BioChip J. (2014) 8(2): 122-128 DOI 10.1007/s13206-014-8207-y Abstract The extent to which the carrier fluid wets the walls of a microchannel is crucial in the droplet formation process for segmented flow microfluidic applications and can be influenced by the use of surfac- tants. Surfactants dynamically modify the microchan- nel surface leading to stabilization of the two phase interface, affecting the droplet formation process. An experimental study of the influence of hydrophobic surfactant (Span 80) during the formation of water-in- oil droplets in a T-shaped microchannel geometry is presented and the wetting properties of the micro- channel walls were characterized. The range of data to be analyzed on the microscale is estimated from the macroscopic interfacial tension and contact angle mea- surements. The critical micelle concentration (CMC) level at the microscale was estimated by observing the trend of droplet length variation with concentration of surfactant in a microchannel. Microchannels used in this work were fabricated using softlithography meth- ods and bonded using a custom-made plasma bonding setup that does not require an ultra high vacuum cham- ber and hence saves the fabrication cost. Keywords: Microdroplets, Microchannel, Double emul- sions, Wettability, Surfactants Introduction Technology based on droplet microfluidics offers a novel approach for the controlled preparation of mono- dispersed microemulsions. These monodispersed mic- roemulsions have been widely used to obtain high qual- ity pharmaceuticals 1 , food products 2 and cosmetics 3 . They also have potential uses in a number of other chemical and biological applications 4,5 . Precise control of droplets containing nanoliter volumes of fluids is required for potential utilization of micro emulsions, coupled with an in-depth knowledge of the dynamical properties of multiphase flows. Many studies have in- vestigated droplet controlling parameters such as flow rate and viscosity ratios between the two immiscible fluids 6,7 however improved understanding of the effects of interfacial tension and wetting properties is still required to fully control and explain the dynamics of droplet formation. This provides the motivation for the present study. T-junction microfluidic devices are among the most popular droplet generation devices. Droplets are form- ed when the dispersed phase stream is sheared off by the carrier fluid at the junction 8-11 . It has been suggest- ed quantitatively that for the stable formation of W/O emulsions, the contact angle of a water droplet with the microchannel surface should be 180� 12 . Typically, the water droplet contact angle with a hydrophobic surface is between 90� and 120� 13 where the oil phase is the surrounding media. If a surfactant is added to the continuous oil phase, the contact angle gradually increases and the surface properties change from hydro- phobic to superhydrophobic 12 . Dreyfus and co-work- ers 12 studied the wetting characteristics of droplet for- mation of deionized water/tetradecane-Span 80 sys- tems in a microfluidic device and identified different Original Article Dynamic Wetting in Microfluidic Droplet Formation Shazia Bashir 1,2, *, Xavier Casadevall i Solvas 3 , Muhammad Bashir 4 , Julia Margaret Rees 1 & William Bauer Jay Zimmerman 4 Received: 24 February 2014 / Accepted: 11 May 2014 / Published online: 20 June 2014 � The Korean BioChip Society and Springer 2014 1 School of Mathematics and Statistics, Hicks Building, Hounsfield Road, University of Sheffield, S3 7RH, U.K. 2 Department of Physics and Applied Mathematics, Pakistan Institute of Engineering and Applied Sciences, Islamabad, Pakistan 3 Institute of Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, HC1 F109, ETH Zurich 4 Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, U.K. *Correspondence and requests for materials should be addressed to S. Bashir ( shazia@pieas.edu.pk, shaziapieas@gmail.com)