Meniscus stability

A two-chamber coating applicator (Figure 9) has been developed to deal with meniscus stability problems (10). The device decouples the free surface at the top of the reservoir from the final coating operation. The first chamber (with the free surface) strips away large air bubbles. The second chamber is pressurized which produces a net flow into the first chamber and strips small bubbles from the fiber. The resultant coating is bubble free, smooth, and regular. 

Momentum Transport The liquid film profile and its thickness 5 depend on the flow rate, the surface tension (liquid, gas and reaction plate material property) and on the channel dimensions (width, depth, length, diameter, etc.), which determine the interfacial area. In mesh reactors, the meniscus stability (depending on the surface tension and pore geometry) plays a certain role. We encounter the following parameters ... 

The onset of flow instability in a heated capillary with vaporizing meniscus is considered in Chap 11. The behavior of a vapor/liquid system undergoing small perturbations is analyzed by linear approximation, in the frame work of a onedimensional model of capillary flow with a distinct interface. The effect of the physical properties of both phases, the wall heat flux and the capillary sizes on the flow stability is studied. A scenario of a possible process at small and moderate Peclet number is considered. The boundaries of stability separating the domains of stable and unstable flow are outlined and the values of the geometrical and operating parameters corresponding to the transition are estimated. 

The capillary flow with distinct evaporative meniscus is described in the frame of the quasi-dimensional model. The effect of heat flux and capillary pressure oscillations on the stability of laminar flow at small and moderate Peclet number is estimated. It is shown that the stable stationary flow with fixed meniscus position occurs at low wall heat fluxes (Pe -Cl), whereas at high wall heat fluxes Pe > 1, the exponential increase of small disturbances takes place. The latter leads to the transition from stable stationary to an unstable regime of flow with oscillating meniscus. 

Chapter 11 consists of following Sect. 11.2 deals with the pattern of capillary flow in a heated micro-channel with phase change at the meniscus. The perturbed equations and conditions on the interface are presented in Sect. 11.3. Section 11.4 contains the results of the investigation on the stability of capillary flow at a very small Peclet number. The effect of capillary pressure and heat flux oscillations on the stability of the flow is considered in Sect. 11.5. Section 11.6 deals with the study of capillary flow at a moderate Peclet number. 

We deal here with the stability of flow in a heated capillary tube when liquid is evaporating on the meniscus. The capillary, as shown in Fig. 11.1, is a straight vertical pipe with diameter d and length 1. The wall heat flux is uniform = const. The thermal conditions on the capillary inlet and outlet are ... 

In this section the influence of the pressure in the capillary and the heat flux fluctuations on the stability of laminar flow in a heated capillary tube is analyzed. All the estimations performed in the framework of the general approach and developed in the previous section are kept also in the present cases. Below we will assume that the single cause for capillary pressure oscillations is fluctuations of the contact angle due to motion of the meniscus, whereas heat flux oscillations are the result of fluid temperature fluctuations only. 

We have observed a dependence of the yield, polymerization degree, and polydispersity of polysilanes on temperature and also on the power of ultrasonication. In the ultrasonication bath the simplest test of the efficiency of cavitation is the stability of the formed dispersion. It must be remembered that the ultrasonic energy received in the reaction flask placed in the bath depends on the position of the flask in the bath (it is not the same in each bath), on the level of liquid in the bath, on temperature, on the amount of solvent, etc. When an immersion probe is used the cavitation depends on the level of the meniscus in the flask as well. The power is usually adjusted close to 50% of the output level but it varies with the reaction volume, flask shape, and other rection conditions. The immersion-type probe is especially convenient at lower temperatures. 

IUPAC defines the lower limit of mesopores as 2 nm [1] which was considered as the limit below which the adsorption will occur by volume filling. However, in our recent article, based on the tensile stress hypothesis, we have shown that this limit is different than IUPAC limit. Using the mechanical stability criterion for the cylindrical meniscus (during adsorption), the critical size is obtained from... 

Similar analyses are available for surface-tension-driven flows in a slender cavity with the additional assumption that the meniscus at the top of the cavity is also flat (36). Smith and Davis (37-39) have used this configuration to study the stability of the flow with respect to wavelike instabilities (see also reference 40). Homsy and co-workers (41, 42) have analyzed the effect of a surface-active agent on the thermocapillary motion in a slender cavity. 

The location and shape of the distribution curve is, of course, dependent on which branch of the hysteresis loop is used to compute the pore size. In spite of the considerable attention given to this problem, in the absence of any detailed knowledge of the pore geometry it is not possible to provide unequivocal general recommendations. In principle, the regions of meta-stability and instability should be established for the liquid/vapour meniscus in the various parts of a given pore structure, but in practice this would be extremely difficult to undertake in any but the simplest types of pore system. 

The dependence of the surfactant concentration at which equilibrium asymmetric aqueous films are formed on the nature of the organic phase is presented in Table 3.18 [551]. It is seen that the stabilising ability of the surfactants strongly reduces with the increase in organic phase polarity. The experiments performed in [552] have shown that at high capillary pressures in the meniscus the stability of films on organic substrate substantially depends on the surfactant concentration. 

The kinetic regularities of heterogeneous defoaming, i.e. the acceleration of foam breakdown with the decrease in hydrocarbon molecular weight (respectively, the decrease in hydrocarbon surface tension and the increase in the contact angle between the meniscus and the substrate) and with the increase in both foam expansion ratio and reduced pressure, indicate that foam breakdown can occur due to the lowered stability of film borders at their contact with the substrate (worsened wetting). The fact that the foam situated between the cuvette wall and the capillary decayed at a considerably slower rate is in benefit to this statement. 

Now consider a tapered tube, wider at the top. This represents a stable equilibrium. For a given pressure applied, the mercury is stable in only one position in the capillary. If the pressure is increased momentarily (or the surface tension is decreased, say, by a fluctuation in the applied potential), the mercury meniscus will move to a lower point, where the radius is a little smaller, to establish a new equilibrium, in accordance with Eqs. 51H-53H. The mechanical equivalent of this configuration is a sphere at the bottom of a concave surface, shown in Fig. 4H(b). Stability is attained by negative feedback. If the... 

Nevertheless, bearing in mind the above-mentioned reservations, it is possible to interpret the pore size distributions obtained using the BJH method with the Harkins and Jura t-curve (Fig. 2). Firstly, it would seem obvious that the peaks centred on 4 nm relate to the closing of the hysteresis at around p/p° = 0.42. One should ignore these, as they are artefacts due to the non-stability of the nitrogen meniscus under these conditions. However, the peaks observed in the case of mlO and mVT4, centered on 40 nm and 100 nm respectively, are significant. It is these peaks that can be confidently used for further comparison. 

Thus Eq. (4) and (5) provide two criticahties based on a purely thermodjmamic analysis of the adsorption in the cylindrical pore. During adsorption or desorption, although the thermodynamic stabihty criteria are satisfied, the condition for mechanical stability of the meniscus has to be satisfied. The mechanical stabihty criteria for the cylindrical meniscus (during adsorption) and hemispherical meniscus (during desorption) are given by [4,6,7,12]... 

The process of nanomanipulation involves control of a number of forces. The main two forces are the force of adhesion between the AFM tip and the adsorbate particle which will be manipulated and that between the particle and the substrate (FaP ). These forces are mediated by the surface forces [98] and depend on the formation of a meniscus on the substrate. While stabilizes the particle on the substrate, F T rnakes the particle stick to the tip. The contact force between the tip and the particle F makes the particle move. The movement of the cantilever from its equilibrium position and the force constant of the cantilever determine the contact force in turn. In addition, since the experiments are carried out in ambient air, there are capillary forces between the tip and the particle as well as the van der Waal forces between the tip and the particle and between the particle and the substrate. It is the balance of these forces that determines the final dynamics of the particle movement. 

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