Perfectly Monodisperse Microbubbling by Capillary Flow Focusing
Original entry: Nan Niu, APPHY 226, Spring 2009
by Dr. Alfonso M. Gañán-Calvo and Dr. José M. Gordillo
This article is published by two Spanish scientists Dr. Alfonso M. Gañán-Calvo and Dr. José M. Gordillo. My decision to spend much time with this article is due to its detailed experimental examination of capillary flow. In recent years, macro-scale gas bubbles have countless applications in science and technology. In particular, fundamental medical applications of micron size bubbles range from ultrasound contrast agents to thrombus destruction, targeted drug delivery, tumor destruction, and even as an enhanced gene vector. Moreover, the size control of the microbubbles produced is critical in all these applications. Many important engineering and science processes are driven by surface tension or physicochemical interface forces and require a mass production of these microsubstrates. In this article, the authors presented a simple microfluidics phenomenon which allows the efficient mass production of micron size gas bubbles with a perfectly monodisperse and controllable diameter. Also, the authors also described in detail the physics of the phenomenon and obtain closed expressions for the bubble diameter from a large set of experimental results.
Introducing the experiment techniques and procedure, the authors describe a capillary flow phenomenon which is based on the focusing effect of a liquid stream through a small orifice that provokes the tip streaming of a gas bubble attached to a feeding capillary tube. In the particular configuration of the experiment set-up, a gas is continuously supplied from a capillary tube positioned upstream in the vicinity of an orifice through which a liquid stream is forced. At the mouth of the capillary tube, a cusp-like attached bubble forms, from whose apex a steady gas ligament issues and is "focused" through the orifice by the surrounding liquid stream as shown in a) of the first picture on the right. Furthurmore, the gas ligament or hollow microjet then breaks up very soon into homogeneous size microbubbles as shown in b) of the first picture.
In the experiment, a gas ligament surrounded by liquid is produced. The authors believe the unstable nature of the gas ligament is indeed the cause of the radically different behavior of a laminar gas ligament from a laminar liquid ligament. Such instability of the gas ligament provokes its rapid breakup into microbubbles. Moreover, the nonlinear evolution of the local breakup of the ligament at the orifice involves a self-excited stable nonlinear saturation state (a limit cycle). This nonlinear phenomenon involves a strong self-locking of the breakup frequency, which yields the observed stunning regularity of the microbubbles produced as shown in the first two pictures blow. The bottom picture is a zoom-in version of the picture right below. In particular, for large enough gas to liquid flow, very light hollow microdroplets are obtained and can provide a small aerodynamic diameter while conveying a large surface area, of interest for many applications.
During the process, the gas flow rates have been introduced by a high precision pump. The liquid flow rate is measured with a digital weight and a clock. Seven different liquids (water-ethanol and waterglycerol mixtures) have been used. Monodisperse microbubbles with diameters ranging from about 5 to 120 mm have been measured.
The findings were quite simple and straightforward. Essential results were reported and plots were generated based on experiments results and are compared with theoretically calculated values. As revealed in the experiment, when the Reynolds number is large, the nondimensional breakup frequency depends on the boundary layer thickness. As a validation, the authors measured bubble diameter, Qg, Ql, and calculated the ligament diameter. The picture above shows graph of obtained experimental results and compares with theoretically calculated values.