This article describes the microfluidic process and parameters to prepare actuating particles from liquid crystalline elastomers. This process allows the preparation of actuating particles and the variation of their size and shape (from oblate to strongly prolate, core-shell, and Janus morphologies) as well as the magnitude of actuation.
The overall goal of this microfluidic synthesis is to set up a capillary based co-flow microfluidic reactor which enables the reproducible production of a large number of liquid crystalline elastomer microparticles with excellent thermal actuating properties. This method can help answer key questions in the field LCE microactuator production, such as how to process liquid crystalline monomers into thermally actuating microparticles via microfluidic reactors. The main advantage of this technique is that a fast particle production and well as different particle shapes, sizes and actuation patterns are accessible by fine-tuning the basic microfluidic setup.
First, equip a glass water bath dish with two septa. Then, pierce with septa with an awl to create a hole to position a tube with an outer diameter of 1/16 inches. Then, attach a fitting and a corresponding ferrule for the 1/16 inches outer diameter tubing to the end of a PTFE tube.
Then, stick the tip of a polyamide coated silica capillary into it. Next, screw the PTFE tube to one of the opposing arms of a polyether ether ketone T-junction for the 1/16 inches outer diameter tube mounted on a small metal table. Next, attach a fitting and corresponding ferrule to the end of a second PTFE tube.
Then, screw the tube to the lateral arm of the T-junction. Next, insert the third PTFE tube through one of the septa. Cut the third PTFE tube to reach a second syringe pump outside the water bath and the end of tube 1.1 inside the water bath.
Then, attach two female Luer locks for the 1/16 inches outer diameter tubing. Next, insert the tube through the second septum. Then, assemble the fourth PTFE tube with a fitting and a ferrule.
After assembling the PTFE tube, connect the tube 1.4 to the remaining arm of the T-junction by carefully sliding it over the capillary. Next, transfer the water bath on a hot plate. Then, fix the tube 1.4 on top of the precision heating plate using adhesive tape.
Also attach a 5-ml glass vial to the end of the tube. Now, connect the end of the tubes 1.2 and 1.3 to syringes filled with continuous phase and hydraulic oil, both operated by syringe pumps. Next, install the stereomicroscope and mount a UV light source.
Add 200 mg of liquid crystalline acrylate and two more reagents in a flask. Then, dissolve the mixture in 1 ml of dicholoromethane. Next, use the vacuum at 40 degrees Celsius to completely remove the solvent and melt the remaining solid at 110 degrees Celsius in an oil bath.
Next, prepare a syringe with a barb to female Luer lock connector for 3/32 inches inner diameter tubing. Attach the syringe and the fifth PTFE tube through a connecting tube. Use the syringe to collect the monomer mixture in the tube.
Use two male Luer locks for a 1/8 inches outer diameter tubing and attach both the locks to the ends of the tube 1.5 containing the monomer mixture. Later on, attach both the ends of the tube 1.5 with the female Luer locks to the end of the tubes 1.1 and 1.3. Then, fill the water bath and set its temperature to 90 degrees Celsius and the precision heating plate's temperature to 65 degrees Celsius.
Once the monomer mixture is melted, set the flow rate of the continuous phase to a value between 1.5 and 2.0 milliliters per hour. And the flow rate of the monomer phase to a reasonable value. Then, center the capillary inside the tube and switch on the UV light when droplets of similar size begin to form.
Collect the fractions of the polymerized particles in the 5-ml glass vial attached at the end of the tube 1.4. Look for a change in the color and the formation of the particles under the UV light. Then, in order to mount the Janus setup, equip a glass water bath dish of 190 millimeter diameter with two septa.
Pierce the septa with an awl to create a hole to position a tube with an outer diameter of 1/16 inches. Next, insert two parallela-lined fused silica capillaries into the tubing sleeve. Use Super Glue to seal the sleeve.
Let the short capillary extend about three millimeters out of one side of the sleeve. Then, attach a fitting and a ferrule to both ends of the tubing sleeve. Then, screw the sleeve to one of the opposing arms of each T-junctions to connect the two T-junctions.
Mount both the T-junctions on a small metal table. Stick a PTFE tube through the water bath septum. Equip it with a fitting and ferrule and connect it to the capillary and leftover opposing arm of T-junction one.
Connect another tube with a ferrule and fitting attached to one end to the freed lateral arm of T-junction one. Next to tube 2.1, attach another PTFE tube through second hole in the septum. Next, add two female Luer locks for 1/16 inches outer diameter tubing to the free ends of the tubes 2.2 and 2.3 inside the water bath.
Connect a fourth PTFE tube with a fitting and a ferrule to the lateral arm of T-junction two. Carefully slide tube 2.5 equipped with a fitting and a ferrule on both side over the capillaries and connect it with the remaining arm of T-junction two. Then, insert the sixth PTFE tube through the other septum and attach a fitting and ferrule to the end inside the water bath.
Connect the fifth and the sixth PTFE tubes using an adaptor. Next, screw the tube containing the monomer mixture between the ends of the tubes 2.2 and 2.3. Next, set up the water bath on a hot plate with a thermometer.
Plug tube 2.1 in the syringe containing the aqueous phase, tube 2.3 in the one containing the hydraulic oil, and tube 2.4 to the continuous phase. Then attach the sixth tube with adhesive tape on top of a precision heating plate. Next, attach a 5-mL glass vial at the end of the tube.
Next, center the capillaries inside the tube and mount a UV light source over the polymerization tube 2.6 on the precision heating plate. For the core shell setup, first attach a fitting and then a ferrule to both the ends of the florinated ethylene propylene tubing. Then, stick a fused silica capillary through the sleeve in a way such that it protrudes about three millimeters out of one side.
Use another thinner capillary and insert it through the bigger capillary such that the thinner one protrudes a few millimeters out of its longer side. Studies show that particles with a diameter between 200 and 400 micrometers are produced at a flow rate of 1.50 to 2.00 mL per hour of the continuous phase whereas the flow rate ratio of the continuous to the monomer phase ranging between 20 and 200. These optical microscopy images show the particle morphology where a prolate-shaped particle elongates its rotational axis when synthesized in a broader polymerization tube.
These rod-like particles are synthesized due to elongation at higher sheer rates during the polymerization in a thinner tube with an internal diameter of 0.5 millimeters. This particle contracts along its rotational axis during the phase transition. Here, the elongation and contraction of actuating core-shell and rod-like shaped Janus particles are represented.
On plotting the actuation properties of single LCE particles, the heating and the cooling curves of the particles show that the strongest actuation of the particles is at the highest flow rate where both the curves form a hystaresis. Interestingly, there is no decrease in the actuation of the particles even over 10 actuation cycles. Once mastered, this microfluidic setup can be prepared in one hour and the particle synthesis takes one to two hours, if it is performed properly.
After its development, this approach opens new possibilities for researchers in the field of liquid crystalline elastomer processing to enable the fabrication of variously shaped microparticles featuring different actuation patterns. While attempting this procedure, it is important to remember to work under UV-free conditions and to use clean microfluidic components in order to prevent clogging during the microfluidic particle production. After watching this video, you should have a good understanding of how to process liquid crystalline monomer mixtures in a capillary based microfluidic reactor, having different temperature zones to address the different liquid crystalline states.
