Introduction: In the past two decades, Micro Fluidic Systems (MFS) has emerged as a powerful tool for biosensing, particularly in enriching and purifying molecules and cells in biological samples. Compared with conventional sensing techniques, distinctive advantages of using MFS for biomedicine include ultra-high sensitivity, higher throughput, in-situ monitoring, and lower cost [1]. Microfluidic devices are an exciting alternative for performing analytical assays, due to the speed in the analyses, reduced sample, reagent and solvent consumption, and less waste generation[2]. Microfluidics has excellent potential, but the complexity of fabricating and operating devices has limited its use[3].
“Open Microfluidics” is a relatively new domain of investigations. Historically it has begun with spatial applications such as vanes and with the first investigations of the capillary flow of liquids older in v-grooves. More recently, it appeared that open microfluidics could be of great interest in biotechnology and biology due to the simplicity of fabrication, easy use, and observation, and to the direct access to the flow, which facilitates addition or withdraw of fluids[4]. In open microfluidics, also referred to as open surface microfluidics or open-space microfluidics, at least one boundary confining the fluid flow of a system is removed, exposing the fluid to air or another interface such as a second fluid[5].
Methods: Physics of Open channel Microfluidic
To confine liquids in open channels, typically a wettability barrier is employed in lieu of solid channel walls (physical confinement). The wettability barrier is achieved by employing either a contrast in solid surface energy and / or texture between the channels used for fluid confinement and the background. Compared to water ( γ lv = 72.1 mN/m), liquids with low surface tension ( γ lv < 30 mN/m) are much more difficult to confine in open channels, as they readily spread on most surfaces. Materials with very low surface energy and precisely engineered textures are necessary to maintain a sufficiently high advancing contact angle with low surface tension liquids. Oil-water interfaces are commonplace in many different chemical reactions and biological systems. This makes methodologies for the fabrication of open-channel microfluidic devices compatible with all liquids relevant to a wide range of academic and commercial interests. Whitesides et al. previously developed a methodology for fabricating paperbased microfluidic devices using fluoro-silanized paper. When the devices were infused with perfluorinated oil, liquids with surface tension as low as 15 mN m-1 could be physically confined within the folded paper.[1]
Results: Mechanism of Open channel Microfluidic
Obtaining a capillary flow depends on channel geometry and contact angle. A general condition for the establishment of a spontaneous capillary flow in a uniform cross section channel has already been derived from Gibbs free energy. Berthier et al. considered spontaneous capillary flows (SCF) in diverging open rectangular channels and suspended channels, and they showed that they do not flow indefinitely but stop at some location in the channel. In the case of linearly diverging open channels, we derive the expression that determines the location where the flow stops. The theoretical approach is verified by using the Surface Evolver numerical program and is checked by experiments. In this work we first consider SCFs in linearly diverging open rectangular channels and suspended channels and they show that they do not flow indefinitely but stop at some location in the channel. In a similar approach to that of uniform channels, using Gibbs free energy, they derived the expression that determines the location where the flow stops. The theoretical approach is verified by using the Surface Evolver numerical program. Experiments using suspended diverging channels are reported here that agree with the theory. In a second step, we extend the theory to large widening angles, and they considered suddenly diverging open rectangular channels and suspended channels. It is shown that such devices may or may not stop the capillary flow, depending on their geometry. It is demonstrated that such geometrical features can act as stop or trigger valves, in a same manner as used in conventional closed systems. [3]
Conclusion: Open channel Microfluidic Advantages
Such open-channel devices have several distinguishing advantages, including their inherently large liquid-vapor interface, easy accessibility to the flowing fluid, relatively rapid and low-cost fabrication methodology, ease of use, and low fluid shear force, making them particularly useful for many biological and biomedical applications. The open-channel approach to droplet-based microfluidics has the potential to enable applications in which each drop can accessed at any time and any location with simple pipettes or other fluid dispensing systems[6]. Open-channel microfluidic devices have shown great potential in achieving a high degree of fluid control, at relatively low-cost, while enabling the opportunity for rapid fabrication. However, thus far, work in open channel microfluidics has mostly focused on controlling the flow of water or other aqueous solutions[7]. Capillary open microsystems are attractive and increasingly used in biotechnology, biology, and diagnostics as they allow simple and reliable control of fluid flows. In contrast to closed microfluidic systems, however, two-phase capillary flows in open microfluidics have mainly remained unexplored[6].