Perfectly Monodisperse Microbubbling by Capillary Flow Focusing

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Abstract

Microscopic gas bubbles appear and 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. 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 to obtain a sufficient yield. This article presents a simple microfluidics phenomenon which allows the efficient mass production of micron size gas bubbles with a perfectly monodisperse and controllable diameter. It also describes the physics of the phenomenon and obtain closed expressions for the bubble diameter as a function of the liquid and gas properties, geometry, and flow parameters, from a large set of experimental results.

Experiment

(a) Cusplike bubble, attached to a capillary gas-feeding tube, from whose cusp a gas ligament issues through the orifice placed in front of the capillary. (b) Stream of gas bubbles issuing from the orifice. Picture taken with an exposure time of 1 ms. (c) Sketch of the region about the exit orifice, showing the steady and absolutely unstable regions of the gas ligament.

Researchers describe a capillary flow phenomenon which resorts on the focusing effect of a liquid stream trough a small orifice which provokes the tip streaming of a gas bubble attached to a feeding capillary tube. In the particular configuration 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 cusplike 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. The gas ligament or hollow microjet then breaks up very soon into homogeneous size microbubbles as shown in b) of the first picture.

A gas ligament surrounded by liquid is produced. The physical explanation of the radically different behavior of a laminar gas ligament from a laminar liquid ligament results on the absolutely unstable nature of the gas ligament. The absolute 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” globally stable nonlinear saturation state (a limit cycle) with a saturated limit cycle amplitude. 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, one can obtain very light hollow microdroplets which provide a small aerodynamic diameter while conveying a large surface area, of interest for many applications.

(a) The liquid used to focus the gas is expelled into the same liquid. (b) The liquid with the gas bubbles is expelled into air, and a gas filled liquid jet with the diameter of the orifice is produced. This liquid jet eventually breaks up into equal-size gas filled microcapsules (c). Pictures taken with an exposure time of 11 ms. Arbitrary parameters (liquid: water +20% ethanol).
Bubbles are virtually equal in size, here 75mm. Arbitrary parameters (liquid: water +45% glycerol).
Picture3.JPG

In the experiment, the gas flow rates have been introduced by a high precision pump In some cases, the liquid is pumped from a pressurized container, whose pressure is controlled by a electronic valve. The liquid flow rate is measured with a digital weight and a clock. In other cases the liquid is injected with another syringe pump. 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. In order to gain knowledge about the fluidic regimes present in this phenomenon, a relevant nondimensional fluidic parameter is defined: a comparison of inertia to viscous forces for both liquid and gas streams.

Analysis

Graph of obtained experimental results

When the Reynolds number is large, the nondimensional breakup frequency depends on the boundary layer thickness. As a validation, researchers measured bubble diameter, Qg, Ql, and calculated the ligament diameter. The picture above shows graph of obtained experimental results.