Interfacial Tension Measurement with an Optofluidic Sensor

Overview & Synopsis

This paper reports a novel optofluidic sensor for measuring dynamic interfacial tensions. The field of micro optofluidics utilizes both microfluidic and microoptic technologies. Thus, our sensor consists of a mirofluidic network and an optical detection system. The sensor is able to measure both surface tension or liquid/liquid interfacial tension. In the case of surface tension measurement, the liquid sample is introduced into a main channel, while air is injected through a T-junction. In the case of liquid/liquid interfacial tension measurement, a second immiscible liquid such as oil is introduced into a main channel, while the sample liquid is injected through the T-junction. The formation frequency of the micro bubbles or micro droplets represent the interfacial tension between the two phases. This frequency can be measured easily by optical detection. Measurements were carried out for aqueous solutions with different concentrations of the ionic surfactant CTAB (Cetyl Trimethyl Ammonium Bromide). The actual interfacial tensions of these solutions were calibrated with a commercial tensiometer (FTA200, First Ten Angstrom). The measurement results show a clear relation between the interfacial tension and the formation frequency. Furthermore, our sensor can be used to identify the critical micelle concentration (CMC) of a surfactant. The sensor potentially allows the use of a minute amount of sample compared to the relatively large amount required for existing commercial systems.