Techniques for accurate measurement of liquid flows involving volumes on the order of nanoliters and picoliters are critical for advancement of microfluidics applications. Although several techniques for measurement of microfluidic flows where a liquid is dispensed on to a target are available today, no proven techniques currently exist for measurement of microfluidic flows where a liquid is injected into a liquid media. Motivated by such a measurement need in a current project, this paper presents a novel experimental technique to measure bulk-flow of a liquid dispensed into another liquid medium with sensitivity down to picoliter range. This technique is based on the principle of titration of two liquids and measurement of hydrogen ions in the resulting solution. The critical components of the system that enable the proposed technique are: A highly sensitive hydrogen ion concentration (pH) sensor and a device to dispense very small, calibrated, volumes of liquid. Since commercially available hydrogen ion sensors are bulky for use in the proposed technique, a hydrogen ion sensor element that provides a sensing tip dimension on the order of a few microns is fabricated from a glass capillary for the current study. Dispensing of calibrated volumes of liquid is accomplished by means of a calibrated syringe for volumes in nanoliter range and a glass capillary for volumes in picoliter range, the latter serving as a dispensing port of a diameter of about 2 μm. The paper includes details of experiments where the proposed technique is first applied to precisely predict injections of 20 and 40 nL volumes and then extended to demonstrate the sensitivity of the sensor element to injections of volumes in the picoliter range. The preliminary experimental results presented generally validate the principle involved in this technique and suggest that the technique is virtually scalable to any volume flows typically encountered in the microfluidics applications.

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