Velocity, particle measurement methods

The 2eta potential (Fig. 8) is essentially the potential that can be measured at the surface of shear that forms if the sohd was to be moved relative to the surrounding ionic medium. Techniques for the measurement of the 2eta potentials of particles of various si2es are collectively known as electrokinetic potential measurement methods and include microelectrophoresis, streaming potential, sedimentation potential, and electro osmosis (19). A numerical value for 2eta potential from microelectrophoresis can be obtained to a first approximation from equation 2, where Tf = viscosity of the liquid, e = dielectric constant of the medium within the electrical double layer, = electrophoretic velocity, and E = electric field. 

The stability of most colloidal solutions depends critically on the magnitude of the electrostatic potential ( /o) at the surface of the colloidal particle. One of the most important tasks in colloid science is therefore to obtain an estimate of /o under a wide range of electrolyte conditions. In practice, the most convenient method of obtaining /q uses the fact that a charged particle will move at some constant, limiting velocity under the influence of an applied electric field. Even quite small particles (i.e. <1 xm) can be observed using a dark-field illumination microscope and their (average) velocity directly measured. The technique is called microelectrophoresis . 

At present there are three basic techniques of measuring particle velocity, namely free surface velocity (FSV) measurements, electromagnetic velocity (EMV) measurements, and flash X-ray measurements. Before proceeding to describe these techniques, it should be noted that agreement among these techniques is not satisfactory. At present there is no consensus as to which technique is best , nor is there any real understanding as to why these different methods give different results... 

Motion analysis and particle tracking methods enable users to follow the movement over time of tagged particles, such as fluorescently labeled cell surface molecules, microtubules, nucleic acids, lipids, and other objects with subpixel resolution.287 These methods allow scientists to measure jc and y coordinates, velocity, mean displacement, mean vector length, and more. 

All that is normally known about a particle is its silhouette, projection, or profile or, in those cases where size is derived from other physical effects, such as, for example, the settling velocity, a dimension related to voliune, mass, or surface texture. Therefore, methods must be found that interpret information from cuts through the particle, scans of portions of the surface area, or information from particle behavior in, for example, fluids and connect it with overall shape. Unless the measured outline of the particle misses a unique, dominant feature of the particle shape, the result will be representative of the particle. The methods are stiU very complicated and require a large number of discrete items of information to describe a particle signature reasonably well. 

Optical particle-based methods offer an elegant way to reliably measure the near wall gradients and thus the wall shear stress for microfluidic applications. Their great benefit is that the flow is not altered by the technique and they can be applied in channels with randomly structured walls. It was also shown that the velocity and particle position information can actually be used to reconstruct the surface in the case of 2D... 

The most popular method of measuring electrophoretic mobility is microelectrophoresis. In microelectrophoresis, particles are placed in a closed capillary with electrodes at either end. When an electric field is applied, the particles migrate towards the electrode and their velocities are measured. Because the capillary walls are charged, the applied electric field will also induce an electroosmotic flow. However, since the capillary is closed, a back pressure creates a net zero flow in the mbe (see Fig. 3). The particle velocity is a combination of the electrophoretic motion and the fluid flow. To obtain the true electrophoretic mobility, the particles must be tracked along the stationary layer where the fluid velocity is zero. For a circular capillary the liquid velocity across the tube is given by the electroosmotic flow and the back pressure flow ... 

Electrophoresis is concerned with the migration of particles under the influence of an external field. First, electrophoretic velocities were measured by the same experimental methods as those used by Hittorf (around 1853) for transport numbers. The method was remarkably developed and refined by Tiselius (1937, Nobel Prize in 1947). Since then, there have been a large number of applications in biology and medicine, in particular in the analysis of sera and the separation of amino acids and proteins. Recently, the technique was improved by coupling capillary electrophoresis to an electrochemical detector (CEEC) that can be implanted in vivo for studying microdialysis [4]. 

Kamiwano M, Saito F. Measurement method of flow velocity of liquid and irregular solid particles using an image sensor with an image fiber. AIChE S5mip Ser 80(241) 122-128, 1984. 

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