This paper presents a theoretical development and critical analysis of the burst frequency equations for capillary valves on a microfluidic compact disc (CD) platform. This analysis includes background on passive capillary valves and the governing models/equations that have been developed to date. The implicit assumptions and limitations of these models are discussed. The fluid meniscus dynamics before bursting is broken up into a multi-stage model and a more accurate version of the burst frequency equation for the capillary valves is proposed. The modified equations are used to evaluate the effects of various CD design parameters such as the hydraulic diameter, the height to width aspect ratio, and the opening wedge angle of the channel on the burst pressure.
Microfluidic discs have been employed in a variety of applications for chemical analyses and biological diagnostics. These platforms offer a sophisticated fluidic toolbox, necessary to perform processes that involve sample preparation, purification, analysis, and detection. However, one of the weaknesses of such systems is the uni-directional movement of fluid from the disc center to its periphery due to the uni-directionality of the propelling centrifugal force. Here we demonstrate a mechanism for fluid movement from the periphery of a hydrophobic disc toward its center that does not rely on the energy supplied by any peripheral equipment. This method utilizes a ventless fluidic network that connects a column of working fluid to a sample fluid. As the working fluid is pushed by the centrifugal force to move toward the periphery of the disc, the sample fluid is pulled up toward the center of the disc analogous to a physical pulley where two weights are connected by a rope passed through a block. The ventless network is analogous to the rope in the pulley. As the working fluid descends, it creates a negative pressure that pulls the sample fluid up. The sample and working fluids do not come into direct contact and it allows the freedom to select a working fluid with physical properties markedly different from those of the sample. This article provides a demonstration of the “micro-pulley” on a disc, discusses underlying physical phenomena, provides design guidelines for fabrication of micro-pulleys on discs, and outlines a vision for future micro-pulley applications.
In traditional centrifugal microfluidic platforms pumping is restricted to outward fluid flow, resulting in potential real estate issues for embedding complex microsystems. To overcome the limitation, researchers utilize hydrophilic channels to force liquids short distances back toward the disk center. However, most polymers used for CD fabrication are natively hydrophobic, and creating hydrophilic conditions requires surface treatments/specialized materials that pose unique challenges to manufacturing and use. This work describes a novel technology that enjoys the advantages of hydrophilic fluidics on a hydrophobic disk device constructed from untreated polycarbonate plastic. The method, termed suction-enhanced siphoning, is based on exploiting the non-linear hydrostatic pressure profile and related pressure drop created along the length of a rotating microchannel. Theoretical analysis as well as experimental validation of the system is provided. In addition, we demonstrate the use of the hydrostatic pressure pump as a new method for priming hydrophobicbased siphon structures. The development of such techniques for hydrophobic fluidics advances the capabilities of the centrifugal microfluidic platform while remaining true to the goal of creating disposable polymer devices using feasible manufacturing schemes.
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