Method Of Observation Initial Stage
METHOD OF OBSERVATION INITIAL STAGE
OF FOAM FORMATION
B. Jachimska, P. Warszynski, K. Małysa
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,
ul. Niezapominajek 8, 30-239 Cracow, Poland
Foams are commonly met in many industrial processes, but a general theory which explains mechanism of foams stability still does not exist. It is rather common approach to discuss formation and stability of various dispersed system in terms of their equilibrium interfacial properties. However, it should always be remembered that generally the dispersion systems are formed under non-stationary conditions. It means that it is rather doubtful that properties of freshly formed interface can always be characterized by their equilibrium values or that a shift from equilibrium is identical for various systems. It should be always taken into account that dispersions are dynamic non-equilibrium systems and therefore the dynamic properties of interface should be considered in discussion of forces determining formation and stability of the dispersions. For example diffusion time of surface active molecules to an interface which depends on surface activity and concentration of solute.
The process of foam formation consists of: i) dispersion of gas phase, ii) bubble motion to the free surface, iii) formation of single foam film, iv) formation of foam. The processes occurring in solution during bubble formation and motion towards the interface have an important influence on properties of the foams, especially for the wet foams.
A new experimental method was elaborated to determine existence of non-equilibrium coverage at interface of the bubbles arriving at the solution surface. Bubbles were formed at the capillary tip connected to the gas-tight syringe. Time of bubble formation was controlled by a precise, computer driven syringe pump. CCD camera and stroboscope lamp were applied to monitor and record the process of the bubble formation and motion was determined by multiframe video analysis.
Formation of single bubbles occurs under dynamic conditions. Attainment of the equilibrium coverage at the bubble surface would depend on
kinetics of adsorption and velocity of formation if the solution/gas interface. Only if adsorption kinetics is fast in comparison to the bubble expansion rate the surfactant adsorption coverage reaches the equilibrium at every stage of the bubble growth.
Measurements of local velocity of bubbles in neighborhood of the capillary orifice supply data confirming that deviation from equilibrium adsorption coverage\’s. Rise velocity of the single bubbles depends on bubble size, viscosity of liquid and also depends strongly on the amount of the active substance adsorbed at the bubble surface. With increased adsorption coverage the velocity of the bubble decreases due to the partial or complete retardation of the bubble surface.
When the bubble reaches the solution surface a foam film is formed. Since the adsorption coverage at the both interfaces of the foam film influence the magnitude of forces stabilizing the foam, any deviation from equilibrium coverage should be reflected in the bubble lifetime. The longest lifetime was observed in case when the equilibrium coverage was attained during bubble formation. The shortest was found when the adsorption equilibrium could not be established.
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