Interface, interfacial mobility

Complementing the equilibrium measurements will be a series of time resolved studies. Dynamics experiments will measure solvent relaxation rates around chromophores adsorbed to different solid-liquid interfaces. Interfacial solvation dynamics will be compared to their bulk solution limits, and efforts to correlate the polar order found at liquid surfaces with interfacial mobility will be made. Experiments will test existing theories about surface solvation at hydrophobic and hydrophilic boundaries as well as recent models of dielectric friction at interfaces. Of particular interest is whether or not strong dipole-dipole forces at surfaces induce solid-like structure in an adjacent solvent. If so, then these interactions will have profound effects on interpretations of interfacial surface chemistry and relaxation. 

Polymers can be confined one-dimensionally by an impenetrable surface besides the more familiar confinements of higher dimensions. Introduction of a planar surface to a bulk polymer breaks the translational symmetry and produces a pol-ymer/wall interface. Interfacial chain behavior of polymer solutions has been extensively studied both experimentally and theoretically [1-6]. In contrast, polymer melt/solid interfaces are one of the least understood subjects in polymer science. Many recent interfacial studies have begun to investigate effects of surface confinement on chain mobility and glass transition [7], Melt adsorption on and desorption off a solid surface pertain to dispersion and preparation of filled polymers containing a great deal of particle/matrix interfaces [8], The state of chain adsorption also determine the hydrodynamic boundary condition (HBC) at the interface between an extruded melt and wall of an extrusion die, where the HBC can directly influence the flow behavior in polymer processing. 

The presence of surfactant adsorption monolayers decreases the mobility of the droplet (bubble) surfaces. This is due to the Marangoni effect (see Equation 5.282). From a general viewpoint, we may expect that the interfacial mobility will decrease with the increase of surfactant concentration until eventually the interfaces become immobile at high surfactant concentrations (see Section 5.5.2, above) therefore, a pronounced effect of surfactant concentration on the velocity of film drainage should be expected. This effect really exists (see Equation 5.286, below), but in the case of emulsions it is present only when the surfactant is predominantly soluble in the continuous phase. 

In addition to the bulk Tg, siower relaxation was assigned to polymer chains close to the polymer-filler interface, whose mobility was restricted by the physical interactions. The existence of an interfacial layer was proposed to explain the DSC results (showing a double step in heat capacity) and TSC/DDS measurements (distinguishing two well-defined dielectric relaxation processes). These results confirmed earlier studies by dynamic mechanical spectrometry, where a second tan 5 peak, observed at 50 to 100°C above the mechanical manifestation of Tg, was attributed to the glass transition of an interfacial polymer layer with restricted mobiUty [Tsagaropoulos and Eisenburg, 1995],... 

SAK Jeelani, S Hartland. Effect of Interfacial Mobility on thin Film Drainage. J Colloid Interface Sci 164 p 296—... 

The critical film thickness for rupture is of the order of 50 A. If the interaction time of the drops is too short to reach the critical film thickness, the drops will not coalesce. The drainage of the film is the rate-determining step in coalescence of deformable drops in polymer blends. Various models have been proposed to describe the film drainage. One model assumes fully mobile interfaces, another model assumes immobile interfaces, and a third model assumes partially mobile interfaces. The mobility of the interfaces is strongly dependent on the presence of impurities, such as surfactants. Surfactants reduce the mobility of the interfaces due to interfacial tension gradients [315]. 

For monodisperse emulsions, the average value, (e ,), and the interfacial mobility parameter, e , are equal. In the special case of completely mobile interfaces, that is, f (j/(ri ,Deff) 0 and (Stiji +... 

A complicated interaction between chemical reaction and mass transport may occur close to the interface between two phases, particularly when the interface is mobile, such as a gas/liquid or a liquidAiquid interface. Because of the transport of at least one component across the interface, there will be considerable concentration gradients perpendicular to the interface. If the interfacial area is changing, e.g., due to gas bubble (or droplet) coalescence or breakup, or simply by deformation of the bubbles (or droplets), there will also be concentration gradients parallel to the interface, and these will result in surface tension gradients which will influence the rates of coalescence and breakup, which has cpnsequences for the rate of mass transport. The situation becomes even more complicated when local gradients in liquid density and viscosity also play a role. 

It shows that, as a rule, the presence of surfactant decreases the interfacial mobility and decreases the velocity of thinning under the action on an outer force, F. The following particular cases are given in the literature (1) tangentially immobile interfaces, (2) partially immobilized interfaces, and (3) completely mobile interfaces. These three cases are considered in Secs. VII.C.1-VII.C.3. 

Indicate by Q the concentration in diffusing species at as X-coordinate and by C, the concentration at X2 as X-coordinate. These two concentrations are maintained constant (fixed by interfacial equilibriums, for example). The two interfaces are mobile in consequence of the growth of the layer. Write the material balance of the amount of diffusing species in the layer. Using the pseudo-steady state condition, we get... 

The phenomena described in this chapter centered on interfacial disorder, sometimes liquid like disorder, of both a thermodynamic and dynamic origin, and the redistribution of both the disordered material and impurities at solid-liquid interfaces. Because of the change in the surface structure created by both surface roughening and surface melting, they have a profound influence on the local attachment kinetics, and ultimately on pattern formation in ice crystals. We have a clear picture of several issues, namely (i) that roughening is a crucial aspect of growth anisotropy, and (ii) that inter-facially melted water at subfreezing interfaces is mobile and responds in a thermodynamically consistent and predictable manner. Less clear however are a number of other issues as discussed below. 

Migration is the movement of ions due to a potential gradient. In an electrochemical cell the external electric field at the electrode/solution interface due to the drop in electrical potential between the two phases exerts an electrostatic force on the charged species present in the interfacial region, thus inducing movement of ions to or from the electrode. The magnitude is proportional to the concentration of the ion, the electric field and the ionic mobility. 

At lower frequencies, orientational polarization may occur if the glass contains permanent ionic or molecular dipoles, such as H2O or an Si—OH group, that can rotate or oscillate in the presence of an appHed electric field. Another source of orientational polarization at even lower frequencies is the oscillatory movement of mobile ions such as Na". The higher the amount of alkaH oxide in the glass, the higher the dielectric constant. When the movement of mobile charge carriers is obstmcted by a barrier, the accumulation of carriers at the interface leads to interfacial polarization. Interfacial polarization can occur in phase-separated glasses if the phases have different dielectric constants. 

Insulators lack free charges (mobile electrons or ions). At interfaces with electrolyte solutions, steady-state electrochemical reactions involving charge transfer across the interface cannot occur. It would seem, for this reason, that there is no basis at this interface for the development of interfacial potentials. 

Key mechanisms important for improved oil mobilization by microbial formulations have been identified, including wettability alteration, emulsification, oil solubilization, alteration in interfacial forces, lowering of mobility ratio, and permeability modification. Aggregation of the bacteria at the oil-water-rock interface may produce localized high concentrations of metabolic chemical products that result in oil mobilization. A decrease in relative permeability to water and an increase in relative permeability to oil was usually observed in microbial-flooded cores, causing an apparent curve shift toward a more water-wet condition. Cores preflushed with sodium bicarbonate showed increased oil-recovery efficiency [355]. 

In general, there is no correlation between the tension and the shear viscosity of an oil-water interface. However, for systems containing demulsifiers, a low interfacial tension (IFT) often leads to a lowering of the shear viscosity. Demulsifiers, in general, are large disordered molecules and when they are present at the interface they create a mobile, low viscosity zone. However, a low IFT is not a necessary condition for a low viscosity interface. A large demulsifier such as PI, although not very surface active, can still lower the shear viscosity to a very low value (Table I). 

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