Reverse meniscus

Physical principles and experimental data presented by LHP and CPL researchers proved a capability to apply such technological designs, where the system of vapor removal channels is located right close to the evaporator wall, providing effective vapor generation process. It is a so-called reverse meniscus thermal regime. 

Technology of the reverse meniscus consists in imposition of micro-porous layer to the PIN-structure that provides high intensity of heat transfer. Using of bidisperse wick structure is also instrumental in intensification of heat transfer. 

In the thermal mode 2, a value Ap ean be eonsidered as auxiliary hydraulic resistance due to concentration of heat and mass fluxes in the zone of vapor generation according to the reverse meniscus principle. 

The formation of a liquid phase from the vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the processes of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. 

Both the cone-shaped and the wedge-like pore give rise to simple, hysteresis-free behaviour. The meniscus is nucleated at the apex of the cone (Fig. 3.14(a)) or at the intersection of the two planes of the wedge (Fig. 3.14(b)), giving a spherical meniscus in the first case and a cylindrical one in the second. In both systems the process of evaporation is the exact reverse of that of condensation, and hysteresis is therefore absent. 

Chapter 3, there is often a region immediately preceding the lower closure point, in which increased adsorption is brought about by reversible capillary condensation. The meniscus now tends to be somewhat ill defined owing to its small dimensions (p. 153), but the mechanism can still be thought of in Kelvin terms, where the driving force is the pressure difference across an interface. 

The Cannon-Fenske viscometer (Fig. 24b) is excellent for general use. A long capillary and small upper reservoir result in a small kinetic energy correction the large diameter of the lower reservoir minimises head errors. Because the upper and lower bulbs He on the same vertical axis, variations in the head are minimal even if the viscometer is used in positions that are not perfecdy vertical. A reverse-flow Cannon-Fen ske viscometer is used for opaque hquids. In this type of viscometer the Hquid flows upward past the timing marks, rather than downward as in the normal direct-flow instmment. Thus the position of the meniscus is not obscured by the film of Hquid on the glass wall. 

What is the order of the reaction and the reaction rate constant The reverse reaction may be neglected. The volume of the solution as determined by the height of the meniscus in the capillary may be assumed to be a measure of the fraction conversion (i.e., the volume change is proportional to the extent of reaction). 

It is important, however, to remove the protein layer from the surface over which the air/liquid meniscus is displaced during the measurement so as to assure that the conditions of flow with and without the layer in the capillary are totally comparable. Using this method we find that in the case of bovine serum albumin very thick layers are formed layers whose thickness grows in direct proportionality to albumin concentration up to 15% w/v, at least. We also find a reversible doubling of layer size as temperature is raised from 7.7 to 15°C in the case of triple helical soluble collagen adsorbed end on to glass. 

The mercury meniscus should not stand still for any long period of time. Therefore the motor of the syringe is reversed immediately after the meniscus has reached the platinum tip. Hence, even if there is no change in volume, the motor buret is always working to and fro, thus keeping the meniscus oscillating around the contact-no contact position. 

In principle, the processes of capillary condensation and evaporation should occur reversibly in a closed tapering pore (Everett 1967). At low relative pressures there is an enhanced concentration of adsorbed molecules in the narrow end of the pore (i.e. a micropore filling effect) as in Figure 7.4. At a certain p/p°, a meniscus begins to form which, with the increase of p/p°, then moves steadily up towards the pore entrance. Evaporation proceeds in the reverse direction but involves the same elemental steps (i.e. the meniscus configurations) and therefore the entire isotherm is reversible. 

Let us imagine two non-interacting rigid plates with zero thickness situated at the surfaces of tension at the film surfaces, i.e. at z = h/2. Besides, the pressure pc the plates are subjected to a variable external force 11/1 (A is the area of the plane-parallel film). This fdrce counterbalances the forces causing the thinning of a non-equilibrium film by liquid drainage from it into the adjacent liquid meniscus. Thus, the film thickness h can change reversibly at fixed values of the independent variables. 

Attractive forces exist between the water in a pore and the solid surfaces when the liquid evaporates, the tension in the meniscus is transferred to the walls, and the pore tends to shrink. The shrinkage is not necessarily reversed when all the liquid has gone, because the pore may collapse. This effect could be important between about 90% and 45% RH. Above 90% it is unlikely to be important because the pores that arc being emptied arc wide, and the resulting stresses are small, and below about 45%, a stable meniscus cannot form. 

While studying adsorption in mesopores using the molecular continuum model we have found [4,6,7] that there exist two critical diameters based on thermodynamic analysis of the adsorption, and two more when the mechanical stabihty of the meniscus is considered. These criticalities refer to the critical pore diameter below which there either exists a different mechanism of adsorption, or the adsorption is reversible. Here we provide a brief outline of these criticalities. The chemical potential of the fluid adsorbed in a cyfindrical pore of radius R can be expressed as [6,7] (r,R) = /jj (r,R) + (f>(R-r,R) = constant(/ ). After considering... 

Further work is being directed to computer-aided analysis of (1) flow in other roll configurations, especially roll-wall combinations and reverse-roll coating (2) stability of the flow to meniscus nonuniformity — "ribbing," for example and (3) effects of viscoelasticity. 

Forward roll, (meniscus roll coating), single layer coating, 1-50 mPa s wet thickness 10-2000 im accuracy + 8% speed 0.05-1 m/s with maximum speed 2.5 m/s shear stress 10-1000 1/s. Not as precise as reverse roll. Thickness dependent on roll rpm, viscosity and gap. 

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