Bubble surface, particle reflection

DUE TO Joint Action of Particle Reflection from a Bubble Surface and Centrifugal Forces... 

Fig. 11.7. Illustration of the mechanism of deposition prevention in the zone 0Q5r<0< t due to the joint action of inertia reflection of a particle tfom the bubble surface and centrifugal forces 1 -grazing trajectory at a single reflection 2 - impossibility of deposition at > oc.

Particle reflection after a collision is described by Eq. (11.55). Particle movement from the bubble surface after its reflection is retarded by a liquid movement with opposite direction, i.e. 

Thus, the retardation of the movement of a reflected particle by the liquid countercurrent is very sensitive to the degree of retardation of the bubble surface, i.e. to the DAL structure in the vicinity of the bubble front pole. The movement is fast at a weak surface retardation and is slow at a strong surface retardation. Thus, at strong surface retardation, the inertial path of the reflected particle can exceed that for the case of weakly retarded surface. The greater the tangential velocity, the shorter is the sliding time. 

There is a direct and an indirect effect of bubble surface retardation on the tangential particle velocity. The direct influence is caused by the dependence of the bubble hydrodynamic fields on the velocity distribution along its surface. The indirect influence is caused by the effect of the inertia path of a reflected particle on its tangential velocity and by the dependence of the path on the bubble surface retardation. The directions of the two effects are opposite. At the transition from a free to a retarded surface, the liquid tangential velocity diminishes at any point and the inertia path grows, which results in an increase in the tangential particle velocity. 

In the gas-liquid two-phase flows illuminated by a laser sheet, for example, the intensity of light reflected from the gas-liquid interface (mostly the gas bubble s surface) not only saturate the CCD camera, but also overwhelm the intensity of light from the seeded tracer particles in its vicinity. Fluorescent particles are often used to realize the laser-induced fluorescence (LIF) technique together with PIV (e.g., Broder and Sommerfeld, 2002 Fujiwara et al., 2004a, b Kitagawa et al., 2005 Liu et al., 2005 Tokuhiro et al., 1998,1999), so that both images of gas-liquid interface (e.g., bubble s geometry) and velocity distribution in the liquid phase around the gas bubbles can be obtained. Issues on PIV measurement of gas-liquid two-phase flows will be further illustrated in the latter sections. 

In 1842, Christian Doppler discovered that the wavelength of sound is a function of the receiver s movement. The transmitter of a Doppler flowmeter projects an ultrasonic beam into the flowing stream and detects the reflected frequency, which is shifted in proportion to stream velocity. The difference between the transmitted and reflected velocities is called the beat frequency, and its value relates to the velocity of the reflecting surfaces (solid particles and gas bubbles) in the process stream. For accurate readings it is important that the ultrasonic radiation be reflected from a representative portion of the flow stream. The main advantage of Doppler meters is their low cost, which does not increase with pipe size, whereas their main limitation is that they are not suitable for the measurement of clean fluids or gases. 

The sea salt particles produced in this way are composed mostly of sodium chloride, which reflects the composition of sea water. Among other substances, marine particulate matter also contains a large amount of sulfates (see Subsection 3.6.2). Furthermore, during their rise through the water, bubbles scavenge a lot of surface active organic materials which are partly injected into the air when the bubbles burst (see Subsection 3.3.3). 

As in Section 11.1, we consider the reflection of a particle from a flat boundary at normal sedimentation. This assumption can be used for a bubble when we are interested in impacts close to the pole, at < ,. Thus, we can simplify the expression for the normal flow of liquid setting cosine to unity and assuming that over whole section of the surface at < j the length of recoil is characterized by a constant value. In dimensionless form, the equation for calculating the inertia path change due to the opposite motion of the liquid which has a velocity distribution expressed by a linear relationship (differs from a linear second-order differential equation with constant coefficients only due to a variation of Reynolds number for a retarded particle). It reads... 

It has been demonstrated that the potential at which the contact angle measurements indicate that abraded mineral surfaces become hydrophobic in the presence of a thiol can be somewhat higher than the value at which chemisorption commences. It was considered that this reflects the presence of a significant induction time in establishing a captive bubble on such a surface. Energy barriers in particle-to-bubble attachment... 

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