1. Synthesis of Single Actuating LCE Particles
2. Synthesis of Core-shell LCE Particles
3. Synthesis of Janus LCE Particles
4. Analysis of the Particles
In this protocol, we present the synthesis of LCE particles with different morphologies via a microfluidic approach. The microfluidic setups for the fabrication of single, core-shell, and Janus particles are shown in Figure 129,38,41. One advantage of the continuous flow production is the very good control over both size and shape of the particles. Figure 2a illustrates the advantage of the single droplet setup: a very narrow size distribution with all particles having the same shape41. Hereby, the size of the spheres can easily be adjusted by changing the ratio of flow rates of the different phases. Following the protocol, particle diameters between 200 and 400 µm can be produced in a well-controlled manner by choosing the flow rate ratios, as shown in Figure 2b1. The best results are obtained for flow rates of the continuous phase (Qc) between 1.5 and 2.0 mL/h and for flow rate ratios of QC/Qd (Qd = the flow rate of the monomer phase) between 20 and 200. For the flow rates of Qc = 1.75 mL/h and Qd = 0.35 mL/h, well-actuating particles with a diameter of 270 µm are observed, for example. If higher ratios Qc/Qd are selected, the droplet formation is less controlled and the particles' size distribution becomes much broader. For lower ratios, the particles are not spherical anymore. In addition to the flow rate adjustments, the distance of the UV-lamp to the polymerization tube as well as the position between the left and the right end of the precision hot plate can change the actuation properties of LCE particles, which happens, for example, if the polymerization kinetics change by reason of choosing monomer mixture compositions or applied polymerization temperatures different from the values described here.
Figure 3a shows an actuating particle which elongates up to 70% when it is heated above its phase transition temperature, which proves that the requirement of inducing an orientation of the liquid crystalline director before polymerization is fulfilled. This alignment of the mesogens results from the shear between the highly viscous continuous phase and the monomer droplets' surface. If silicon oils of lower viscosity are used, the particle's actuation is reduced.
Furthermore, the microfluidic device allows the control over different kinds of actuation patterns, such as elongation or contraction during the phase transition, by varying the shear rate acting on the droplets during the polymerization. This can be processed easily at constant flow rates of the continuous phase by using different inner diameters of the polymerization tube. Figure 3a shows a prolate shaped particle, which elongates along its rotational axis and was synthesized at lower shear rates in a broader polymerization tube (ID: 0.75 mm). The liquid crystalline molecules (mesogens) are aligned along a concentric director field in this case. On the other side, rod-like particles (as illustrated in Figure 3b) feature a contraction during the phase transition and a bipolar alignment of the mesogens' director field. This particle was produced at higher shear rates in a thinner polymerization tube (ID: 0.5 mm).
The protocol describes another advantage of the microfluidic process. Besides single particles, samples of more complex morphologies can also be synthesized. Figure 3c shows an actuating core-shell particle and Figure 3d a Janus particle which both were produced following part 2 and 3 of the protocol29,30.
If all steps of the protocol are done correctly, particles having the properties shown in Figure 4 should be obtained3,41. In Figure 4a, the heating and cooling curves are plotted for single particles synthesized at different flow rates. By heating the particle from room temperature, the liquid crystalline order is – at first – reduced for a little bit, resulting in a small deformation of the particle. However, close to the phase transition temperature, all orientation is suddenly lost and the particle shows a strong elongation just by heating it up for a few degrees. By cooling the particle down, a hysteresis can be observed, and the original shape is obtained. This process is reversible over many actuation cycles, as shown in Figure 4b.
Figure 1: Microfluidic setups. (a) The general setup includes three syringes, which contain the hydraulic silicone oil (1), the aqueous monomer mixture (3), and the continuous phase silicone oil (4). The liquid crystalline monomer mixture (2) is placed in the water bath (5) at 363 K, which heats up the liquid crystal to the isotropic state. The droplet's polymerization is initiated on the hot plate (6) at 338 K in the nematic state of the liquid crystal by UV-irradiation (7). (The single particle setup equals the general setup, but lacks the second capillary, syringe (3) and the second T-junction). (b) This panel shows a setup containing two capillaries side by side to each other, which allows the Janus droplet formation. (c) The core-shell setup is composed of a capillary which is telescoped into a broader second capillary. Please click here to view a larger version of this figure.
Figure 2: Representative particles obtained in the microfluidic single particle setup. (a) This panel shows a microscopy image of monodisperse LCE particles prepared in the microfluidic single particle setup. Scale bar = 200 µm. (b) This panel shows the dependence of the particles' diameter with respect to the ratio of the oil's flow rate (Qc) to the monomer mixture's flow rate (Qd). The size of the obtained particles is only dependent on the velocity ratio of both phases and not on their absolute values. (This figure has been modified from Ohm, Fleischmann, Kraus,Serra, and Zentel1 and Ohm, Serra, and Zentel41.) Please click here to view a larger version of this figure.
Figure 3: Optical microscopy images of four different particle morphologies in the nematic state (at 353 K) and after phase transition in the isotropic state (at 413 K). These panels show (a) the elongation of an oblate-shaped LCE particle (concentric director field), (b) the contraction of a rod-like shaped LCE-particle (bipolar director field), (c) the elongation of an oblate-shaped core-shell particle, and (d) the contraction of a prolate-shaped Janus particle (left part: LCE, right part: acrylamide hydrogel). Scale bars = 100 µm. Please click here to view a larger version of this figure.
Figure 4: Actuation properties of representative single particles. (a) This panel shows the heating and cooling curves of LCE particles being prepared in the single particle microfluidic setup at different flow rates for the continuous phase. The particles prepared at the highest flow rate show the strongest actuation (about 70%) and both curves form a hysteresis, respectively. (b) This is a plot of 10 actuation cycles of LCE particles showing no decrease of their actuation over the cycle number. This proves that the particles are crosslinked, and the actuation is completely reversible. Note: This graph was drawn for a particle made from a main-chain LCE system but looks the same for the LCE system used in this article. (This figure has been modified from Ohm, Serra, and Zentel41.) Please click here to view a larger version of this figure.
NanoTight fitting for 1/16'' OD tubings | Postnova_IDEX | F-333N | |
NanoTight ferrule for 1/16'' OD tubings | Postnova_IDEX | F-142N | |
PEEK Tee for 1/16” OD Tubing | Postnova_IDEX | P-728 | T-junction |
Female Fitting for 1/16” OD Tubing | Postnova_IDEX | P-835 | female luer-lock |
Male Fitting for 1/8” OD Tubing | Postnova_IDEX | P-831 | male luer-lock |
Female Luer Connectors for use with 3/32” ID tubings | Postnova_IDEX | P-858 | for the syrringe's tip |
NanoTight FEP tubing sleeve ID: 395 µm OD: 1/16'' | Postnova_IDEX | F-185 | |
Fused Silica Capillary Tubing ID: 100 µm OD: 165 µm | Postnova | Z-FSS-100165 | glass capillary |
Fused Silica Capillary Tubing ID: 280 µm OD: 360 µm | Postnova | Z-FSS-280360 | glass capillary |
‘‘Pump 33’’ DDS | Harvard Apparatus | 70-3333 | syringe pump |
Precision hot plate | Harry Gestigkeit GmbH | PZ 28-2 | |
Stereomicroscope stemi 2000-C | Carl Zeiss Microscopy GmbH | 455106-9010-000 | |
Mercury vapor lamp Oriel LSH302 | LOT | Intensity: 500 W | |
Teflon Kapillare, 1/16'' x 0,75mm | WICOM | WIC 33104 | teflon tube |
Teflon Kapillare, 1/16'' x 0,50mm | WICOM | WIC 33102 | teflon tube |
Teflon Kapillare, 1/16'' x 0,17mm | WICOM | WIC 33101 | teflon tube |
Silicion oil 1.000 cSt | Sigma Aldrich | 378399 | |
Silicion oil 100 cSt | Sigma Aldrich | 378364 | |
1,6-hexanediol dimethacrylate | Sigma Aldrich | 246816 | Crosslinker |
Lucirin TPO | Sigma Aldrich | 415952 | Initiator |
Polarized optical microscope BX51 | Olympus | For analysis | |
Hotstage TMS 94 | Linkam | For analysis | |
Imaging software Cell^D | Olympus | For analysis |
This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.
This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.
This paper focuses on the microfluidic process (and its parameters) to prepare actuating particles from liquid crystalline elastomers. The preparation usually consists in the formation of droplets containing low molar mass liquid crystals at elevated temperatures. Subsequently, these particle precursors are oriented in the flow field of the capillary and solidified by a crosslinking polymerization, which produces the final actuating particles. The optimization of the process is necessary to obtain the actuating particles and the proper variation of the process parameters (temperature and flow rate) and allows variations of size and shape (from oblate to strongly prolate morphologies) as well as the magnitude of actuation. In addition, it is possible to vary the type of actuation from elongation to contraction depending on the director profile induced to the droplets during the flow in the capillary, which again depends on the microfluidic process and its parameters. Furthermore, particles of more complex shapes, like core-shell structures or Janus particles, can be prepared by adjusting the setup. By the variation of the chemical structure and the mode of crosslinking (solidification) of the liquid crystalline elastomer, it is also possible to prepare actuating particles triggered by heat or UV-vis irradiation.