Abstract
Pressure is one basic parameter involved in microfluidic systems. In this study, we developed an easy capillary-based method for measuring fluid pressure at one or multiple locations in a microchannel. The principal component is a commonly used capillary (inner diameter of 400 μm and 95 mm in length), with one end sealed and calibrated scales on it. By reading the height (h) of an air-liquid interface, the pressure can be measured directly from a table, which is calculated using the ideal gas law. Many factors that affect the relationship between the trapped air volume and applied pressure (papplied) have been investigated in detail, including the surface tension, liquid gravity, air solubility in water, temperature variation, and capillary diameters. Based on the evaluation of the experimental and simulation results of the pressure, combined with theoretical analysis, a resolution of about 1 kPa within a full-scale range of 101.6–178 kPa was obtained. A pressure drop (Δp) as low as 0.25 kPa was obtained in an operating range from 0.5 kPa to 12 kPa. Compared with other novel, microstructure-based methods, this method does not require microfabrication and additional equipment. Finally, we use this method to reasonably analyze the nonlinearity of the flow-pressure drop relationship caused by channel deformation. In the future, this one-end-sealed capillary could be used for pressure measurement as easily as a clinical thermometer in various microfluidic applications.
Highlights
With the rapid development of microfluidics or lab-on-a-chip systems [1,2,3,4], pressure measurement in microchannels is becoming more and more crucial for fundamental understanding and precise control of fluid flows on microscales and for further development of microfluidic devices [5,6,7,8,9,10,11]
Pressure can be used to characterize the mechanical properties in microfluidic systems, such as bacterial biofilms deflection and cell transformation [17,18,19,20,21,22]
We demonstrate an easy one-end-sealed capillary pressure sensor using trapped air compression, which is inspired by the previous studies of References [53,54,55]
Summary
With the rapid development of microfluidics or lab-on-a-chip systems [1,2,3,4], pressure measurement in microchannels is becoming more and more crucial for fundamental understanding and precise control of fluid flows on microscales and for further development of microfluidic devices [5,6,7,8,9,10,11]. The second category is based on a sealed side channel connected to the microchannel at the location where the pressure is to be measured, for example, the flexible liquid metal electrodes-based capacitive method [52] and trapped air compression methods [30,53,54,55,56,57]. Srivastava and Burns [54] developed an easy method for measuring the pressure of liquid and air by monitoring the movement of a liquid–air interface as it compresses air trapped inside a one-end-sealed side channel connected with a chamber. It is low-cost, electronic-free, and image-free and does not require auxiliary equipment for signal processing. (c) The h–pressure relationship is independent of the capillary diameter and the physical and chemical properties of the liquid. (d) The major advantage is that, once a bubble or liquid plug enters the capillary, it is easy to clear them, as the capillary can be pulled out from the microfluidic chip, which means that it is robust and can be used for other microfluidic chips many times without recalibration
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