Vapor and gas bubbles of micrometric or nanometric dimensions in aqueous solutions are pivotal for an array of applied research areas including micro-optics, micro-hydraulic manipulation, liquid thermodynamic investigations, and biomedical applications such as drug delivery and therapy, and cell investigations. Therefore, the importance of controlled bubble generation is out of question. Fortunately, laser radiation offers appropriate methods for this fulfillment.
Large amount of energy from short laser pulses leads to rapid evaporation of water as well as bubble growth and subsequent high speed collapse and energy release in the form of shock waves (optical cavitation). On the other hand, bubble generation by CW laser radiation is interesting for a number of applications. For this case, complex dynamic fluid behavior with superheating as well as bubble formation is possible. Above all, availability of near-stationary bubble evolution regimes allowing for their analysis and control is quite attractive.
Near-stationary bubbles with longer lifespans offer possibilities for highly selective chemical actions, accurate mass transfer, precise transport of biological species, and production of adjustable micro-optical devices. In a recent collaboration Ukrainian researchers at Chernivtsi National University and Odessa Mechnikov National University with their colleagues at DTU Fotonik in Denmark demonstrated a clear-cut approach for microbubbles generation and their ensembles in water suspension with colloidal light-absorbing nanoparticles, which was based on suspension heating by a focused beam of the near-infrared laser. Their work is published in Optics Express.
The authors focused the laser radiation within a 5° cone angle in the cuvette with the water suspension, and without spatial filtering, a focal spot with nearly 100 mm diameter was formed. The authors visualized the cuvette processes by white light illumination and recorded them with a camera. During this time, the infrared-protective cover was removed allowing the authors to view bubbles illuminated by the visible light and the focal spot due to scattered light from the infrared laser beam.
The infrared laser power could be adjusted within 0.1 and 3 W. The cuvette contained water suspension of black ink pigment for jet printers. Every ink pigment particle had a spherical polymer core with a resin cover.
After superheating the suspension, threshold conditions for generation of bubbles were realized in the illuminated area, and they appeared at random times consecutively. When the heating was reduced below the threshold, new bubbles formation was suppressed, but existing bubbles remained stable and could increase in size, shrink or collapse counting on the power of the incident laser. This provided an efficient route for controlling the number of bubbles and their sizes.
The researchers observed that the bubbles remained together in the near-surface liquid layer within an area corresponding to the spot of the focused laser beam. This was linked to the excess temperature needed for the existence of bubbles, and the surface tension gradient pushing the bubbles to the most heated area. Simultaneously, electric double layers were developed at liquid-gas interfaces. These double layers generated repulsive interaction that prevented bubbles coalescence and adhesion to the water surface. As a result, quasi-ordered spatial distributions of bubbles are formed that are stable against mechanical perturbations. It was observed that convectional currents did not destroy adherence of bubbles to the bright spot, but affected particular bubbles redistribution.
The outcomes of this study by Professor Oleg Angelsky and colleagues are helpful for various applications involving microbubbles, particularly in association with accurate transportation and delivery of species in biomedical and nano-engineering.
O. V. Angelsky,1, A. Ya. Bekshaev,2 P. P. Maksimyak,1 A. P. Maksimyak,1 S. G. Hanson,3 and S. M. Kontush2. Controllable generation and manipulation of micro-bubbles in water with absorptive colloid particles by CW laser radiation. Vol. 25, No. 5 | 6 Mar 2017 | OPTICS EXPRESS 5232.Show Affiliations
1Correlation Optics Department, Chernivtsi National University, 2, Kotsyubinsky Str., Chernivtsi 58012, Ukraine.
2Research Institute of Physics, Odessa I.I. Mechnikov National University, Dvorianska 2, Odessa 65082, Ukraine.
3DTU Fotonik, Department of Photonics Engineering, DK-4000 Roskilde, Denmark.
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