a) Inertial Two-Phase Gas-Liquid Droplet Microflows
Microfluidics has broadened its impact on biological, physical, and chemical applications due to the consumption of small samples, precise controllability, and possibility of device miniaturization. In these applications, rapid and efficient mixing of multiple species is often required to understand their characteristics. Among many known methods of advanced mixing at microscale, we experimented droplet collision mixing with air liquid system. Different from a diffusion driven mixing, droplet collision promotes mixing by utilizing the inertia of two approaching droplets; therefore, it enhances the efficiency of mixing.
High inertial droplet collision micromixing is demonstrated in microchannel using laser-induced fluorescence (aka. LIF). LIF is an inelastic radiative process and is based on the absorption and subsequent emission of a photon following quantum energy state interactions. It provides a significantly greater collision cross section such that the excitation energy required for a detectable signal is significantly less and in many cases a conventional white light source is sufficient.
Our major objective is to construct a spatial concentration map and provide better mixing information by controlling environmental conditions and using spectral filtering with two color laser induced fluorescence. Using two kinds of fluorescence rather than one kind, we will be able to cancel out the environmental effects on the outcome by taking the ratio of two results (pixels) from dual view camera.
The emission intensity is given by the following equation,
If εCt <<1, we can approximate,
I_em = ϕ(T)I_0 (1-e^(-ε(λpH)Ct)) ≈ ϕ(T)I_0 εCt
Ф: Quantum yield
I0: Excitation intensity
ε: Adsorption (extinction) coefficient
C: Concentration of fluorescent solution
t: Solution thickness
By taking a ratio of the two images, we can get rid of the effects from ϕ(T),I_0, ε, and, t; therefore, we can construct a pure spatial concentration map.
Experimental Setup
The diagram above demonstrates the experimental setup. Droplet images will be revealed as two different images of the same droplet in CMOS camera using two different emission filters and dichromatic beam splitter.
The other two images show our microchip channels we made using soft lithography with plasma bonding. There are two inlets for liquid and two for air. The air will split the liquid input into droplets, and these droplets will collide at the Y junction in the center of the chip. We input air and liquid at the constant rate for all inlets.
Experimental Results
Prior to producing droplet collision with two fluorescence dye, we ran our experiment with pure water in order to prevent the waste from expensive fluorescence.
When the water is injected at a rate of 5 µl/min into water reservoir by a syringe pump and the air is injected at a rate of 1.6V by air pump, a nice water droplet was created. The following images are describing the process of the droplet generation. As the water is entering the air flowing channel, it starts to create a droplet and once a specific size of droplet is reached, it leaves the entrance and moves to the junction part.
Videos demonstrate the droplet collision in the junction part of our channels when the water is injected at a rate of 5 µl/min into water reservoir by a syringe pump and the air is injected at a rate of 1.6V by air pump.
This collision is widely classified into two types. The first type is when the droplets from two channels reaches the junction at the same time. The two droplets creates the collision and leaves the junction right away. The image below depicts the progression.
The second type is when the droplets reach the junction at different times. Therefore, the droplet which reaches the junction at first waits for the other droplet to come. When they finally meet, the collision takes place and the mixed droplet leaves the junction. The images below also show this progression.
The above image shows the droplet collision processes using one fluorescence dye. The left is directly acquired from CMOS camera, and the right is image processed by Matlab. Although they show a general spatial concentration map and gives an overall idea of mixing process, it still included the environmental effects such as the intensity of the light source. Therefore, we need to use two dyes and cancel these effects by taking their ratios.
Once taking a video of droplet mixing with allowable and expected intensities at dual views, we can proceed to image processing of two recordings. Taking a ratio of the corresponding pixels of the dual views, therefore controlling the environmental effects, we can finally construct a spatial concentration maps and provide better mixing information.