Particle fluid assemblies

Particles immersed in a fluid with a different dielectric constant or conductivity (electrorheological fluid) assembled into chains, columns of chains, and glasses in the presence of an electric field Electric dipole interactions nm to (Jim 55... 

At the nanoscale, colloidal assembly systems rely on fluid flows caused by natural COTivection for positioning colloidal particles. Convective assembly patterns are controlled by evaporation rate and contact line geometry. Coatings with monolayer precision have been created through... 

Clearly, the inclusion of the liquid phase (xi) will tend to reduce the number of fines in the system. Thus, a diminution in particle size must be effected to provide an equivalent particle population in the fluid-particle assembly. The particle size ratio normally used in practical systems tends to be somewhat lower than the one computed from Eq. (8). 

Particle packings (random) are usually (not alwa) ) less efficient than the pre-packaged/preformed assemblies however, particle types are generally more flexible in loading and the ability to handle dirty fluids. 

Figure 4.1.2 is a photograph of a coimterflow burner assembly. The experimental particle paths in this cold, nonreacting, counterflow stagnation flow can be visualized by the illumination of a laser sheet. The flow is seeded by submicron droplets of a silicone fluid (poly-dimethylsiloxane) with a viscosity of 50 centistokes and density of 970 kg/m, produced by a nebulizer. The well-defined stagnation-point flow is quite evident. A direct photograph of the coimterflow, premixed, twin flames established in this burner system is shown in Figure 4.1.3. It can be observed that despite the edge effects.

Compare this with Eq. (4.88). Coinciding in linear parts—a fact that has been mentioned in Ref. 14—the susceptibilities of a fluid and random solid assemblies differ in the cubic contributions unless one deals with magnetically isotropic particles for which Si = 0. This important fact has been overlooked in Refs. 64 and 65, where the authors have taken Eq. (4.89) as a starting point to study a solid system. Right from the comparison of the static formulas (4.88) and (4.89) for X 3, underestimation of the predicted values, when Eqs. (4.89) are used, becomes apparent. 

So we see that in the limit d>l, which corresponds to magnetically rigid particles, the cubic susceptibility of a solid random assembly is thrice that of a magnetic fluid containing the same amount of identical particles. In this connection we recall a well-known fact [14,67,68] that for the cases compared, the respective static linear susceptibilities % identically coincide, that is, are completely indistinguishable whatever a. This confirms surmise that in nanoma-gnetic systems the cubic susceptibility is much more selective than the linear one. 

Equations (4.329) for a solid assembly and (4.332) for a magnetic suspension are solved by expanding W with respect to the appropriate sets of functions. Convenient as such are the spherical harmonics defined by Eq. (4.318). In this context, the internal spherical harmonics used for solving Eq. (4.329) are written Xf (e, n). In the case of a magnetic fluid on this basis, a set of external harmonics is added, which are built on the angles of e with h as the polar axis. Application of a field couples [see the kinetic equation (4.332)] the internal and external degrees of freedom of the particle so that the dynamic variables become inseparable. With regard to this fact, the solution of equation (4.332) is constructed in the functional space that is a direct product of the internal and external harmonics ... 

Figure 4.32. Linear dynamic magnetic susceptibilities of a randomly oriented super-paramagnetic assembly (a, b) and of a magnetic fluid of the same particles (c, d) for all the graphs the ratio TdAb = 10-4. Vertical axes for % are scaled in the units of cp2/r so that at to —> 0 both % tend to 1/3.

Inorganic nanoparticles themselves can be assembled into mesoscopic structures. Dinsmore et al. proposed an approach for the fabrication of solid capsules from colloidal particles with precise control of size, permeability, mechanical strength, and compatibility (Fig. 2.9).44 This unusual mesoscopic structure is called colloidosome and is prepared through emulsion droplets at a water-oil interface. Following the locking together of the particles to form elastic shells, the emulsion droplets were transferred to a fresh continuous-phase fluid identical to that contained inside the droplets. The resultant structures are hollow, elastic shells whose permeability and elasticity can be precisely controlled. 

Operation. In a diffusion pump, the pump fluid is heated so that a vapour pressure of 1-10 mbar is established in the boiler. The vapour rises in the jet assembly where it is expanded through nozzles and enters the space between the nozzle and the cooled wall of the pump at high supersonic velocity. Pumping action is based on the transfer of momentum in collisions between the high speed (several times the speed of sound) pump fluid vapour molecules and particles that have entered the vapour jet. 

We developed systems where we could conveniently control the parameters affecting the assemblies and characterize them. These parameters include the shapes, surface properties, densities, and colors of the objects the directionality of the forces between objects and the densities and surface properties of the fluids. Some of these systems allow quick examination of tens to thousands of assembling particles. Agitation is normally in the form of fluid shear or gravity. The following sections describe some of the successes and failures in these experiments in self-assembly. 

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