Fabricating High-viscosity Droplets using Microfluidic Capillary Device with Phase-inversion Co-flow Structure

A phase-inversion co-flow device is demonstrated to generate monodisperse high-viscosity droplets above 1 Pas, which is difficult to realize in droplet microfluidics.

The overall goal of this procedure is to generate monodispersed high-viscosity droplets with viscosities above one pascal seconds and a low viscosity continuous phase via phase-inversion in a simple co-flow microfluidic device. This method can solve the key problem in the droplet microfluidics field about manipulation of inherently high-viscosity fluids such as high glycerol polymer solutions and nanoparticle suspensions. The main advantage of this technique is that it initially generates low-viscosity droplets which are easier to form and control than high-viscosity droplets.

The subsequent phasing version to generate high-viscosity droplets is induced when the low-viscosity droplets flow from the exit of a co-flow structure. Demonstrating the procedure will be Mr.Zhongnan Li, a graduate student from Tsinghua University. To begin preparing the device, use a tungsten carbide scriber to cut a three centimeter length of a round glass tube with an inner and outer diameter of 580 micrometers and one millimeter respectively.

This will form the middle tube of the device. To make the inner tube of the device, cut a two centimeter length of a round glass tube with an inner and outer diameter of 200 and 330 micrometers respectively. Place one milliliter of ODTS in a 1.5 milliliter centrifuge tube.

Immerse one end of the middle tube in the ODTS for 10 seconds. Then flush the tube with nitrogen gas from the untreated end until the tube is dry. Next, use a blade to cut a 0.5 millimeter by 0.5 millimeter notch.

On the edge of the plastic Luer hub of a 20 gauge half-inch blunt needle, cut a 0.5 millimeter by 0.5 millimeter notch and a one millimeter by one millimeter notch directly across from each other on the hub of a second needle. Then place the middle tube lengthwise on a standard glass microscope slide with the hydrophobic ODTS-coated end extending about one centimeter passed the narrow end of the slide. Insert the inner tube into the untreated end of the middle tube leaving about one centimeter of the inner tube outside the middle tube.

Use epoxy to fix the tubes in place along the center line of the slide. For better performance, adjust the position of the inner tube to be approximately concentrate to the middle tube. Once the epoxy has set, place the singly-notched needle over the end of the inner tube so that the tube fits in the notch.

Fix the needle in place with epoxy to form the low-viscosity oil inlet. Then fix the double-notched needle over the junction between the inner and middle tubes to form the high-viscosity aqueous solution inlet. Use epoxy to seal the needle hubs both around the tubes and to the glass substrate.

To ensure that the high-viscosity inlet needle sits firmly on the substrate, fit the middle tube into the big notch and the inner tube into the small notch. After the epoxy has dried, fit a 20 millimeter length of polyethylene tubing with an inner diameter of 0.86 millimeter over the hydrophobic end of the middle tube to complete the device. To begin the process, draw 0.8 milliliters of glycerol tinted with blue dye into a one milliliter syringe.

Draw 0.8 milliliters of light paraffin oil into a second one milliliter syringe. Connect the glycerol syringe to the high-viscosity aqueous solution inlet of the device via polyethylene tubing with an inner diameter of 0.86 millimeters. Connect the liquid paraffin syringe to the low-viscosity oil inlet.

Mount both syringes on syringe pumps. Then use a two-finger clamp and a lab stand to fix the device vertically over a 35 millimeter Petri dish. Adjust the device position so that the end of the outlet tubing is about two millimeters above the bottom of the dish.

Pour enough liquid paraffin into the Petri dish to just immerse the device outlet. Add the same quantity of liquid paraffin to a second 35 millimeter Petri dish. Set the glycerol syringe pump flow rate to two microliters per minute.

Set the liquid paraffin syringe pump flow rate to six microliters per minute. Run both pumps to begin generating the glycerol droplets. Monitor the droplet generation with a camera if desired.

Wait about one minute for the glycerol and liquid paraffin flows to stabilize sufficiently to form uniform glycerol droplets. Then exchange the Petri dish under the device outlet for the second liquid paraffin-filled dish to collect the uniform droplets. Monodispersed glycerol droplets were generated by phase-inversion co-flow devices with middle tubes having diameters of either 200 or 500 micrometers.

Monodispersed droplets were also generated from other high-viscosity fluids including honey, a starch solution, and a polyvinyl alcohol solution. Glycerol droplets generated by the 500 micrometer device with oil and glycerol flow rates of 30 and 10 microliters per minute respectively had an average diameter of 521 micrometers. Glycerol droplets generated by the 200 micrometer device at the same flow rates had an average diameter of 212 micrometers.

The size of the glycerol droplets was found to change with variations in the ratio of the oil flow rate to the glycerol flow rate. Increasing the oil flow rate while holding the glycerol flow rate constant resulted in a decrease in particle size. After watching this video, you should have a good understanding of how to make monodispersed high-viscosity droplets with a phase-inversion co-flow device.

In contrast to common co-flow devices, you do not need to taper the inner glass capillary to a sharp tip to make the phase-inversion co-flow device. The phase-inversion co-flow device can be used to generate high-viscosity droplets with dynamic viscosities of up to 12 pascal seconds. Once mastered, this technique can be done in 30 minutes if it is performed properly.