On-chip droplet characterisation using impedance-based measurement

• Main Author: Low, Jerome Fang Yao
• Other Authors: Yoon Yong Jin
• Format: Final Year Project
• Language: English
• Published: 2017
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/70706
• Institution: Nanyang Technological University

This paper investigates an impedance-based on-chip droplet characterisation and the suitability for black silicon to be used as biological sensors. As black silicon is superhydrophobic, black silicon substrates can be used instead to reduce the “hugging” of the dispersed phase to the substrate’s surface. The use of black silicon substrates may even lead to decreased reaction times and higher sensitivity for the lab-on-chip devices that are currently using other substrates. Droplets were generated using 90° cross-flowing streams and it was found that the droplet sizes vary linearly with the flow rates of the dispersed and continuous phases. The droplet diameter from an increase in the flow rate by a unit for the dispersed phase is approximately the same if the flow rate for the continuous phase were to be decreased by the same unit instead. Surfactant concentration was tested to be an important factor in the stability of the droplet as well. Concentrations significantly higher than 1.5% (v/v) Span 80 led to the formation of micelles and resulted in microscale tip-streaming of the dispersed phase. On the contrary, low concentrations of surfactant resulted in the instability of the droplets where the droplets break up and merge if two droplets are too close to each other. Both 31 µm beads and Jurkat cells were used for the encapsulation within droplets. Experiments show that the number of empty droplets increased to 2% if the concentration of the dispersed phase was less than 4 million cells per millilitre. This concentration was subsequently used to ensure that the impedance measurements recorded were for droplets containing at least a single cell. It was also demonstrated that impedance-based measurements can be used for droplet characterisation. Results showed that the conductivity of the medium changes the current response when a different medium was used for the droplet generation. The presence of cells being encapsulated in the droplets also caused the current response in the impedance measurement to be lesser and led to a smaller change in the current response.