's shape is transformed due to diffraction.

Negative propagation of vector Bessel beams

Energy characteristics of the superposition of TE- and TM-polarized electromagnetic Bessel beams are studied. For some phase differences between TE and TM waves the components of the Poynting vector vary in sign. We call this situation "negative propagation," because locally the beam may behave like a wave propagating in the direction opposite to the conventional one. We predict the following phenomena, which should confirm negative beam propagation: reflection of the beam from a circular aperture and unusual movement of microparticles in the beam.

Knee loading dynamically alters intramedullary pressure in mouse femora

Dynamic mechanical loads have been known to stimulate bone formation. Many biophysical factors such as number of daily loading cycles, bone strain, strain-induced interstitial fluid flow, molecular transport, and modulation of intramedullary pressure have been considered as potential mediators in mechanotransduction of bone. Using a knee loading modality that enhances anabolic responses in mouse hindlimb, we addressed a question: Do oscillatory loads applied to the knee induce dynamic alteration of intramedullary pressure in the femoral medullary cavity? To answer this question, mechanical loads were applied to the knee with a custom-made piezoelectric loader and intramedullary pressure in the femoral medullary cavity was measured with a fiber optic pressure sensor. We observed that in response to sinusoidal forces of 0.5 Hz and 10 Hz, pressure amplitude increased up to 4-N loads and reached a plateau at 130 Pa. This amplitude significantly decreased with a loading frequency above 20 Hz. To confirm alteration of intramedullary pressure, real-time motion of microparticles in a glass tube inserted to the femoral medullary cavity ex vivo was visualized. Taken together, these data reveal that knee loading dynamically alters intramedullary pressure as a function of loading intensities and frequencies.

Plasma dragged microparticles as a method to measure plasma flows

The physics of microparticle motion in flowing plasmas is studied in detail for plasmas with electron and ion densities ne,i∼1019m−3, electron and ion temperatures of no more than 15eV, and plasma flows on the order of the ion thermal speed, vf∼vti. The equations of motion due to Coulomb interactions and direct impact with ions and electrons, of charge variation, as well as of heat exchange with the plasma, are solved numerically for isolated particles (or dust grains) of micron sizes. It is predicted that microparticles can survive in plasma long enough, and can be dragged in the direction of the local ion flow. Based on the theoretical analysis, we describe a new plasma flow measurement technique called microparticle tracer velocimetry (mPTV), which tracks microparticle motion in a plasma with a high-speed camera. The mPTV can reveal the directions of the plasma flow vectors at multiple locations simultaneously and at submillimeter scales, which is hard to achieve by most other techniques. Thus, mPTV can be used to study plasma flows produced in the laboratory.

Finite element modeling of a microparticle manipulator

The contactless movement of microparticles and cells to known locations within a fluid volume is of interest in the fields of microtechnology and life sciences. A device which can position such inhomogeneities suspended in a fluid at multiple locations is described and modeled. The device consists of a thin fluid layer contained in a channel etched into a silicon wafer. Waves are excited by a macro-piezoelectric plate with electrodes on the top and bottom surfaces and, as a result, waves propagate into the adjacent fluid. The result is a pressure field throughout the fluidic volume. When an inhomogeneity in a fluid is exposed to an ultrasonic field the acoustic radiation force results; this is found by integrating the pressure over the surface of the particle, retaining second order terms, and taking the time average. Thus, due to the presence of a pressure field in the fluid in which the particles are suspended, a force field is created. The particles are then collected at the locations of the force potential minima. In the device described here, the force field is used to position particles into lines. The locations of the particles are predicted by using a finite element model of the system. The experimental and modeling results, presented here, are in good agreement.

Microelectrode arrays for two-dimensional polar movement of microparticles in water

This article demonstrates the circular movement of microparticles in water using radial microelectromechanical system electrode arrays. The particles are moved in two polar dimensions, θ and r, by ac electro-osmosis. It is observed that initially particles circle rapidly above the array and move slowly radially to finally form a thin ring at an equilibrium radius. Three-dimensional computational fluid dynamics simulations are performed, cross sectional information is extracted to obtain tangential (Uθ) velocity as a function of r. Orbital velocity and equilibrium radius measurements are performed at various voltages. Velocities and radii are found to be in general agreement with the simulated results.

Motion of Charged Micro-particle Immersed in Liquid Crystal Controlled by In-plane Field for Electro Paper Display

We have studied the motion of charged micro-particles that are immersed in a nematic liquid crystal (LC) and controlled by in-plane field. The LC is an anisotropic liquid such that the viscosity of the LC depends on flow direction, phase of the LC, and temperature, which affects the motion of the charged particles under the influence of electric field. This study shows that the motion of charged particles mainly depends on the applied voltage and the LC phase, but does not show any significant influence from the initial alignment of LC, although one may expect directional difference in drag force due to interaction between LC and particle. The viscosity changes due to temperature variations in nematic phase also show no signification influence on particle velocity when compared to the effect from varying in-plane field strength.

The influence of the magnetic field on the electrical magnetoresistance of magnetorheological suspensions

The magnetoresistance R of magnetorheological suspensions (MRS) is established by solving the equation of motion of iron micro-particles under the influence of the external magnetic field. We found that R depends on the mean diameter of the micro-particle, on the volume concentration of the solid phase as well as on the rheological properties of MRS. The mechanisms by virtue of which the magnetic field leads to the onset of conduction processes are discussed.

A System for Real-Time Analysis of Capillary Blood Microcirculation Based on the Low-Frequency Processing of Scattered-Laser-Beam Speckle Structures

A method based on laser speckle interferometry with a semiconductor laser and using spectral–temporal algorithms of low-frequency (0–35 kHz) data processing to analyze the dynamic parameters of blood microcirculation is considered. The described method for measuring the velocity of motion of blood microparticles is simple to implement and allows the noninvasive monitoring of blood circulation dynamics with a fairly high sensitivity. The test results demonstrate that the design of the device presented ensures the possibility of developing an analyzer meeting the technical requirements for this type of device.

Microparticle detachment from surfaces exposed to turbulent air flow: microparticle motion after detachment

The motion of heavy microspheres after their detachment from flat surfaces is considered. The microspheres initially are in static contact equilibrium, embedded fully in the viscous sublayer and subjected to a slowly accelerating fully developed turbulent flow. The equations of motion of the microspheres are presented, including surface roughness effects. The equations are solved numerically for detachment with and without consideration of a burst–sweep event initiating detachment. The microsphere velocity along the surface after detachment is measured using a strobed laser-light sheet and compared to the numerical solutions. Results indicate that the microspheres undergo pure rolling along the surface before possible entrainment and that the sweep plays a role in the detachment process. A model for predicting the microsphere velocity along the surface after detachment is presented.

Observation of multiple scattering of laser radiation from a light-induced jet of microparticles in suspension

Variation in the correlation function of light multiply scattered by a random medium was observed with increasing the incident beam power. The light-induced motion of microparticles in suspension, caused by a high-power laser radiation, serves as an additional factor in the decorrelation of the scattered light. The experimental data are in good agreement with the results of theoretical analysis.

Direct observation of microparticle gyromotion in a magnetized direct current glow discharge dusty plasma

Laboratory observations of oscillatory motion of charged microparticles have been made in an argon dc glow discharge plasma created within a strong dc magnetic field. Measurements of the oscillation frequency and amplitude are consistent with the gyromotion of magnetized dust grains under the ambient plasma conditions. The measurements provide an effective method for the noninvasive determination of the charge on the observed microparticles. The observations also seem to indicate that the neutral drag force on the dust grains may be smaller than anticipated from the classical estimation.

Direct manipulation and observation of the rotational motion of single optically trapped microparticles and biological cells in microvortices.

This paper describes a method for manipulating and monitoring the rotational motion of single, optically trapped microparticles and living cells in a microvortex. To induce rotation, we placed the microparticle at the center of rotation of the vortex and used the recirculating fluid flow to drive rotation. We have monitored the rotation of single beads (which ranged in diameter from a few micrometers to tens of micrometers) and living cells in a microvortex. To follow the rotation of a smooth and symmetrically shaped bead, we first ablated a small region ( approximately 1 microm) on the bead. An Ar(+) laser was then tightly focused ( approximately 0.5-microm spot size) onto the bead, and rotation was tracked by recording changes in the level of backscattered laser light as the ablated region repeatedly transited the laser focus. Using this method, we have followed bead rotation that varied in frequency from 0.15 to 100 Hz and have studied the effect of bead diameter on the rate of rotation at a given fluid flow rate. To monitor the rotation of single living cells, we selectively stained portions of B-lymphocytes with the fluorescent dye DiOC(6). We observed rotation by following changes in the fluorescence signal as the dye-stained region transited the laser focal volume. This technique provides a simple and sensitive method for controlling and monitoring the rotational motion of microparticles in a microfluidic environment.

Motion of micro-particles in channel flow

Abstract The motion of micro-particles in channel flow was investigated using the DNS method for Re=3300 (based on center velocity and half channel width). The calculations used the fractional projection method to directly solve the Navier–Stokes equations, and the explicit third-order Runge–Kutta method for the time integration. A higher order finite difference scheme was used to discretize the fluid dynamics equations. The numerical results provided the two-phase flow field in the near-wall region with micro-particles. The simulation results showed that particles with Stokes number on the order of 0.1 tend to congregate near the wall while the micro-particles 1 μm in diameter distribute homogeneously in the whole flow field. Simulations of micro-particulate matter movement using the κ–e turbulence model applied to the same problem as the DNS analysis showed that the turbulence model could not accurately predict the micro-particle motion in the turbulent flow near the wall, with higher particle concentrations close to the centerline and lower concentrations near the wall.

Modeling and optimization of colloidal micro-pumps

Manipulating micro-particles is very important in microfluidic applications, such as biomedical flows and self-assembled structures. Here, flows generated by the forced motion of colloidal micro-particles in a microchannel are investigated. The force coupling method combined with the spectral/hp element method is used to numerically simulate the dynamics of the flow, while a penalty method is used to determine the required forces on the particles. The pumping motion is investigated for two specific systems: a peristaltic micro-pump and a gear micro-pump. We verify the accuracy of the simulations and then for each system, we investigate the net flow rate as a function of pump frequency and channel dimension, and present optimization results. The results for the net flow rate are comparable to and within the range of the experimental data.