Microfluidic Investigation of the Ion-Specific Effect on Bubble Coalescence in Salt Solutions.

A microfluidic method was developed to study the ion-specific effect on bubble coalescence in salt solutions. Compared with other reported methods, microfluidics provides a more direct and accurate means of measuring bubble coalescence in salt solutions. We analyzed the coalescence time and approach velocity between bubbles and used simulation to investigate the pressure evolution during the coalescence process. The coalescence time of the three salt solutions decreased initially and then increased as the concentration of the salt solution was increased. The concentration with the shortest coalescence time is considered as the transition concentration (TC) and exhibits ion-specific. At the TC, the change in coalescence time indicates a shift in the effect of salt on bubble coalescence from facilitation to initial inhibition. Meanwhile, it can be seen that the sodium halide solutions significantly inhibit the bubble coalescence and the inhibition capability follows the order NaCl > NaBr > NaI. The results of the approach velocity show that the coalescence time decreases with increasing approach velocity, as well as the approach velocity was strongly influenced by concentration. The approach velocity undergoes a significant change at the TC. Furthermore, simulations of bubble coalescence in the microchannel indicate that the vertical pressure gradient at the center point of the bubble pairs increases as bubbles approach, driving liquid film drainage until bubble coalescence. The pressure at the center of the bubble pair reaches the maximum when the bubbles have first coalesced. It was further revealed that the concentration of the salt solution has a significant impact on the maximum pressure, as evidenced by the observed trend of decreasing pressure values with increasing concentrations.

Study on the Strengthening Mechanism of a MIBC-PEG Mixed Surfactant on Foam Stability.

In the flotation process, the frother, which is typically a surfactant, can be added to the pulp to reduce the surface tension and create stable foam. Currently, the nonionic mixed surfactant is widely employed as the frother for fine coal flotation. In this study, we focused on examining the foam properties of a mixed surfactant comprising short-chain methyl isobutyl carbinol (MIBC) and long-chain polyethylene glycol-1000 (PEG). Analytical techniques such as surface tension measurement, dynamic foam stability measurement, bubble morphology observation, and foam film drainage measurement were used to investigate the foam properties in single and mixed surfactant solution from a macroscopic scale to a microscopic scale. The surface tension results indicated that PEG exhibited higher surface activity than MIBC, and the addition of PEG to MIBC resulted in a significant reduction in solution surface tension. The dynamic foam stability analysis revealed that the incorporation of a small amount of PEG into MIBC solution notably improved foam stability. Furthermore, the addition of PEG to the MIBC solution led to a shift in the bubble size distribution curve from a "double peak" to a "single peak" shape. This shift indicated a substantial reduction in bubble size, indicating an enhanced inhibition of bubble coalescence. Additionally, the liquid film drainage rate was significantly slowed down, and the stability of the liquid film was improved upon the addition of PEG to MIBC. This improvement can be attributed to the synergistic effect of MIBC and PEG molecules adsorbed at the gas-liquid interface. The synergistic effect of mixed MIBC-PEG was due to the additional surface tension gradient created by the difference in surface activity between PEG and MIBC. This surface tension gradient enhances the Marangoni flow of surfactant molecules, thereby improving the self-healing ability of the liquid film and increasing its stability.

New Insight into Temperature Effects on Low-Rank Coal Flotation Using Diesel as a Collector.

Efficient flotation of low-rank coal is of great significance for the development of green and low-carbon cycles. Temperature is a crucial parameter of flotation, but the mechanism of its effect on flotation lacks understanding. In this paper, the mechanism was studied by kinetic flotation, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, low-temperature liquid-nitrogen adsorption (LP-N2A), X-ray photoelectron spectroscopy (XPS), and molecular dynamics simulation. The flotation combustible recovery gradually decreases as temperature rises. Compared with 60 °C, the combustible recovery at 5 °C increases by 18.13%. The desorption energy for oil droplets decreases as the temperature rises. As a result, the oil droplets are easier to desorb at high temperatures. The SEM and LP-N2A results demonstrate that the pores and fractures of the coal sample are well developed. Also, the oil-water interfacial tension and viscosity of oil droplets decrease as the temperature rises, while the diffusion ability increases. These increase the volume of oil droplets that penetrate into the pores, resulting in poor spreadability of oil droplets on the coal surface. The average volume of bubbles gradually increases as temperature rises, which renders the flotation foam unstable and worsens the flotation. Therefore, the flotation performance is better at low temperatures.