How the capillary burst microvalve works
Graphical abstract
We provide a theoretical framework to explain how the capillary burst valve, as shown in this figure, works.
Introduction
It is currently of a great interest to integrate microfluidic systems for biochemical analysis with other functional components, such as sensors and reaction chambers [1], [2], [3], [4]. The major advantage of the integrated microfluidic systems is that they consume only a minute amount of biochemical fluid while keeping a high sensitivity. Hence they are expected to be widely used in many applications including proteomics, rapid drug screening, and portable disease-detecting or therapeutic systems. The microfluidic systems are mainly composed of micropumps, microvalves and microchannels. It is difficult to integrate such components of microscales with other biochemical components such as sensors and catalytic device fabricated by different processes and made of different materials. Therefore, simplifying the design and fabrication process of microfluidic systems is crucial in achieving the integration of various functional parts.
Recently, simple driving and regulating systems for microliquid flow have been suggested to substitute for such complex handling systems as those based on piezoelectric materials [5] and electrokinetics [6]. The focus of this work is on the flow-regulating device that involves no complex design and fabrication processes. One of the candidates is the capillary burst valve (referred to as CBV hereafter) that regulates the liquid flow only by giving a microchannel a simple geometric or surface-chemical variation. The geometrical CBV designates a sudden expansion of the microchannel where the liquid meniscus is trapped at the beginning of the expansion. The surface-chemical CBV stops the liquid flow where the sudden change of the wetting properties occurs [7]. In either case, the liquid meniscus is stopped at the valve until the driving force overcomes the resisting capillary force, thus the flow regulation is easily achieved by controlling the driving force. When the valve bursts, the remaining channel is wetted by advancing liquid.
Here we consider the geometric CBV that involves no chemical treatment. The concept of the geometric CBV was suggested and tested earlier [8], [9]. The most important performance measure of the valve is how much pressure it can withstand before bursting. In that aspect, earlier researchers showed that the bursting pressure of the geometric CBV is proportional to the surface tension of liquid and inversely proportional to the channel dimension. However, no further rigorous theoretical development was made to determine the empirical constants appearing in their empirical relationship for the maximum pressure difference that the valve can withstand [10]. Therefore, here we aim to elucidate the fundamental working principle of the geometrical CBV and propose the relationship of the maximum pressure to the valve dimensions and the liquid properties.
We first start with a brief introduction of the basic theory of contact line movement needed for the problem of geometric CBV. Then we thoroughly analyze the working principle of the valve to predict the bursting condition of the valve both in circular and rectangular channels. We then compare the theoretical results with the experimental measurements using a centrifugal microfluidic valve system fabricated by soft lithography [11].
Section snippets
Theory for a round tube
We consider a circular tube meeting an abruptly diverging section as shown in Fig. 1 as the simplest geometry. When the liquid meniscus is in the straight section, the pressure jump, i.e., the difference between the pressure inside and outside the liquid, and , respectively, under equilibrium is given by the Young–Laplace equation, , where σ is the surface tension, the equilibrium contact angle, and D the tube diameter. When is less than the right angle
Experiments
We fabricated the microchannel having the capillary burst valve by means of soft lithography. The fabrication process is illustrated in Fig. 5. A 150 μm-thick negative photoresist (SU-8 100, Microchem, corp.) was spin-coated onto a silicon substrate, and was left on a flat surface for 10 min for stress relaxation. Then the photoresist was soft-baked at 65 °C for 20 min and at 95 °C for 50 min on a hotplate. UV (ultraviolet) exposure was carried out with a dose of 255 mJ/cm2 and a post-exposure
Results and discussion
Although the centrifugal disk as shown in Fig. 7 has three valves in series for versatile biosample delivery functions, we used a single valve in the experimental measurements reported here. In addition, the valve location, the liquid amount and the rear width were kept constant. Hence we have the following constant values to determine the bursting pressure: , and . Therefore, for each liquid/solid combination ( and ) and β, the bursting speed can be plotted as a
Summary
Capillary burst microvalves based on the diverging channel section had been suggested previously, but no rigorous theoretical explanation was devoted to their operations so far. Thus we have reported the theoretical analysis of the capillary burst valve to provide the maximum pressure difference that the valve can withstand. Such prediction requires the valve dimensions and the wetting properties of the liquid/solid combination. Therefore, there is no need to find fitting constants for each
Acknowledgements
This work was supported by the Intelligent Microsystem Center sponsored by the Korea Ministry of Commerce, Industry and Energy through 21st century's Frontier Project. We thank Jong-In Hong and Heon Ju Lee for the help with the measurement of surface tensions and contact angles, respectively. H.Y.K. acknowledges administrative support from the Institute of Advanced Machinery and Design at Seoul National University.
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