Phenomena occurring when a gas bubble approaches the solution surface were revealed using high speed camera (1182 frames/sec). It was found that the ...
M. Krasowska, M. Krzan, K.Lunkenheimer, K. Małysa ”Influence of n-pentanol concentration on the bubble pulsation and bouncing” in “Surfactants and Dispersed Systems in Theory and Practice” , Materiały konferencji SURUZ, Polanica Zdrój 2003, pp. 241-245. bubble, shape pulsation, bubble bouncing, bubble rupture, foam film, solution surface, n-pentanol, adsorption
Marta KRASOWSKA*, Marcel KRZAN*, Klaus LUNKENHEIMER** and Kazimierz MAŁYSA*
INFLUENCE OF N-PENTANOL CONCENTRATION ON THE BUBBLE PULSATION AND BOUNCING Phenomena occurring when a gas bubble approaches the solution surface were revealed using high speed camera (1182 frames/sec). It was found that the bubble approaching the solution surface bounced backwards from the free surface and its shape pulsated rapidly with frequency over 1000Hz. Number of the bouncing cycles and magnitude of the shape pulsations were decreasing with increasing n-pentanol concentrations.
INTRODUCTION Immediate rupture of a bubble reaching the surface of distilled water is a commonly accepted and used criterion that the water didn’t contain any surface active impurities. The presence of surfactant prolongs the lifetime of the bubble at the solution surface as a result of formation of adsorption layers at the interfaces of the foam film formed by the bubble at the solution surface. Bikerman wrote in his textbook [1] that: “… When the bubble reaches the upper surface of the liquid, and the liquid has no foaming tendency, the bubble burst at once; that is the film separating it from the bulk gas phase immediately ruptures. When the liquid contains a foaming agent, the above film has a significant persistence, and the bubble lifts a “dome” ...”. Thus, it is rather a commonly shared understanding that the bubble is stopped at the solution surface and ruptures immediately in the case of clean water or prolongs its life for a definite time in the case of surfactant solutions. Recently, however, we recorded that during the first fractions of a second of the bubble contact with the liquid surface a lot of fascinating things can occur. The paper presents data showing phenomena occurring when the bubble arrives at the liquid surface. Rapid pulsations of the bubble’s shape occurring within the time frame below 1ms and multiple bouncing of the bubble from the solution surface are shown. The influence of adsorbed n-pentanol on the shape pulsation and bouncing of the bubble is described. Bouncing of bubbles from water surface has already been reported earlier [2,3], but obviously due to too small magnification of the images it was not noticed that the bubble shape can pulsate so rapidly. *
Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, Cracow, Poland ** Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Potsdam/Golm, Germany
EXPERIMENTAL A square glass column of cross-sectional area 40x40 mm was used to avoid optical distortions of the bubble shape recordings. At the bottom of the column a capillary of inner diameter of 0.075 mm was fixed. Single bubbles were formed at the capillary orifice using a high precision syringe pump. The phenomena occurring at the moment of the bubble contact with the solution surface were recorded at a frequency of 1182 frames/sec, using a high speed camera Speedcam 512+. The movies recorded were transformed into BMP pictures and analyzed using a PC with SigmaScanPro Image Analysis Software. The subsequent positions of the bubble, the bubble’s vertical and horizontal diameters were measured in time intervals of 0.845 ms. Details of the method of analysis and experimental set-up are described elsewhere [4]. High purity n-pentanol and four times distilled water was used for preparations of the solutions. The experiments were carried out at room temperature. RESULTS AND DISCUSSION Figure 1 shows the sequence of frames illustrating the phenomena occurring when the rising bubble approached the surface of n-pentanol solutions of concentrations
a)
b)
Fig. 1. Images of the bubble bouncing at the surface of n-pentanol solutions of concentrations: a) 0.001 M, and b) 0.005 M. Time interval between subsequent photos was 0.845 ms.
0.001 and 0.005 M. Both sequences of the photos refer to the first approach of the bubble to the solution surface. It can be straightforward seen on both photo sequences that: i) the bubble approaching the surface of the n-pentanol solution neither ruptured nor „was stopped“ immediately, ii) after contact with the solution surface (a „dome“ formation) the bubble started to move backward, i.e. opposite to the direction of the
buoyancy force, iii) the bubble started to pulsate very rapidly, changing its shape during time intervals shorter than 0.845ms. Moreover, it can be observed that at higher n-pentanol concentration the bubble’s shape pulsations and the distances of the bouncing were lower. Quantitative data on variations of the bubble velocity in the immediate vicinity of the surface of the n-pentanol solutions of concentrations 0.001 and 0.005 M are presented in Fig.2. As seen there the velocity of the bubble approach to the interface was constant, prior to deformation of the solution surface (the dome formation). The local velocity U of the bubble at a given position was calculated as:
U =
(x 2 - x 1) 2 + (y2 - y1 ) 2
t (1) where (x2, y2) and (x1, y1) are coordinates of the subsequent positions of the bubble, and t (= 0.845 ms) is the time interval between subsequent frames. In 0.001M npentanol solution this velocity of the approach was ca. 30 cm/sec, while in 0.005 M the velocity was much lower (ca. 15 cm/s). As seen in Fig. 2a the bubble bounced at least 6 times before it was stopped. During the first bounce the bubble started to move backward within a time period of ca. 1-2 ms and its maximum velocity of the backward motion was 26 cm/sec. Next, the bubble started its second approach to the surface reaching the maximum approach velocity of 16 cm/sec. The amplitude of the
[cm/s] Velocity
Fig. 2. Variations of the bubble velocity at surface of n-pentanol solutions of concentrations: a) 0.001M, and b) 0.005M
C5OH 0.001 M
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velocity variations and the bubble’s shape pulsations were decreasing with every cycle as a result of energy dissipation. In 0.005M n-pentanol solution (Fig.2b) the bubble bounced only twice and simultaneously its shape pulsations were much smaller. In distilled water the velocity of
approach of the bubble was ca. 35 cm/s and the bubble shape pulsations are the largest. The bubble bounced 4 times from the water surface within a time period of 0.08 s and ruptured. Coalescence in pure liquids was considered theoretically by Chester and Hofman [5] as a competition of two processes: i) the thinning of the liquid between the bubbles, and ii) the increase of the free energy of the system resulting from the increase of the interface area. The free energy of the system increases, at the expense of the kinetic energy and the bubbles therefore decelerate and can bounce if the thinning liquid layer didn’t reach earlier a critical thickness of rupture. The presence of surfactant (n-pentanol) prolongs the lifetime of the bubbles as a result of formation of much more stable foam films. Therefore, after a certain number of bouncing cycles accompanied by rapid shape pulsations, the bubble was stopped and the foam film began to be formed. Note, please, that high frequency of the bubble pulsation means a rapid variation of its surface area and in the consequence a lack of the equilibrium adsorption coverage. It shows how rapid the processes proceed that occur during the formation of various dispersed systems (e.g. foams, emulsions). This should always be taken in into account. Summarizing, we can say that increasing n-pentanol concentration caused: i) a decrease of the bubble velocity, ii) a diminishing of the bubble’s bouncings and of its shape pulsations, and iii) a prolongation of the bubble lifetime at the free surface, as a result of the increased stability of the thin liquid layer (foam film). Surface tension gradients induced at the bubble surface as a result of its motion through the viscous medium [6,7] are the force lowering the mobility of the gas/solution interface leading to lowering of the bubble rising velocity [7-9]. We think that also the induced gradients of the surface tension caused the damping of the bubble’s shape pulsations and bouncing. ACKNOWLEDGEMENTS
Skillful assistance of Eng. M. Barańska with the experiments is gratefully acknowledged. REFERENCES [1] Bikerman, J.J. Foams, Springer-Verlag Berlin, Heidelberg, New York, 1973; Chapter 2. p.57. [2] Kirkpatrick, R. D.; Lockett ,M. J.; Chemical Engin. Sci., 29 (1974) 2363. [3] Duinveld, P. C. Ph. D. Thesis, University of Twende, Enschede, Netherlands, 1994. [4] Krzan, M., Lunkenheimer, K., Malysa, K. Langmuir, (submitted). [5] Chesters, A.K.; Hofman, G. Appl. Sci. Research, , 38 (1982) 353. [6] Dukhin, S.S.;.Deryaguin, B.V. Zh. Fiz. Khim., 35 (1961) 1246, 1453. [7] Levich, V.G. Physico Chemical Hydrodynamics, Prentice Hall., 1962, Chapter VIII. [8] Clift, R.; Grace, J.R.; Weber, M.E. Bubbles, Drops and Particles, Academic Press, 1978. [9] Krzan, M; Malysa, K Colloids Surfaces A:, 207 (2002) 279. STRESZCZENIE Przedmiotem badań były procesy zachodzące w momencie gdy następuje kontakt wypływającej bańki z powierzchnią roztworu. Badania wykonano z zastosowaniem super szybkiej kamery cyfrowej, rejestrującej 1182 klatki na sekundę. Stwierdzono, że bańki odbijają się od powierzchni i równocześnie ich kształt ulega deformacjom z bardzo dużą częstotliwością (>1000Hz). Badania wykazały, że wszystkie parametry ruchu, takie jak liczba kolejnych odbić, ich czas trwania, prędkości lokalne baniek, odległości na jaką zostają cofnięte do roztworu bańki i rozmiar występujących deformacji, maleją wraz ze wzrostem stężenia n-pentanolu.
