Interparticle attraction

Surface Coating. A dense surface coating (encapsulation) that contains no occluded solvent decreases interparticle attraction provided that the coating has a Hamaker constant intermediate between the particle and the Hquid. This is called semisteric stabilization (ST). The energy of interaction between coated spheres is as follows (26) ... 

The attenuation of ultrasound (acoustic spectroscopy) or high frequency electrical current (dielectric spectroscopy) as it passes through a suspension is different for weU-dispersed individual particles than for floes of those particles because the floes adsorb energy by breakup and reformation as pressure or electrical waves josde them. The degree of attenuation varies with frequency in a manner related to floe breakup and reformation rate constants, which depend on the strength of the interparticle attraction, size, and density (inertia) of the particles, and viscosity of the Hquid. 

Dispersion of a soHd or Hquid in a Hquid affects the viscosity. In many cases Newtonian flow behavior is transformed into non-Newtonian flow behavior. Shear thinning results from the abiHty of the soHd particles or Hquid droplets to come together to form network stmctures when at rest or under low shear. With increasing shear the interlinked stmcture gradually breaks down, and the resistance to flow decreases. The viscosity of a dispersed system depends on hydrodynamic interactions between particles or droplets and the Hquid, particle—particle interactions (bumping), and interparticle attractions that promote the formation of aggregates, floes, and networks. 

Individual particle surfaces can be lubricated by an adsorbed film that produces a smoother surface and/or decreases interparticle attraction. A plasticized binder may serve this purpose. Forming surfaces can be lubricated by coating with a film of low viscosity Hquid such as water or oil. Die surfaces can also be coated with a solution of stearic acid dissolved in a volatile Hquid that rapidly evaporates to leave a lubricating film. 

Our model predicts restabilisation at very high polymer concentrations. It is shown that this restabilisation is a thermodynamic effect, resulting from a decreased interparticle attraction, and is not kinetically determined, as proposed by Feigin and Napper. 

Particles attract whenever they approach to within a minimum distance. Whatever the magnitude of the interparticle attraction, energetic molecules will separate and continue moving after their encounter but, conversely, molecules of lower energy do not separate after the collision because the attraction force is enough to overwhelm the momentum that would cause the particles to bounce apart. The process of coalescence has begun. 

Figure 9 Properties of the attractive colloidal fluid investigated in Ref. 75 (a) self-diffusivity and (b) average free volume versus strength of the interparticle attraction (c) self-diffusivity versus average free volume for the hard-sphere fluid (open circles) and the attractive colloidal fluid (closed circles). Data compiled from Ref. 75.

Figure 10 (a) Free-volume persistence time extracted from the free-volume autocorrelation function (Eq. [9]) for an attractive colloidal fluid as a function of the strength of the interparticle attraction, (b) Comparison of colloidal self-diffusivity (closed symbols) with that estimated using the free-volume scaling relationship D — A(v )2 /tf discussed in the text (open symbols). Data taken from Ref. 75. 

Fig. 54 quantifies the relation between the yield value and a number of system-inherent parameters. The curves show that the yield value t decreases with the cube of the particle size and increases drastically with the pigment volume concentration. Since the yield value is an indication of pigment-vehicle interaction, it is also proportional to the degree of pigment dispersion [114], Interparticle attractions have received considerable theoretical and experimental treatment, and a large number of original publications and reviews are available (such as [115-117]). 

In order to understand how the addition of small amounts of organic materials can reduce this interparticle attraction to an extent that considerable quantities of water can be removed from the system whilst mamtaining the same rheological characteristics, it is necessary to consider a large amount of experimental data ... 

B. by promoting interparticle attraction. These materials are styrene copolymers with carboxyl groups, synthetic polyelectrolytes and natural gums. 

Class Emulsions of various organic materials, which increase interparticle attraction and also supply... 

The pivotal role that superplasticizers play in the formulation of self-leveling mortars is due to the dramatic effects they produce on flow behavior. Such effects are believed to be derived by the adsorption of the admixture on the surfaces of cement grains, thereby providing surfaces of a similar or zero charge which are mutually repulsive. They thus fully disperse cement particles, freeing more water for lubrication and reducing interparticle attraction. Both yield stress and plastic viscosity are decreased and the decrease is greater for yield stress it may be completely eliminated if sufficient admixture is added so that Newtonian behavior is observed (Fig. 7.25) [75, 76]. 

All gas particles have some volume. All gas particles have some degree of interparticle attraction or repulsion. No collision of gas particles is perfectly elastic. But imperfection is no reason to remain unemployed or lonely. Neither is it a reason to abandon the kinetic molecular theory of ideal gases. In this chapter, you re introduced to a wide variety of applications of kinetic theory, which come in the form of the so-called gas laws. ... 

Formation of agglomerates by powder compaction may involve rearrangement of particles to increase their packing efficiency resulting in the enhancement of interparticle adhesion forces [89]. Furthermore, particle deformation at the point of contact between particles can greatly increase both the contact surface area and interparticle attraction [84]. 

An extreme in sediment volume was usedt as a criterion for the effective cancellation of interparticle attraction by the continuous phase. Nylon-6,6 dispersions consisting of 1.0 g of solid in 10 ml of /2-propanol-thiodiethanol mixtures of various compositions were allowed to settle to sedimentation equilibrium. Listed here are the equilibrium sediment volumes, the volume/volume compositions, and the surface tensions of the media ... 

We now explore these interparticle attractions in order of relative strength, beginning with the strongest. 

Was this your answer The polar oxygen-hydrogen covalent bond in each methanol molecule leads to hydrogen bonding between molecules. These relatively strong interparticle attractions hold methanol molecules together as a liquid at room temperature. 

Ai 2 will be positive (interparticle attraction) where Au and A22 are either both greater than or both less than A33. However, in the unusual situation where A33 has a value intermediate between those of An and A22, then A132 is negative - i.e. a repulsive van der Waals interaction between the particles is predicted,... 

Class C. Emulsions of various organic materials, which increase interparticle attraction and also supply additional superfine particles in the cement paste. These are materials consisting of acrylic emulsions and aqueous clay dispersions. 

To an extent that increases with the w/c ratio, fresh cement pastes exhibit the phenomenon of bleeding, i.e. settlement of the solid particles. The interparticle attractions are sufficiently strong that particles of all sizes settle at the same rate, typically about 2pms . Settlement also tends to increase the w/c ratio at the top and to decrease it at the bottom of the sample. It decreases with increased fineness or increased early hydration rate of the cement. In a concrete, it can produce layers of water beneath aggregate particles or reinforcing bars. 

The purpose of adding a polymer is not always to stabilize colloidal particles in other cases one aims at imparting an effective attraction which will bring the particles together in a floe or a network. The Interparticle attraction is then caused by the formation of so-called polymer bridges, strands of pol rmer that are adsorbed simultaneously onto two or more particles, in this way bridging the gap between them. In these applications kinetic aspects, which have not received much attention in the present chapter, may be crucial. We mention a number of examples of this type of destabilization. 

Reduction of interparticle attraction by creating a physical barrier between particles. 

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