Reflection fluid-solid interface

Fig. 6.3. Reflected pressure amplitude and transmitted longitudinal and shear stress amplitudes at a fluid-solid interface (a) water-PMMA (b) water-fused silica (i) reflected wave in the fluid (ii) transmitted longitudinal wave (iii) transmitted shear...

In micro- and nanoscale fluid mechanics, measurements of mass transport and fluid velocity are used to probe fundamental physical phenomena and evaluate the performance of microfluidic devices. Evanescent wave illumination has been combined with several other diagnostic techniques to make such measurements within a few hundred nanometers of fluid—solid interfaces with a resolution as small as several nanometers. Laser Doppler velocimetry has been applied to measure single-point tracer particle velocities in the boundary layer of a fluid within 1 pm of a wall. By seeding fluid with fluorescent dye, total internal reflection fluorescence recovery after photobleaching (FRAP) has been used to measure near-wall diffusion coefficients and velocity (for a summary of early applications, see Zettner and Yoda [2]). 

The optical reflection constant is calculated by dividing the averaged pixel intensity of the reflectance region by the averaged pixel intensity of the incidence region. The laser beam hit the fluid-solid interface and reflected off the... 

An additional advantage to neutron reflectivity is that high-vacuum conditions are not required. Thus, while studies on solid films can easily be pursued by several techniques, studies involving solvents or other volatile fluids are amenable only to reflectivity techniques. Neutrons penetrate deeply into a medium without substantial losses due to absorption. For example, a hydrocarbon film with a density of Ig cm havii a thickness of 2 mm attenuates the neutron beam by only 50%. Consequently, films several pm in thickness can be studied by neutron reflecdvity. Thus, one has the ability to probe concentration gradients at interfaces that are buried deep within a specimen while maintaining the high spatial resolution. Materials like quartz, sapphire, or aluminum are transparent to neutrons. Thus, concentration profiles at solid interfaces can be studied with neutrons, which simply is not possible with other techniques. 

Let us start with the case of pure phases, when surfactant is missing and the fluid-liquid interfaces are mobile. Under these conditions, the interaction of an emulsion droplet with a planar solid wall was investigated by Ryskin and Leal, and numerical solutions were obtained. A new formulation of the same problem was proposed by Liron and Barta. The case of a small droplet moving in the restricted space between two parallel solid surfaces was solved by Shapira and Haber. ° These authors used the Lorentz reflection method to obtain analytical solutions for the drag force and the shape of a small droplet moving in Couette flow or with constant translational velocity. 

The correlations for solid fluid interfaces in Table 8.3-3 are much like their heat transfer equivalents. More significantly, these less important, fluid-solid correlations are analogous but more accurate than the important fluid-fluid correlations in Table 8.3-2. Accuracies for sohd fluid interfaces are typically average 10% for some correlations like laminar flow in a single tube, accuracies can be 1 %. Such precision, which is truly rare for mass transfer measurements, reflects the simpler geometry and more stable flows in these cases. Laminar flow of one fluid in a tube is much better understood than turbulent flow of gas and liquid in a packed tower. 

Devices such as ultrasonic flow equipment use the Doppler frequency shift of ultrasonic signals reflected from discontinuities in the fluid stream to obtain flow measurements. These discontinuities can be suspended solids, bubbles, or interfaces generated by turbulent eddies in the flow stream. The sensor is mounted on the outside of the pipe, and an ultrasonic beam from a piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle to the flow stream. Signals reflected off flow disturbances are detected by a second piezoelectric crystal located in the same sensor. Transmitted and reflected signals are compared in an electrical circuit, and the corresponding frequency shift is proportional to the flow velocity. 

The use of an evanescent wave to excite fluorophores selectively near a solid-fluid interface is the basis of the technique total internal reflection fluorescence (TIRF). It can be used to study theadsorption kinetics of fluorophores onto a solid surface, and for the determination of orientational order and dynamics in adsorption layers and Langmuir-Blodgett films. TIRF microscopy (TIRFM) may be combined with FRAP ind FCS measurements to yield information about surface diffusion rates and the formation of surface aggregates. 

The contact angle exhibited at a stationery contact line, where a gas-liquid interface intersects a solid surface, is a unique characteristic of the three materials involved namely, the gas, the liquid, and the solid. It reflects the nature of their interaction across the various surfaces that intersect at the contact line. It is known from thermodynamics that a fluid-fluid... 

Major applications of SFE-SFC are somewhat limited at the moment to the analysis of lipids and pesticides from foods and similar matrices and different types of additives used in the production of polymers [79,146,188-194]. The approaches used cover a wide range of sophistication and automation from comprehensive commercial systems to simple laboratory constructed devices based on the solventless injector [172,174,175,188]. Samples usually consist of solid matrices or liquids supported on an inert carrier matrix. Aqueous solutions are often analyzed after solid-phase extraction (SPE-SFE-SFC) to minimize problems with frozen water in the interface [178,190]. The small number of contemporary applications of SFE-SFC reflects a lack of confidence in supercritical fluid chromatography as a separation technique and competition for... 

When three phases are present, three different interfaces are possible, one for each pair of fluids. Sometimes all three interfaces meet, the jimction forming a curve known as a contact line. If one phase is a sohd, the contact line lies along its surface. In this case, the angle that the fluid interface makes with the solid surface is called the contact angle. Since it reflects the wetting properties of liquids on solids, the contact angle is a second fundamental property important in interfacial phenomena. 

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