Neutron radiography of an anisotropic drainage flow.

Liquid drainage through foam is dominated by gravity, capillary, and viscous forces. The liquid is conducted by an isotropic network of Plateau borders; however, imposed stress changes the alignment of the foam's structural elements. Previous numerical simulations predicted that a vertical drainage flow will be deflected horizontally if the foam is sheared. We investigated this phenomenon by measuring the distribution of the liquid fraction within a foam formed in a flat rectangular cell. The foam was subjected to shear stress under a forced liquid supply from the top of the cell. Neutron radiographies of unchanged and sheared foam were analyzed to extract measurements of the liquid fraction. Deflections in the distribution of the drainage liquid were detected and found to be positively correlated with increasing foam shear.

A methodology to implement a closed-loop feedback-feedforward level control in a laboratory-scale flotation bank using peristaltic pumps.

This paper describes the implementation of a level control strategy in a laboratory-scale flotation system. The laboratory-scale system consists of a bank of three flotation tanks connected in series, which mimics a flotation system found in mineral processing plants. Besides the classical feedback control strategy, we have also included a feedforward strategy to better account for process disturbances. Results revealed that the level control performance significantly improves when a feedforward strategy is considered. This methodology uses peristaltic pumps for level control, which has not been extensively documented even though: (1) peristaltic pumps are commonly used in laboratory-scale systems, and (2) the control implementation is not as straightforward as those control strategies that use valves. Therefore, we believe that this paper, which describes a proven methodology that has been validated in an experimental system, can be a useful reference for many researchers in the field.•Preparation of reagents to ensure that the froth stability of the froth layer is representative of an industrial flotation froth.•Calibration of instruments - convert the electrical signal from PLCs to engineering units.•Tuning PI parameters using SIMC rules by performing step-changes in each flotation cell.

Effect of Particle Size on the Rising Behavior of Particle-Laden Bubbles.

The rising behavior of bubbles, initially half and fully coated with glass beads of various sizes, was investigated. The bubble velocity, aspect ratio, and oscillation periods were determined using high-speed photography and image analysis. In addition, the acting forces, drag modification factor, and modified drag coefficient were calculated and interpreted. Results show that the aspect ratio oscillation of the rising bubbles is similar, irrespective of the attached particle size. As the particle size is increased, the rising bubbles have a lower velocity and aspect ratio amplitude, with the time from release to each aspect ratio peak increasing. Higher particle coverage is shown to decrease the bubble velocity and dampen the oscillations, reducing the number of aspect ratio peaks observed. The highest rise velocities correspond to the lowest aspect ratios and vice versa, whereas a constant aspect ratio yields a constant rise velocity, independent of the particle size. Force analysis shows that the particle drag modification factor increases with the increased particle size and is greatest for fully laden bubbles. The modified drag coefficient of particle-laden bubbles increases with the increased particle size, although it decreases with the increased Reynolds number independent of the particle size. The drag force exerted by the particles plays a more dominant role in decreasing bubble velocities as the particle size increases. The results and interpretation produced a quantitative description of the behavior of rising particle-laden bubbles and the development of correlations will enhance the modeling of industrial applications.

A model for the stability of films stabilized by randomly packed spherical particles.

Particle stabilized thin films occur in a range of industrial applications where their properties affect the efficiency of the process concerned. However, due to their dynamic and unstable nature they are difficult to observe experimentally. As such, a tractable way of gaining insight into the fundamental aspects of this complicated system is to use computer simulations of particles at interfaces. This paper presents modeling results of the effect of nonuniform packing of spherical particles on the stability of thin liquid films. Surface Evolver was used to model cells containing up to 20 particles, randomly packed in a thin liquid film. The capillary pressure required to rupture the film for a specific combination of particle arrangement, packing density, and contact angle was identified. The data from the periodic, randomly packed models has been used to find a relationship between particle packing density, contact angle, and critical capillary pressure which is refined to a simple equation that depends on the film loading and contact angle of the particles it contains. The critical capillary pressure for film rupture obeys the same trends observed for particles in regular 2D and 3D packing arrangements. The absolute values of P*(crit), however, are consistently lower than those for regular packing. This is due to the irregular arrangement of the particles, which allows for larger areas of free film to exist, lowering the critical capillary pressure required to rupture the film.

The size of films in dry foams.

The relationship between the size of films in a dry foam and the size of the bubbles is investigated. The study was carried out using foam structures simulated using Surface Evolver, with the structures covering a wide range of poly-dispersities. It was found that the most important factor influencing the size of a film is the size of the smaller bubble to which it is attached. The larger bubble does have an influence, but it is much smaller, with the film size increasing by approximately 80% as the larger bubble goes from the same size as the smaller bubble to infinitely large. The relationship between a film's size and the size of the two bubbles to which it is attached was found to be independent of the underlying bubble size distribution, with the probability distribution for the size of the film depending only on the size of the neighbouring bubbles.