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The rapid development of microfluidic devices in recent years has led to a huge number
of applications in chemistry, biology and interdisciplinary areas. This is because they
act as miniaturized platforms in which sorting, mixing, reaction and measurement can
be achieved in a precise and rapid manner. Being able to both understand and measure
the pressure of fluids inside these devices is very important, especially in the cases
where multiphase flows are involved. For example, certain advanced micromixing
technologies demand accurate evaluations of bubble-induced extra pressure, since the
pressure contribution from one bubble is likely to impact the velocity and residence time
of others, affecting the mixing efficiency and quality in a complicated manner. Similarly,
in some microfluidics-based biochemical analysis, extra pressure brought about by
droplets is a critical factor in the design of on-chip pumping, as high throughput
experiments involving continuous supply of large numbers of droplets often require a
considerable enhancement in the pumping pressure necessary to maintain the droplet
flow3. Last, state-of-the-art microfluidic logic devices rely heavily on the pressure
distribution inside the channels, which automatically controls the paths of each droplet
in the microfluidic network and as a result determines the "on" and "off" of each switch.
A few techniques to measure pressure change or pressure drop in microfluidic channels
have been developed. Examples include connecting the device to commercially
available pressure sensors and comparing pressures of different areas by analyzing the position of fluid-fluid interface. However, all of those methods have intrinsic drawbacks in one or more aspects that considerably limit their applications. A significant one is that they are primarily aiming at measuring or comparing pressures over relatively long channels (~10 mm), and are hence only designed to work in the highpressure range, i.e. to detect a pressure change on the order of tens or hundreds of Pascals. Moreover, the long channels make it rather challenging to look into the detailed dynamics of pressure variations caused by inhomogeneous emulsions, since such a long section invariably contains multiple elements, for instance droplets, of the emulsion flow, and the measurements average out the behavior of one single element. Consequently, to further reveal the characteristics of flows in microfluidics, it is highly desirable for a pressure measurement device to work in the low-pressure range, and to resolve pressure changes "locally", i.e. within small spatial regions.
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