Drying menisci

Mechanical stresses are necessary for the crystallization contacts to be established in hydration structures in the course of hydration hardening. These stresses are either applied externally or originate from the directional growth of the crystals inside the already formed network. Mechanical stresses may also be caused by the weight of powders or result from the action of capillary forces acting in the drying menisci. 

For a cylindrical meniscus, r, = r, V2 = 00, and the contractive force F = nrQO (and not 2nrod). When the particles are completely immersed in the liquid, rj = rj = r, and F = 0. The opposite limiting case of a nearly dry meniscus, that is, when rj 0 and rj 0 (r, > 0 and rj < 0), yields rf = 2ro I r21 (from the Pythagorean theorem) and F 2 nr o. Under incomplete wetting conditions, F= cos 9(1 - rfr. These types of meniscus attractive forces between particles are frequently encountered in nature and in many practical applications. These forces play a critical role in the structural and rheological properties of soils and the ground. 

A modification of the foregoing procedure is to suspend the plate so that it is partly immersed and to determine from the dry and immersed weights the meniscus weight. The procedure is especially useful in the study of surface adsorption or of monolayers, where a change in surface tension is to be measured. This application is discussed in some detail by Gaines [57]. Equation 11-28 also applies to a wire or fiber [58]. 

Capillarity. The outer surface of porous material has pore entrances of various sizes. As surface Hquid is evaporated during constant rate drying, a meniscus forms across each pore entrance and interfacial forces are set up between the Hquid and material. These forces may draw Hquid from the interior to the surface. The tendency of Hquid to rise in porous material is caused pardy by Hquid surface tension. Surface tension is defined as the work needed to increase a Hquid s surface area by one square meter and has the units J/m. The pressure increase caused by surface tension is related to pore size ... 

To test the delivery time, again separate the components of the stopcock, dry, grease and reassemble, then fill the burette to the zero mark with distilled water, and place in the holder. Adjust the position of the burette so that the jet comes inside the neck of a conical flask standing on the base of the burette stand, but does not touch the side of the flask. Open the stopcock fully, and note the time taken for the meniscus to reach the lowest graduation mark of the burette this should agree closely with the time marked on the burette, and in any case, must fall within the limits laid down by BS 846 (1985). 

Chitosan freeze-dried fleeces support chondrocyte attachment and synthesis of extracellular matrix [344]. Chitosan was used to assist the spontaneous tissue repair of the meniscus [345]. The repair of the cartilage and the prevention of its degradation in osteoarthritis is, however, possible with the association of glucosamine sulfate salt and chondroitine sulfate, the latter being particularly effective [346]. 

Fig. 6.22 Effect of pore meniscus and surface tension on the drying shrinkage of mortar (Baiogh [122]).

Gels dried by conventional methods, known as xerogels (xero- = dry), generally have lost the very open structure of the wet gel. The reason for this is that the liquid phase, as it evaporates and shrinks, exerts powerful capillary forces on the pore walls through the surface tension at the meniscus, resulting in collapse or cracking of the large-scale structure of the gel. 

This test method is applicable to coal of any size but is limited by the diameter of the neck of the flask. The apparatus must be scrupulously clean [usually by a mixture of sulfuric acid and potassium dichromate, after which the apparatus is washed with distilled water and dried in an oven at about 110°C (230°F)]. The condenser should be cleaned periodically in the same manner, and the receiver should be capable of collecting up to 10 mL of water in graduations of 0.1 or 0.05 mL, with a calibration for the water-toluene meniscus. 

Copies of Boris Cahan s thesis were circulated among many working in the fundamentals of the field in the 1970s. The thesis gave its readers access to the details of the elegant experimental studies it reported on the interplay between meniscus shapes and the thickness of the boundary layer, i0, electrolyte conductivity, and the local heat developed in porous electrodes used in fuel cells (and which tends to dry out thin menisci). 

Finally, we note the value of rinsing the water-deposited film with methanol. As methanol is more volatile and has a lower surface tension, film cracking due to the capillary stresses exerted by the meniscus of the drying solvent on the walls of nanosized pores is reduced. Similar results have been reported by Goh.22 Films thicker than a few hundred nm exhibit cracks due to stresses that result during drying of the film, even after methanol treatment. Eliminating these cracks will require other modifications of the drying process. 

Fig. 20a-c Schematic representation of the thermal conductances of tip (f()> tip-sample-contact (f,s) and sample material (fs) [215], Along with the frequency of the heating current, the penetration depth D, of the thermal wave depends on the thermal resistance of the configuration. E, and Es denote the Young s modulus of the tip and the sample, respectively. The enlarged contact situations represent a a dry and stiff b a dry and compliant c a moist and stiff contact, a is the mechanical contact radius and r2 is the radius of the liquid bridge. R and ri denote the radius of curvature of the tip and of the liquid meniscus, respectively... 

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