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Method of capillary self-oscillations for monodispersed aerosol generation
J. Aerosol Sci. Vol. 30, Suppl. I, pp. $389-S390, 1999 © 1999 Published by Elsevier Science Ltd. All fights reserved Printed in Great Britain 0021-8502/99/$ - see front matter
Pergamon
METHOD OF CAPILLARY SELF.OSCILLATIONS FOR MONODISPERSED AEROSOL GENERATION S.M. KONTUSH t, K.V.ROMANOV I, A. Ya. BEKSHAEV2, S.S. MIKHAILOVSKY 3 ,S.S. RYBAK 1 tOdessa State Academy of Refrigeration, Dvorianskaya, l/3,Odessa, 270100, Ukraine 2Odessa State University, Dvoriansk~tya, 2, Odessa, 270026, Ukraine 3Odessa State Institute of Hydro-Meteorology, Odessa, Ukraine KEYWORDS Aerosol generation; Monodispersed droplets; Capillary waves; Self-stabilization Obtaining single droplets or one-dimensional fluxes of identical droplets is very important for a lot of experimental problems in the aerosol physics, hydro-meteorology, chemical and biological industry. One of commonly known processes to create the droplets is based on gas bubble destruction near the surface with consequent formation of the cumulative splash and ejection of liquid particles. Such a method is often realized in nature but seldom in laboratory practice; nevertheless it was used, e.g., for the study of droplets fusion (Whelpdale, List, 1969). The main question relating to this method is how wide is the droplet size spectrum. It is quite natural to expect that the droplets will be monodispersed with high accuracy if the reproducibility of the particle formation conditions, first of all bubble sizes and figures, is provided. It is the approach that was realized in the work (Kontush, Romanov, 1971) for generation of monodispersed droplets (Fig. 1). Vn, kHz O
4
o o a
\\.
h,
lie
10
5 ',
II
8
k~
II IW II //t-'//~
~1.~
~
/
,
3/
,
l
; ~, ,. / , . - / > / . .
~2
2
P
0
Fig. 1
II
It
10
20
30
Sn
Fig. 2
Here a thin layer of liquid 3 is placed on the surface of substrate 1 with round channel 2. At the little extra air pressure within the channel a bubble 4 is formed over it. Obviously, the bubble decay occurs when its size becomes approximately equal to liquid layer thickness, and this may guarantee necessary reproducibility. Under proper selection of the cannel and the liquid film parameters and proper air pressure at the channel input such a generator, indeed, can produce a "fountain" of droplets (see Fig. 1), but sizes as well as velocities of the droplets turn out to be not fully identical. This happens due to the fact that a bubble destruction generates within the film capillary waves that irregularly disturb the formation process of the following droplet.
$389
$390
Abstractsof the 1999EuropeanAerosolConference
In order to overcome this drawback some ways of damping the film oscillation seem to be reasonable, but perturbation caused by the bubble destruction is generally too strong to achieve noticeable success by this manner. Under such conditions it seems relevant to struggle for the most regularity of the film oscillations, which may be attained, in particular, by the use of a resonator for capillary waves. Such a resonator can be created by surrounding the channel with coaxial metallic ring 5 as is depicted in Fig. 1 (Gabrusenok, Kontush 1968). In this system only distinct set of eigen oscillatory modes can be excited, to which the discrete series of eigen frequencies corresponds. These frequencies can be calculated according to the theory of capillary waves (Rayleigh, 1926) and are determined by the relation
v" = 2n ~k RJ p
k, R J
where R is the ring radius, T is the liquid film surface tension, h 0 - its nominal thickness, p - the liquid density and s, is n -th root of the Bessel function Jl(x). For the case of water layer with h 0 = 60 }tm and R = 1 mm eigenfrequencies are given by Fig. 2. Repeating events of bubble emergence and destruction constitute the periodic action that excites the resonator mode whose frequency is closest to frequency of the bubbles' formation. On its part, this mode provides peculiar self-support and self-tuning, creating in some moments conditions that force bubbles to burst concertedly with the film motion (e.g., due to reducing the film thickness over the channel). Therefore, the self-osciUatory mode of operation with high reproducibility of all parameters of the system action. As a result, the generated droplets have strictly alike sizes and velocities. The generator tuning is rendered by changing the air pressure at the channel input p or the nominal liquid film thickness h o. In our experiments following values of the system parameters were obtained: p = (1.2 - 1.5).105 Pa, h0=40 - 150 pro, the channel diameter 10 - 20 ~tm; droplet diameters varied within 6 to 30 pro, their velocity lay between 0.5 and 3.0 m/s and frequency v = 1 - 10 kHz. The monodispersity of the droplets was estimated with microscope to be within few percents. This principle of monodispersed aerosol generation has a sensible advantage over all other ones due to absence of mechanically moving details. Its main shortcoming, revealed by experiments, consists in practical difficulty to achieve the stable working regime but we hope this obstacle is ofteclmical character and further investigations will eliminate it. Thus, the method proposed is expected to be perspective and useful in various applications. REFERENCES
Whelpdale D.M, List R. (1969) J. Attn. Sci., 26, 305. Kontush S.M., Romanov K.V. (1971) The monodispersed droplets jet generation under the gas blowing through a thin liquid layer, Physics of Air Dispersed Systems, No. 4, 38-43, Kiev (in Russian). Gabrusenok P.S., Kontush S.M. (1968) Method of aerosol generation, USSR Patent, J~_ 273511. Rayleigh (1926) The Theory of Sound, McMillan & Co, London.
Pergamon
METHOD OF CAPILLARY SELF.OSCILLATIONS FOR MONODISPERSED AEROSOL GENERATION S.M. KONTUSH t, K.V.ROMANOV I, A. Ya. BEKSHAEV2, S.S. MIKHAILOVSKY 3 ,S.S. RYBAK 1 tOdessa State Academy of Refrigeration, Dvorianskaya, l/3,Odessa, 270100, Ukraine 2Odessa State University, Dvoriansk~tya, 2, Odessa, 270026, Ukraine 3Odessa State Institute of Hydro-Meteorology, Odessa, Ukraine KEYWORDS Aerosol generation; Monodispersed droplets; Capillary waves; Self-stabilization Obtaining single droplets or one-dimensional fluxes of identical droplets is very important for a lot of experimental problems in the aerosol physics, hydro-meteorology, chemical and biological industry. One of commonly known processes to create the droplets is based on gas bubble destruction near the surface with consequent formation of the cumulative splash and ejection of liquid particles. Such a method is often realized in nature but seldom in laboratory practice; nevertheless it was used, e.g., for the study of droplets fusion (Whelpdale, List, 1969). The main question relating to this method is how wide is the droplet size spectrum. It is quite natural to expect that the droplets will be monodispersed with high accuracy if the reproducibility of the particle formation conditions, first of all bubble sizes and figures, is provided. It is the approach that was realized in the work (Kontush, Romanov, 1971) for generation of monodispersed droplets (Fig. 1). Vn, kHz O
4
o o a
\\.
h,
lie
10
5 ',
II
8
k~
II IW II //t-'//~
~1.~
~
/
,
3/
,
l
; ~, ,. / , . - / > / . .
~2
2
P
0
Fig. 1
II
It
10
20
30
Sn
Fig. 2
Here a thin layer of liquid 3 is placed on the surface of substrate 1 with round channel 2. At the little extra air pressure within the channel a bubble 4 is formed over it. Obviously, the bubble decay occurs when its size becomes approximately equal to liquid layer thickness, and this may guarantee necessary reproducibility. Under proper selection of the cannel and the liquid film parameters and proper air pressure at the channel input such a generator, indeed, can produce a "fountain" of droplets (see Fig. 1), but sizes as well as velocities of the droplets turn out to be not fully identical. This happens due to the fact that a bubble destruction generates within the film capillary waves that irregularly disturb the formation process of the following droplet.
$389
$390
Abstractsof the 1999EuropeanAerosolConference
In order to overcome this drawback some ways of damping the film oscillation seem to be reasonable, but perturbation caused by the bubble destruction is generally too strong to achieve noticeable success by this manner. Under such conditions it seems relevant to struggle for the most regularity of the film oscillations, which may be attained, in particular, by the use of a resonator for capillary waves. Such a resonator can be created by surrounding the channel with coaxial metallic ring 5 as is depicted in Fig. 1 (Gabrusenok, Kontush 1968). In this system only distinct set of eigen oscillatory modes can be excited, to which the discrete series of eigen frequencies corresponds. These frequencies can be calculated according to the theory of capillary waves (Rayleigh, 1926) and are determined by the relation
v" = 2n ~k RJ p
k, R J
where R is the ring radius, T is the liquid film surface tension, h 0 - its nominal thickness, p - the liquid density and s, is n -th root of the Bessel function Jl(x). For the case of water layer with h 0 = 60 }tm and R = 1 mm eigenfrequencies are given by Fig. 2. Repeating events of bubble emergence and destruction constitute the periodic action that excites the resonator mode whose frequency is closest to frequency of the bubbles' formation. On its part, this mode provides peculiar self-support and self-tuning, creating in some moments conditions that force bubbles to burst concertedly with the film motion (e.g., due to reducing the film thickness over the channel). Therefore, the self-osciUatory mode of operation with high reproducibility of all parameters of the system action. As a result, the generated droplets have strictly alike sizes and velocities. The generator tuning is rendered by changing the air pressure at the channel input p or the nominal liquid film thickness h o. In our experiments following values of the system parameters were obtained: p = (1.2 - 1.5).105 Pa, h0=40 - 150 pro, the channel diameter 10 - 20 ~tm; droplet diameters varied within 6 to 30 pro, their velocity lay between 0.5 and 3.0 m/s and frequency v = 1 - 10 kHz. The monodispersity of the droplets was estimated with microscope to be within few percents. This principle of monodispersed aerosol generation has a sensible advantage over all other ones due to absence of mechanically moving details. Its main shortcoming, revealed by experiments, consists in practical difficulty to achieve the stable working regime but we hope this obstacle is ofteclmical character and further investigations will eliminate it. Thus, the method proposed is expected to be perspective and useful in various applications. REFERENCES
Whelpdale D.M, List R. (1969) J. Attn. Sci., 26, 305. Kontush S.M., Romanov K.V. (1971) The monodispersed droplets jet generation under the gas blowing through a thin liquid layer, Physics of Air Dispersed Systems, No. 4, 38-43, Kiev (in Russian). Gabrusenok P.S., Kontush S.M. (1968) Method of aerosol generation, USSR Patent, J~_ 273511. Rayleigh (1926) The Theory of Sound, McMillan & Co, London.