Polydimethylsiloxane water interface

A second example involves reflectance infrared spectroscopic structural analysis of polydimethylsiloxane at the air-water interface. Surface pressure versus surface area or surface concentration isotherms of polydimethylsiloxane on water have been studied since 1947 at least (223). Upon compression, the isotherm begins at zero surface pressure at surface concentrations significantly below 0.75 mg/m. Around f 1 0.75 mg/m, the surface pressure tt jumps substantially to about 9 mN/m, where it exhibits a plateau until about T2 1.6 mg/m, where a small Tt jump occurs followed by a smaller rise (Fig. 31). Structural features associated with the various transitions have often been debated. Particular controversy is associated with the ix plateau aroimd 9 mN/m between Fi and T2 (224,225). In conjimction with other techniques, such as epifluorescence microscopy, external reflectance infrared spectroscopy was used to study microstructural features (coexistence of two phases) of polydimethylsiloxane CH3—[Si(CH3)2—Oln—SKCHala, spread at the air-water interface in the vicinity of the n plateau at 9 mN/m (226). A broad band containing several components is foimd in the 1000-1100 cm ... 

FIGURE 6.9 Photomicrograph of rupture of water drop containing silylated polystyrene resin particles dispersed in polydimethylsiloxane in a Couette device. Shear rate, 1.32 s viscosity of polydimethylsiloxane, 10 Pa s resin particle radius, 86 10 microns and initial radius of drop, 0.11 cm. Particle volume fraction in debris formed by expulsion from the main drop is seen to be much higher than that of the latter. Particles are also seen to adhere to oil-water interface. (Reprinted from Colloids Surf., 15, Smith, P.G., van de Ven, T.G.M., 191. Copyright 1985, with permission from Elsevier.)... 

The use of neutron reflectivity at liquid interfaces, which is a method sensitive to both surface roughness and surfactant layer thickness, was reviewed with the examples of polydimethylsiloxane-surfactant layers.633 Sum-frequency generation (SFG) vibrational spectroscopy was applied to study surface restructuring behavior of PDMS in water in an attempt to understand antifouling properties of silicones.6 ... 

We will describe two mesoscale, self-assembling systems in which the interactions between objects are based on capillary forces. The first is based on polyhedral polydimethylsiloxane (PDMS) objects at a perfluorodecalin (PFD)/H20 interface. These objects have their faces patterned to be either hydrophobic or hydrophilic, and they assemble via lateral capillary forces that originate from interactions between these faces (Fig. 4. la). The second system uses polyhedral objects that are suspended in water and have selected faces covered with a water-insoluble liquid - either a hydrophobic organic liquid or a liquid metal solder these objects assemble via capillary forces into three-dimensional (3D) structures (Fig. 4.1b). 

Recently, capillary action has been used to self-assemble macroscopic objects. Objects of various shapes were cut from polydimethylsiloxane, a polymer that is not wettable by water but is wetted by fluorinated hydrocarbons. Designated surfaces were then made wettable by water by using controlled oxidation. These objects were then floated at an interface between perfluorodecalin (CioFig) and water. When two non-oxidized surfaces (wettable by CioFis) approached each other within a distance of approximately 5 mm, they moved into contact, which with time created an ordered, self-assembled pattern of the objects. The movement and self-assembly was driven by the solvent adhesive forces that produce the capillary action, thereby leading to an elimination of the curved menisci between non-oxidized surfaces. One such pattern is shown to the right. 

Figure 2.20 shows the images of a small water droplet resting on a flat horizontal polydimethylsiloxane (PDMS) surface but with surface modifications. Note that the contact angles are quite different, corresponding to the different surface properties. In Chapter 3, we will further discuss surface modification. The contact angle is not limited to a liquid-vapor interface it is also applicable to the interfaces of two liquids. 

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