Glass-Based Devices to Generate Drops and Emulsions

This protocol describes how to fabricate glass based microfluidic devices in detail. Following this protocol will allow anyone without experience to build and use these devices. With glass, we can easily render surfaces hydrophobic or hydrophilic thus making glass based devices attractive for generating stable emulsion drops.

There are multiple applications for drops and emulsions in fields as diverse as cosmetic, the food industry, and drug delivery. In this manuscript, we emphasize a bit more on their use in intensive agriculture. We recommend being patient at each step.

Also check that the tip is clean and not broken. Do not wait until the device is completed to check these. For making simple drops, use a glass base made with a microscope slide to build the device.

For the tip, use one millimeter diameter round glass capillaries. Next, use a 20 gauge syringe needle to facilitate introducing the liquid into the capillary. Using a razor blade or a scalpel, carve a hole equal to the size of the outer diameter of the capillary at the base of the needle.

Rinse the needle with water to remove any dust and fibers from cutting and allow the needle to air dry. To assemble the device, glue the round capillary to the microscope slide by placing the tip of the capillary one to two centimeters outside the end of the microscope slide and adding a dab of epoxy at the center of the capillary such that it will not interfere with the field of vision or with the syringe needle. Next, place the syringe needle such that the end of the capillary sits at the center of the needle and put a small amount of hardened epoxy around the rim at the bottom of the needle.

After a few minutes, apply the second layer of fresh epoxy, covering the base of the needle and avoiding the hole. Then, cover the hole with hardened epoxy to prevent the epoxy from flowing inside the needles. For generating drops, place the device in a vertical position using a clamp such that the tip is facing down like a kitchen faucet.

Use a syringe pump or a pressure driven setup to pump the liquid into the device. For making emulsion drops, use a five centimeter long square capillary for the outer liquid and a one millimeter diameter round capillary. Ensure that the length of the round capillary is several centimeters longer than the square capillary.

Match the outer diameter of the round capillary to the inner size of the square capillary to ensure that both capillaries are coaxially aligned. Next, to assemble the device, glue the square capillary to the microscope slide by placing the tip of the capillary one to two centimeters outside the end of the microscope slide and applying a dab of epoxy at the center of the capillary. Wait until the epoxy cures completely.

Then, introduce the round capillary into the square capillary such that the end of the round capillary on the slide is a few centimeters from the end of the square capillary. The other end of the round capillary which is inside the square capillary should be at a distance approximately equal to the diameter of the round capillary from the end of the square capillary. Glue the round capillary using a dab of epoxy at mid-distance between the end of the round capillary and the beginning of the square capillary and wait until the epoxy cures completely.

To house the capillary in the needle base, carve a hole at the base of the round cap which is the size of the outer diameter of the capillary. Then, to fit the other needle at the end of the square capillary, carve a round and a square hole at the base of the needle to accommodate the joint. After cutting, rinse the needles with water and let them air dry.

Then glue them as demonstrated earlier. To build the microfluidic device, cut two small pieces of a slide and using epoxy, glue them to two microscope slides to keep the slides together. Wait until the epoxy cures completely.

Using a diamond scribe, cut the square capillary to a length of approximately four centimeters. Use the square capillary with an inner side that matches the outer diameter of the round capillary to align the capillaries coaxially. For this protocol, use a two millimeter side capillary.

Then, using a pipette pulling machine, pull a round capillary to obtain two half capillaries with a thin tip. Next, using a micro forge, cut the tip of one of the half capillaries to the desired diameter. To use the other half as a collector capillary, cut the pulled tip so that the original flat ends are recovered.

Glue the square capillary to the slides. Do not put the capillary in the middle of the slides as the joints of the slides should not be in the viewing region. Then, place the tip and collector capillary inside the square capillary around two millimeters apart.

To avoid the joint, place the ends of both the tip and collector capillaries on the same side. To fabricate connections to the open ends of the capillaries, place four needles covering these ends and glue the needles as demonstrated earlier. To test the device for leaks, close two of the needles using a bent piece of tubing held by a binder clip.

Then, using a syringe, manually pump deionized water into the device through one of the needles using the last needle as an exit. If no leakages are observed, pump the water through the next needle repeating the process for all four needles. If a leak is found, thoroughly dry the device, apply epoxy, and wait for one hour before testing again.

After confirming no leakage, fill the device with experimental liquid and remove air bubbles. Then connect the inner liquid or dispersed phase syringe to needle one, the outer liquid or continuous phase to needle two, and the collector liquid or counter electrode to needle four. Needle three is for the exit.

Finally, connect the power supply to the needles feeding the inner and collector liquids to set a potential difference between the tip and collector liquid. In this study, approximately 3.3 millimeter silicone oil drops were generated using a 580 micrometer tip. And 1.75 millimeter drops were generated using an 86 micrometer tip.

Additionally, emulsion drops were generated using electro co flow devices in the dripping, cone jet, and whipping modes. In these results, the same liquid is used as inner and collector liquids. However, if the experiments goal is collecting these drops, a different conducting liquid should be used as a collector.

Be patient and wait until the epoxy cures. If it gets wet, it will never dry and you will need to start the entire procedure again. Multiple emulsions can be generated using in combinations of flow focus in a scheming series.

Each extra scheme will add a shell to the motion drop. Flow focusing sections can also be used.