Microfluidic droplet control by photothermal interfacial flow

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Droplet-based microfluidics is an emerging field that can perform a variety of discrete operation of tiny amount of reagent or individual cell. Noncontact manipulation of droplets in a microfluidic platform can be achieved by using the Marangoni convection due to a local temperature gradient given by the irradiation of heating light. This method provides noncontact, selective and flexible manipulation for droplets flowing in microfluidic network. Although the potential of this selective operation method of droplets was confirmed, the driving force exerted on droplets has not been quantitatively obtained. In this study, we have developed a measurement system of the temperature field around droplets during the manipulation by light irradiation and evaluated the manipulation force. In O/W emulsion system with oleic acid and buffer solution, oleic acid for droplet and buffer solution for continuous phase, the temperature distribution around the droplets was measured by laser-induced fluorescence. From the balance of drag force and photo-induced Marangoni force, the driving force was determined. From the results, we confirmed the applicability of the noncontact droplet manipulation using the photothermal Marangoni effect