Showing papers on "Laplace pressure published in 1999"

Thermodynamic equilibrium and stability of liquid films and droplets on fibers

Abstract: The modeling of liquid spreading and penetration into fibrous materials requires a better understanding of the interactions of thin liquid films and small droplets with single fibers. The wetting properties of fibers may differ significantly from those of plane solid surfaces. Convex surfaces of fibers imply a positive Laplace pressure acting on the liquid-gas interface. This effect causes liquid film instability and hinders droplet spreading. Liquid films on fibers arc stable when the destabilizing action of the Laplace pressure is balanced by liquid-solid adhesion. Equilibrium configurations of liquid droplets and films are determined by the competition between capillary and adhesion forces. A general analytical solution is presented for the equilibrium profile of the transition zone between a film and a droplet residing on a cylindrical fiber. A new equation for apparent contact angles on fibers is derived. Adhesion forces. including van der Waalls and polar interactions, are expressed in terms of disj...

The influence of relative humidity on interparticle force.

Abstract: Forces acting between individual grains in a powder can have a critical and controlling effect on powder bulk behaviour. Operations such as powder flow, fluidisation, compaction, agglomeration and mixing are all influenced significantly by the intensity of interparticle forces. This is especially true when the particle size falls below around 100 mum at which point the surface forces outweigh the force due to gravity acting on a single particle. Studies of cohesion using bulk powder samples are of limited use because it is difficult to decouple the fundamental mechanisms of interparticle force from other contributions to cohesion such as variations in the powder microstructure, or geometric interlocking of individual particles. A review of the relevant literature has unearthed conflicting evidence associated with the influence of relative humidity (RH) on both bulk powder cohesion and interparticle force. Therefore there is a need for experimental force studies at the scale of the individual particle to identify the fundamental mechanisms that prevail and resolve some of the apparent uncertainty that currently exists. A custom built force instrument, incorporating Atomic Force Microscope (AFM) technology, was designed, constructed and commissioned. This instrument was used to quantify the interactions between particles of around 40 mum in diameter and flat surfaces as a function of the relative humidity of the surrounding air. Interactions between soda-lime glass surfaces, gold surfaces and amorphous quartz surfaces were studied. Striking results were obtained on soda-lime glass surfaces upon decreasing the RH from > 70% to around 40%. At this point the glass surfaces suddenly exhibited a strong repulsion upon approach. The range of this repulsion was observed at separation distances as great as 250 nm. Once the surfaces were brought into contact the strong repulsion was accompanied by a very large force of adhesion. This strong repulsion and associated peak value of adhesion was not observed at other RH values and was specific to desorption rather than adsorption. Force curves for gold and quartz surfaces showed no such repulsion and peak adhesion. It is thought that the critical humidity coincides with the formation of a complete monolayer of adsorbed water molecules. A number of possible explanations have been offered for the effect and its uniqueness to soda-lime glass in the present experiments. Theoretical calculations of adhesion force have been performed based on the concept of capillary meniscus formation. Calculations give values of around 17000 nN for a sphere 40 mum in diameter and a contact angle of 20°. These values are somewhat larger than measured values in all cases apart from peak adhesion. It is thought that at low humidities there is insufficient water adsorbed to overcome the effect of surface roughness. Contact occurs at asperities, which reduces the expected contact area and hence leads to an adhesive force that is lower than predicted. At humidities > 80% the experiments show evidence of capillary elongation upon surface separation. This implies that the surface adsorbed film is mobile with bulk liquid being drawn into the bridge under the action of the surface tension force. The associated increase in bridge volume and the change in bridge curvature with I elongation will tend to equalise the Laplace pressure inside the bridge and therefore give a value of adhesion that is lower than predicted.

A Model for Grain Junction in Polycrystalline Material and Effect of Temperature Changes on Related Phenomena

Abstract: Experimental evidences show that the material properties in an immediate vicinity of a Grain Junction (GJ) significantly differ from those in the bulk material. It is assumed that the material near the GJs is in some sort of a specific phase, and there is an interface between the GJ and the bulk material. Hsieh and Balluffi’s (1989) experimental data allows us to calculate the Laplace pressure at the interface. We consider the stress field induced by the Laplace pressure at the interface between the GJ and the bulk domain. The stresses extend the GJ domain and compress the material of adjacent grains. In the case of equilibrium they decrease as r−2 for r→∞ where r is the distance from the geometrical center of the GJ to the point under consideration, and as r−1 in the nonequilibrated case. We discuss the effect of grain anisotropy on the generation of shear stresses at grain boundaries.

Determination of Real Three Dimensional Foam Structure Using Optical Tomography

Abstract: Foams are presently a very active field of research as it is commonly encountered in the daily life. Until now, many studies have been devoted to the morphology of two dimensional foams, which are much more simple than the three dimensional ones. A 2D foam consists of a single layer of bubbles between two flat and parallel glass plates. The liquid films are perpendicular to the glass plates, and the 2D foam appears as a network of polygons. Its structure is very easy to observe and the way the 2D foam structure evolves is now well-known. The film network reorganises during time because of film ruptures (coalescence) or because of gas diffusion from a small bubble towards a bigger neighbour due to the difference of Laplace pressure (coarsening or disproportionation).