Investigation of a microfluidic sensor for surface tension measurement

• Main Author: Thevaraja Ramu.
• Other Authors: Nguyen Nam-Trung
• Format: Final Year Project
• Language: English
• Published: 2009
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: http://hdl.handle.net/10356/16170
• Institution: Nanyang Technological University

This report describes the experiments conducted using a microfluidic surface tension sensor made from Polydimethylsiloxane (PDMS). The device is integrated with a bubble / droplet formation system and an optical detection system. As the bubble or droplet passes by, it disrupts the light signal and the change in signals is captured by an oscilloscope and the frequency is obtained. Bubble formation experiments were carried out using Distilled water, pure Mineral Oil and Mineral Oil with various Span 80 surfactant concentrations as the continuous phase and Nitrogen gas as the dispersed phase. Droplet experiments were carried out using pure Mineral Oil and Mineral Oil with various Span 80 surfactant concentrations as the continuous phase and Distilled water as the dispersed phase. The effects of varying the pressure, flow rate, type of fluid and the surfactant concentration on the bubble or droplet size, shape and formation frequency were collected and correlated. For the bubble experiments it was found that DI water, when used as the continuous phase, cannot form bubbles due to the hydrophobicity of the channel walls. When Mineral Oil was used, monodispersed bubbles were easily formed. It was observed that as the oil flow rate increased or when the pressure was decreased, the bubble size decreased. When Span 80 surfactant was introduced into the Mineral Oil, there was no change in bubble size regardless of concentration level. Furthermore, when the fluids were tested using a LAUDA Bubble Pressure Tensiometer MPT C, there was little change in their surface tension. Thus, this proved that Span 80 surfactant does not change the surface tension of Mineral Oil. For the droplet experiment, as Mineral Oil and DI water was used, monodispersed droplets were formed. The size of the droplet increases with increasing of the water flow rate or the decreasing of the oil flow rate. Furthermore, as the Span 80 surfactant concentration increases the droplet size decreases thus, showing the surfactant can affect the interfacial tension at the oil-gas interface. It was observed that the droplet formation frequency increased as the oil flow rate was increased. This shows that the frequency is inversely proportional to the droplet diameter when changing the oil flow rate. However, when the DI water flow rate was increased, the frequency increased although the droplet size increased as well. Thus, when changing the water flow rate the frequency is proportional to the droplet diameter. This report shows the experiments conducted and the results gathered in detail. It also includes some possible reasons to explain the effects due to varying the experimental conditions.