Capillary phenomena dynamics

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Capillary effects are encountered in many areas of interface and colloid science, with its importance relative to other processes (e.g., fluid dynamics) depending on the exact situation. For example, when two spherical drops of a liquid in an emulsion make contact and coalesce to form a larger drop (Fig. 6.2a), the extent and duration of flow due to the capillary phenomenon is limited and fluid dynamics is of little practical importance. When there is an extensive amount of flow, on the other hand, such as in capillary imbibition, wicking processes, or capillary displacement (Fig. 6.2b) fluid dynamics may become important. 

Our overall conclusion, therefore, is that for mesoporous glasses adsorption, hysteresis is a dynamic phenomenon that is not simply related to a capillary vapor-liquid phase transition. Slow dynamics for long times makes the states accessible in experiments in the hysteresis loop appear equilibrated and quite reproducible. Mean field theory and Monte Carlo simulations in the grand ensemble provide a physically realistic description of these phenomena. 

However, for precise absolute measurements [1.6.1] is Inadequate. When the droplet falls down, part of the liquid remains on the capillary and the counteracting surface force is not vertical. The breakaway phenomenon is rather com-plicated and may rather look like fig. 1.17. Using high speed cinematography, details of the detachment phenomena can be seen. As the expansion of the interface is a dynamic process, the rate at which the interfacial tension can adjust. 

The dynamic interaction between flow and drops and bubbles floating in the flow may deform or even destroy them. This phenomenon is important for chemical technological processes since it may change the interfacial area and the relative velocity of phases and cause transient effects. In this case, the viscous and inertial forces are perturbing actions, and the capillary forces are obstructing actions. The bubble shape depends on the Reynolds number Re = aeU,p/p and the Weber number We = aeU2p/cr, where p, and p are the dynamic viscosity and the density of the continuous phase, a is the surface tension coefficient, and ae is the radius of the sphere volume-equivalent to the bubble. 

The capillary condensation phenomenon is of course not exclusive to water. It can be found in any confined system, where the surfaces prefer one phase over another and there is a first order phase transition between the phases of the material between the surfaces. A nematic liquid crystal is an example of such a system exhibiting a first order phase transition between the isotropic and the nematic phase. For this system, the nematic capillary condensation has been predicted by P. Sheng in 1976 [17]. Since the isotropic-nematic phase transition is only weakly first order, the phenomenon is not easy to observe. One has to be able to control the distance between the surfaces with a nanometer precision and the temperature within 10 K, which is unachievable to methods like NMR, SEA, DSC, etc., and very difficnlt to achieve in dynamic light scattering experiments [18,19]... 

On the other hand, capillary breakup of sufficiently viscous liquid jets is a longwave phenomenon, and its description in the framework of the quasi-one-dimensional equations of the dynamics of liquid jets is sufficiently accurate. The effect of the viscosity on the capillary breakup of highly-viscous liquid jets was studied numerically by Yarin [29]. The initial perturbation of the jet surface was imposed as a harmonic... 

Impregnation is yet another classic phenomenon in the field of interfacial dynamics. We restrict ourselves to the example of a capillary tube of radius R placed in contact with a reservoir of liquid (Figure 5.12). 

For polyacrylamide there are two rheological effects which can be explained in terms of its random coil structure. Firstly, it was discussed above that polyacrylamide is much more sensitive than xanthan to solution salinity and hardness. This is explained by the fact that the salinity causes the molecular chain to collapse, which results in a much smaller molecule and hence in a lower viscosity solution. The second effect which can be explained in terms of the polyacrylamide random coil structure is the viscoelastic behaviour of this polymer. This is shown both in the dynamic oscillatory measurements and in the flow through the stepped capillaries (Chauveteau, 1981). When simple models of random chains are constructed, such as the Rouse model (Rouse, 1953 Bird et al, 1987), the internal structure of these bead and spring models gives rise to a spectrum of relaxation times, Analysis of this situation shows that these relaxation times define response times for the molecule, as indicated in the simple Maxwell model for a viscoelastic fluid discussed above. Thus, because of the internal structure of a flexible coil molecule, one would expect to observe some viscoelastic behaviour. This phenomenon is discussed in much more detail by Bird et al (1987b), in which a range of possible molecular models are discussed and the significance of these to the constitutive relationship between stress and deformation rate and deformation history is elaborated. 

Light control of dynamic surface tension was demonstrated in a mixture of sodium dodecylsulfate and 4,4 -bis(trimethylammonium)hexyloxyazobenzene bromide. A decrease of 25 mN/m was determined on E—>Z isomerization by UV irradiation. Aggregation in the bulk and mass transport of the surfactant to the surface of the solution differ between the isomers. This phenomenon can be used to trigger the release of droplets of aqueous solutions from capillaries. Calix[4]resocinarene derivatives substituted with azobenzenes exhibited reversible precipitation/dissolution in dilute methanol solution upon alternate UV and VIS light irradiation due to the low solubility of the -trans isomer. ... 

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