Pearl chain

Fig. 50. Diagrammatic representation of mutual dielectrophoresis 61K If the particles approach during dielectrophoresis they are attracted to each other due to their dipoles. This leads to the formation of pearl chains of cells...

M 6] [P 5] The particles are trapped at the edges of the electrodes, align with field lines and form pearl chains at a frequency of 100 kHz [48], At a frequency of 10 M Hz, particles are repelled from the edges of the electrodes towards the field gradient minima. 

Table VI summarizes observed manifestations of field-generated forces- The field effects may manifest themselves as an orientation of particles in the direction of the field or perpendicular to it or, "pearl chain" formation, i.e., the alignment of particles in the field direction may occur. This has long been considered a mysterious demonstration of microwave induced biological effects. Deformation or destruction of cells can be achieved with fields. The movement of cells in inhomogeneous electrical fields can be affected.

The solute particles are held at the accumulation wall by a DEP force which depends on the dielectric properties of the particles and the surrounding medium, the frequency and magnitude of the electrical field, and the electrode geometry. DEP-FFF is an unconventional FFF technique in that the DEP force is inherently non-uniformly distributed over the channel, not only in the plane of the electrodes/channel wall, but also across the channel above the electrodes [282]. Since solute particles themselves are a source of local field non-uniformities, mutual attraction occurs due to DEP forces between the particles which in extreme cases can lead to what is called pearl-chain formation. As a consequence, DEP-FFF can be considerably disturbed by interparticular interactions [57]. 

Hyperlayer-DEP-FFF has the advantage of making better use of the parabolic velocity profile of the fluid since flow at different heights in the channel is exploited as in steric-DEP-FFF the particles essentially stay in the layer near the channel wall. In addition, the hyperlayer mode minimizes the adhesion of the particles to the channel wall, which also suppresses the aggregation of cells into pearl-chains. 

When a DC pulse is applied to a couple of fluid-phase vesicles, which are in contact and oriented in the direction of the field, electrofusion can be observed. Vesicle orientation (and even alignment into pearl chains) can be achieved by application of an AC field to a vesicle suspension. This phenomenon is also observed with cells [164, 165] and is due to dielectric screening of the field. When the suspension is dilute, two vesicles can be brought together via the AC field and aligned. A subsequent application of a DC pulse to such a vesicle couple can lead to fusion. The necessary condition is that poration is induced in the contact area between the two vesicles. The possible steps of the electrofusion of two membranes are schematically illustrated in Figure 7.8a. In Sections 7.5.2.1 and 7.5.2.2, consideration will be given to the fusion of vesicles with different membrane composition or different composition of the enclosed solutions. 

The mixture was subjected to B = 400 mT uniform magnetic field. The imposed field orients the magnetic dipoles and if the particles are spaced closely enough, mutual particle interactions occur. Due to the attractive forces, a pearl chain structure develops as shown in Fig. 5. This phenomenon is called the magneto-rheological effect [53,55,57]. 

The experimental data were analyzed on the basis of Eq. 7. It is seen that the slopes of the straight lines (elastic moduli, G) are direction-dependent. The elastic modulus is larger if the compression force and the direction of pearl chain structure are parallel. This finding indicates a strong mechanical anisotropy. It can be concluded that the spatial distribution of the solid particles has a decisive effect on the stress-strain dependence. 

When the direction of the compressive force is parallel and perpendicular to the pearl chain structure, a deviation has been found from the ideal mechanical behavior. The nominal stress does not obey a linear dependence with the quantity D, as demonstrated in Fig. 14. This kind of mechanical behavior can be described by the Mooney-Rivlin equation with C2 < 0. 

When the deformation ratio reaches the value 0.9, the pearl chain structure starts to bend under compression, because the polymer chains in the mPVA network interact with the magnetic particles, as shown in Fig. 17. The... 

In contrast to the magnetic mPVA gels, compression of the carbonyl iron-loaded mPDMS network results in a break-point in the stress-strain curve if the deformation of the sample is parallel to the pearl chain structure. Figure 16b shows that the nominal stress increases with the compression up to a deformation ratio of 0.95 in every case. On increasing the compression above this ratio, the columnar structures of the iron particles are destroyed (Fig. 18). 

Figure 19b shows the results of mPDMS composites filled with carbonyl iron. The concentration of filler particles is 30 wt %. One can see the breaking of the pearl chain structure at X = 0.95. 

FIGURE 3. Diagram of the yield spectrum, or dielectrophoretic collection rate onto a wire-wire electrode of various cells as affected by the frequency of the applied field. The ordinate as shown is given in relative values as the average length of the pearl chains of cells gathered onto the wire after a definite period (say, 2 min) of field at a given maximum value. 

A dielectric particle disturbs the local E-field, so even in a homogeneous external field there is a local gradient. Two neighboring dielectric particles will therefore be attracted to each other. They form a dipole and wiU be oriented in the direction of the E-field. In a homogeneous field, cells will therefore tend to form pearl chains in the direction of the E-field. In a gradient field the pearl chains will protrude from the electrode surface (Schwan and Sher, 1969). 

Pearl chains of nanoparticles can be arranged and fused together to create a nanowire or oflier similar structures. However, other dielectrophoresis techniques can be utilized to assemble a variety of geometries. Dielectrophoretic traps and optical dielectrophoresis can assemble groups of like particles. Electro-orientation techniques have been used to align and connect nanowires between electrodes. Functionalized particles can be implemented with dielectrophoresis Lab-on-a-Chip systems to create biological sensory or assembly systems. Dielectrophoresis, therefore, is a veiy versatile engineering tool. 

When an AC field is applied, the cells introduced into the channel can be guided to the areas with the highest electric field with positive dielectrophoresis. This facilitates self-assembly of the cells at the micro orifice and results in cell contact and pearl-chain formation (Fig. 2b). When the diameter of the micro orifice is smaller than that of the cells, one-to-one cell contact is guaranteed as there is space for only one cell to fit in the orifice. 

When a DC pulse is applied, the membrane potential can be induced to initiate cell fusion (Fig. 2c) only for the cells close to the micro orifice. This ensures that the other cells in the pearl chain are not influenced by the DC pulse, which allows realization of one-to-one cell fusion. 

We constructed a model (Figure 1.1a) according to which the fibrillar morphology, which was seen by many researchers, is due to an oriented arrangement of these particles in a pearl-chain-like structure. We... 

Pearl chains of nanoparticles can be arranged and fused together to create a nanowire or other similar stmc-... 

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