Soft Solid Interfaces

All the measurement techniques reviewed so far have one common thread They are all based on monitoring the distortions of interfaces. The key is to determine how surface tension resists such distortions. The fundamental reason that these methods are so simple to use is that up to this point we have been dealing exclusively with fluids. The interfaces considered so far have been between a liquid and a vapor or between two liquids. In such cases, surface tension is the dominant mechanism, even though gravity may at times complicate matters. 

As such, these methods cannot be readily applied to hard interfaces, whether between a solid and a vapor or between a solid and a liquid, because the elastic energy stored in the solid far exceeds the interface energy associated with any distortion. As a result, measuring the surface tension of solids is generally perceived as an impossible task. Nevertheless, there exist a few examples of direct or indirect measurements that are worth mentioning. 

While the surface energy of the solid favors an extended contact, elasticity strenuously opposes it. The JKR formula expresses the fact that the radius I results from a compromise between these two forms of energy. It reads 

FIGURE 2.25. The JKR test. A sphere and a plane are brought into contact. The contact surface area is a function of the load on the sphere. 

W per unit area, since K is known at this point. When the surrounding medium is air, the following simple relation holds  

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The major differences between polymer and liquid electrolytes result from the physical stiffness of the PE. PEs are either hard-to-soft solids, or a combination of solid and molten in phases equilibrium. As a result, wetting and contact problems are to be expected at the Li/PE interface. In addition, the replacement of the native oxide layer covering the lithium, under the... 

The changes in polymer appearance after exposure to aqueous media were evaluated using light and SEM microscopy. Before exposure to buffer, the polymer is transparent. After exposure to buffer the polymer became opaque, and when it was cut, two different regions were found the outer region which is gel and the core, which appears as a soft solid matrix. SEM analysis of the interface of the polymer exposed to buffer showed that a kind of a rigid network was formed across the polymer sample. This network causes the polymer drop to keep its shape in water. 

The microlens is an important and key optical component for focusing and collimating light in the microfluidic optical detection system, and can be made up of a hard solid surface or interface (soft solid/liquid, liquid/liquid/...). Hard solid state lenses on-chip are usually fixed focal length lenses similar to the miniaturized traditional lenses used in the free-space detection system. They can be fabricated... 

Industrial emulsions are usually prepared as concentrated systems, containing ( ) < 0.94. Owing to interface interactions and deformabil-ity of droplets, these systems behave rather like elastic, soft solids without any sign of Newtonian behavior. Between the highly concentrated and dilute regions, there is a wide zone of structural change reflected in a spectrum of non-Newtonian behavior. 

For instance, if the contribution iJbd were important, it would be unreasonable not to include it directly in the analysis of the membrane-mediated interaction between the insertions, and in the energetics of channel formation. However, this has never been attempted. Also, the interactions responsible for this surface contribution cannot be localized at the geometrical surface, but must affect the membrane s properties in the vicinity of the insertion. This poses a question of how valid it is to describe the membrane with Eq. (16), which ignores these interactions. It is well known that even well-defined solid interfaces are actually transition regions where the properties of both phases are strongly modified. This should be even more the case for the soft contacts involving membranes. 

In this Chapter we want to introduce some basic concepts and present an overview of the mechanisms, that are in our opinion responsible for the alignment of liquid crystals on solid and soft-matter interfaces. In addition we will present and discuss recent developments and results in surface sensitive... 

The major differences between PEs and liquid electrolytes result from the physical stiffness of the PE. PEs are either hard-to-soft solids, or a combination of solid and molten in phases equilibrium. As a result, wetting and contact problems are to be expected at the Li/PE interface. In addition, the replacement of the native oxide layer covering the lithium, under the OCV conditions, by a newly formed SEI is expected to be a slow process. The SEI is necessary in PE systems in order to prevent the entry of solvated electrons to the electrolyte and to minimize the direct reaction between the lithium anode and the electrolyte. SEI-free Li/PE batteries are not practical. The SEI cannot be a pure polymer, but must consist of thermodynamically stable inorganic reduction products of PE and its impurities. 

JKR type mea.surement.s on monolayers depo.sited on. soft elastomers. The recent interest in the JKR experiments has been stimulated by the work of Chaudhury and coworkers [47-50J. In a 1991 paper, Chaudhury and White-sides [47] reported their extensive studies on the measurement of interfacial work of adhesion and surface energies of elastomeric solids. The motivation for this work was to study the physico-organic chemistry of solid surfaces and interfaces. 

One of the major themes of boundary lubrication is to transfer the shear stress at the interface of direct solid contact to somewhere inside the lubricating layer, to achieve low friction and high wear resistance. In this sense, materials with low shear strength, such as liquid films, soft metals, and lamella solids, can be employed as candidate lubricants. 

We can consider the spreading of a sessile drop on a soft, lossy substrate rather like the advance of a negative crack and thus use fracture mechanics concepts, as was the case in the derivation of Eq. (15) for the separation of an elastomer from a rigid solid. The term negative is used since the spreading of a drop leads to the creation of solid/liquid interface rather than separation. 

The desire to realise technological goals has spurred the discovery of many new solid electrolytes and intercalation compounds based on crystalline and amorphous inorganic solids. In addition an entirely new class of ionic conductors has been discovered by P. V. Wright (1973) and M. B. Armand, J. M. Chabagno and M. Duclot (1978). These polymer electrolytes can be fabricated as soft films of only a few microns, and their flexibility permits interfaces with solid electrodes to be formed which remain intact when the cells are charged and discharged. This makes possible the development of all-solid-state electrochemical devices. 

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