Formation and influence of the dynamic adsorption layer on kinetics of the rising bubble collisions with solution/gas and solution/solid interfaces.

BACKGROUND: The DAL (dynamic adsorption layer) formation, that is, the establishment of uneven distribution of adsorption coverage over the rising bubble surface, with significantly diminished coverage at the upstream pole, is the factor of crucial importance for the bubble motion parameters and kinetic of the bubble collisions with various interfaces. The DAL presence can influence the stability of the thin liquid films formed by the colliding bubble at solution/gas and solution solid interfaces. AIM: The purpose of this paper is to critically review the existing state of art regarding the influence of the DAL formation and existence on the bubble motion parameters as well as kinetics of coalescence at free solution surface and three phase contact (TPC) formation at solid/liquid interfaces of different hydrophilic/hydrophobic properties. CONCLUSIONS: Despite the fact that up to now there is no direct experimental evidence showing DAL existence, it is documented by experimental data showing clear correlation between bubble local velocity variations and shape pulsations as well as lifetimes of the liquid film formed by the colliding bubble at gas/liquid and gas/solid interfaces.

Dynamics of Rear Stagnant Cap formation at the surface of spherical bubbles rising in surfactant solutions at large Reynolds numbers under conditions of small Marangoni number and slow sorption kinetics.

On the surface of bubbles rising in a surfactant solution the adsorption process proceeds and leads to the formation of a so called Rear Stagnant Cap (RSC). The larger this RSC is the stronger is the retardation of the rising velocity. The theory of a steady RSC and steady retarded rising velocity, which sets in after a transient stage, has been generally accepted. However, a non-steady process of bubble rising starting from the initial zero velocity represents an important portion of the trajectory of rising, characterized by a local velocity profile (LVP). As there is no theory of RSC growth for large Reynolds numbers Re ¼ 1 so far, the interpretation of LVPs measured in this regime was impossible. It turned out, that an analytical theory for a quasi-steady growth of RSC is possible for small Marangoni numbers Ma « 1, i.e. when the RSC is almost completely compressed, which means a uniform surface concentration Γ(θ)=Γ(∞) within the RSC. Hence, the RSC angle ψ(t) is obtained as a function of the adsorption isotherm parameters and time t. From the steady velocity v(st)(ψ), the dependence of non-steady velocity on time is obtained by employing v(st)[ψ(t)] via a quasi-steady approximation. The measurement of LVP creates a promising new opportunity for investigation of the RSC dynamics and adsorption kinetics. While adsorption and desorption happen at the same localization in the classical methods, in rising bubble experiments desorption occurs mainly within RSC while adsorption on the mobile part of the bubble surface. The desorption flux from RSC is proportional to αΓ(∞), while it is usually αΓ. The adsorption flux at the mobile surface above RSC can be assumed proportional to ßC0, while it is usually ßC0(1-Γ/Γ(∞)). These simplifications may become favorable in investigations of the adsorption kinetics for larger molecules, in particular for globular proteins, which essentially stay at an interface once adsorbed.

Novel method and parameters for testing and characterization of foam stability.

The paper presents novel parameters which can be used for a swift characterization of all kinds of liquid foams. The procedure of the automated method developed consists of introducing a predefined volume of gas into the test solution contained in a cylindrical glass column at constant flow rate. The levels of the foam and of the solution are recorded simultaneously in dependence on time using a photosensor system. Two novel parameters, called time of deviation and time of transition, have been derived on the basis of simultaneous measurements of the changes in the foam volume (DeltaV(F)) and the corresponding volume of the drained solution (DeltaV(S)). These parameters enable one to distinguish three different stages of the foam decay, and on their basis the foam stability can be predicted, irrespective of whether constituting an unstable (wet) or a (meta)stable (dry) foam system. The validity of the method elaborated is demonstrated by applying various unstable and stable foam systems, including biological surfactants such as sugar and lung surfactants.

Air at hydrophobic surfaces and kinetics of three phase contact formation.

This review focuses on the importance of air presence at hydrophobic solid surfaces for wetting film rupture and kinetics of three phase contact formation. Affinity to air is a typical feature of hydrophobic surfaces, but it has been often either overlooked or not taken into consideration. When the hydrophobic surface, contacted earlier with air, is immersed into water then air can stay attached to the surface. The origin of long range hydrophobic forces and data showing that these interactions were due to the bridging of nanobubbles attached to the hydrophobic surfaces are discussed. A major part of the review is devoted to the description and analysis of data showing that air (nano-, micro-bubbles and/or air film) present at a hydrophobic surface facilitated rupture of the liquid film and three phase contact formation during bubble collisions with flat Teflon plates of different surface roughness. Although all Teflon plates were highly hydrophobic (contact angles ca. 100 degrees -130 degrees ) the time of the three phase contact (TPC) formation and attachment of the colliding bubble was strongly affected by the plate surface roughness. The time of the TPC formation was shortened from over 80 down to 2-3 ms when the roughness was increased from below 1 microm to over 50 microm. Higher surface roughness means that larger amounts of air was entrapped during the Teflon plates' immersion in water. Additional experimental evidence is given, showing that facilitation of the TPC formation and the bubble attachment was due to air presence and re-distribution over the Teflon surfaces: i) prolonging the plate immersion time resulted in quicker attachment; ii) irregular and disappearing air pockets were recorded at a Teflon surface; iii) a satellite bubble left at a Teflon surface during the first collision facilitated the attachment; iv) attachment always occurred during the first collision in the case of a very rough "Teflon V" surface, but in highly concentrated n-octanol and n-heptanol solutions there was bouncing and attachment occurred during the second collision, moreover; v) the degree of bubble kinetic energy transferred into surface energy was significantly smaller during collisions with hydrophobic (Teflon) surfaces than with the hydrophilic ones. The mechanism of air entrapment and redistribution over Teflon plates immersed in water is presented.

Wetting films in attachment of the colliding bubble.

The importance of wetting films in three phase contact formation and attachment of the bubble colliding with different solid surfaces is described The paper reviews main factors determining stability, drainage, rupture, three phase contact (TPC) formation and expansion of the TPC perimeter under dynamic conditions. There are shortly reviewed specific forces of interactions (DLVO and non-DLVO), kinetics of drainage and mechanisms of the wetting film rupture, as well as the TPC formation and expansion. The review is focused on the role of hydrophobic/hydrophilic properties, surface roughness and heterogeneity of the solid substrates for the wetting film stability and rupture under dynamic conditions. Phenomena occurring during collisions of the rising bubble with solid plates of different surface properties are discussed in relation to the kinetics of the wetting films drainage and TPC formation. It is showed that stability and drainage kinetics of the wetting films are decisive for the TPC formation and attachment of the colliding bubble.

Air-facilitated three-phase contact formation at hydrophobic solid surfaces under dynamic conditions.

The paper presents results documenting the mechanism of facilitation of the three-phase contact (TPC) formation due to gas entrapped during immersion of hydrophobic (Teflon) plates into distilled water and n-octanol solutions. Collisions, bouncing, the time scale of the TPC formation, and bubble attachment to Teflon plates of different surface roughness were studied using a high-speed camera. Processes occurring during the microscopic wetting film formation at the Teflon plates were monitored using the microinterferometric method (Scheludko-Exerowa cell). A strong relation between the time necessary to form a stable TPC and the roughness of the Teflon was observed. The higher the Teflon roughness was the shorter the time for the TPC formation. This effect can be attributed to two factors: (i) local differences in the thickness of the thinning intervening liquid layer (quicker attainment of rupture thickness at pillars of rough surface) and/or (ii) the presence of gas at the hydrophobic surface. Experimental findings, that (i) prolongation of the plate immersion time resulted in quicker TPC formation, (ii) white irregular and disappearing spots (air pockets) were recorded during the wetting film formation, and (iii) high n-octanol concentration caused prolongation of the time of the TPC formation, show that attachment (TPC formation) of the colliding bubble to hydrophobic surfaces was facilitated by air entrapped at the Teflon plates (and re-distributed) during their immersion into water phase. Thus, on collision instead of solid/gas wetting liquid film a thin gas/liquid/gas foam film was formed which facilitated the TPC formation.

Influence of surface active substances on bubble motion and collision with various interfaces.

Bubble motion as a function of distance from a point of its detachment and phenomena occurring during the bubble approach and collision with liquid/gas and liquid/solid interfaces in pure water and solutions of various surface active substances are described and discussed. It is showed that presence of surface active substance has a profound influence on values of the terminal velocity and profiles of the local velocity. At low solutions concentrations there are three distinct stages in the bubble motion: (i) a rapid acceleration, (ii) a maximum velocity value followed by its monotonic decrease, and (iii) attainment of the terminal velocity, while at high concentrations (and in pure water) there are only stages (i) and (iii). It is showed that the bubble terminal velocity decreases rapidly at low surfactant concentration, but there can be found some characteristic concentrations (adsorption coverage's) above which the velocity almost stopped to decrease. Immobilization of the bubble surface resulting from adsorption of the surface active substances (surface tension gradients inducement) causes over twofold lowering of the bubble velocity. Presence of the maximum on the local velocity profiles is an indication that a stationary non-uniform distribution of adsorption coverage (needed for immobilization the bubble interface) was not established there. When the rising bubble arrives at liquid/gas interface or liquid/solid interface there can be formed either foam or wetting film or three-phase contact (TPC). It is showed that prior to the foam and/or wetting film formation the bubble colliding with the interfaces can bounce backward and simultaneously its shape pulsates rapidly with a frequency over 1000 Hz. It is rather unexpected that even in the case of the free surface the bubble's shape and consequently its surface area can vary so rapidly. It shows straightforward that on such a rapidly distorted interface the adsorption coverage can be very different from that at equilibrium. This fact should be taken into account more appropriately in the discussion of the mechanism of formation and stabilization of various dispersed systems (e.g. foams, emulsions). Bubble collision with solids and formation of the three-phase contact is a necessary condition for flotation separation. It is rather common understanding that immediate attachment should occur in the case of hydrophobic surface, while there should be no attachment in the case of the hydrophilic ones. It is reported that even in the case of such hydrophobic solid surface as Teflon, the bubble attachment did not need to occur at first collision and in distilled water the bubble can bounce a few times without attachment. Presence of frother facilitates the bubble attachment to hydrophobic solid surface. Time scale of the TPC formation is very short, of an order of single ms. It was observed that presence of a micro-bubble at the solid surface facilitated drastically an attachment of the colliding bubble. Roughness of Teflon surface increases probability of the bubble attachment-most probably-as a result of higher probability of micro- and/or nano-bubbles presence at the solid surface.