Comparison of monodisperse droplet generation in flow-focusing devices with hydrophilic and hydrophobic surfaces

Abstract

A thin flow-focusing microfluidic channel is evaluated for generating monodisperse liquid droplets. The microfluidic device is used in its native state, which is hydrophilic, or treated with OTS to make it hydrophobic. Having both hydrophilic and hydrophobic surfaces allows for creation of both oil-in-water and water-in-oil emulsions, facilitating a large parameter study of viscosity ratios (droplet fluid/continuous fluid) ranging from 0.05 to 96 and flow rate ratios (droplet fluid/continuous fluid) ranging from 0.01 to 2 in one geometry. The hydrophilic chip provides a partially-wetting surface (contact angle less than 90°) for the inner fluid. This surface, combined with the unusually thin channel height, promotes a flow regime where the inner fluid wets the top and bottom of the channel in the orifice and a stable jet is formed. Through confocal microscopy, this fluid stabilization is shown to be highly influenced by the contact angle of the liquids in the channel. Non-wetting jets undergo breakup and produce drops when the jet is comparable to or smaller than the channel thickness. In contrast, partially-wetting jets undergo breakup only when they are much smaller than the channel thickness. Drop sizes are found to scale with a modified capillary number based on the total flow rate regardless of wetting behavior.

Drop mass transfer in a microfluidic chip compared to a centrifugal contactor

“…34 The device consisted of a cross geometry with the continuous phase fluid flowing past the dispersed phase, generating a jet which breaks up into drops due to capillary instability. Glass chips were purchased from Translume (Ann Arbor, MI) and were made hydrophobic using a 2 vol % octadecyltrichlorosilane (OTS) (Sigma-Aldrich, St. Louis, MO, > 90% pure) in hexadecane (Sigma-Aldrich, St. Louis, MO, anhydrous > 99%) wash. Chip orifice widths were 50 and 100 lm, and the chip depth was always 100 lm.…”

“…The production of these drops is discussed elsewhere. 34 To obtain the change of concentration in the drops as they traveled down the channel of the microfluidic chip, images were taken along the length of the microfluidic chip channel (see Figure 1). Image locations are known through use of a motorized stage (ProScan, Prior Scientific, Cambridge, UK).…”

“…Droplet production in a similar system was discussed previously, including control of size and frequency of droplets. 34 Flow was visualized using an inverted microscope (Leica DM IRB, Germany), and recorded using a high speed CCD camera (Phantom v 9.1, Vision Research, Wayne, NJ), with frame rates as high as 5000 frames per second. Distances are calibrated using Klarmann Rulings, (Litchfield, NH) KR-868 reticules.…”

Multiscale models of nuclear waste reprocessing : from the mesoscale to the plant-scale.

“…A more detailed description of the work that is summarized here was published in Lab on a Chip [Roberts et al 2012A]. Since we have a journal article on the subject, here we will just summarize the results.…”

“…Three journal articles arose from this work. The first one on droplet generation on wetting and non-wetting surfaces was published in Laboratory on a Chip [Roberts et al, 2012A], the second paper on the detailed fluid dynamics at the droplet-scale examined with experiments and finite element simulations will be submitted to Physics of Fluids [Roberts et al, 2012B], and the third on mass transport, using the microfluidic device and scaled-up to the centrifugal contactor will be submitted to Chemical Engineering Science [Roberts et al, 2012C]. The mass transport in the contactor is discussed in the contactor experimental chapter, while the base neodymium mass transport in the microfluidic device is summarized here.…”