Fabrication of Tunable Spherical Colloidal Crystals Immobilized in Soft Hydrogels
Original entry: Darren Yang, AP225, Fall 2010
T. Kanai, D. Lee, H.C. Shum, D.A. Weitz, “Fabrication of Tunable Spherical Colloidal Crystals Immobilized in Soft Hydrogels,” Small, 6, 807–810 (2010).
colloids, hydrogels, microfluidics, photonic crystals, stimuli-responsive materials
Background and Summary
Traditionally, spherical colloidal crystals are fabricated using droplets as templates. In such method, droplets of aqueous colloidal suspension are dispensed on a hydrophobic surface then dried. The evaporation of the liquid medium leads to the assembly of particles into close-packed structures. In this process, the particles touch one another, and the crystal lattice constant is uniquely determined by the diameter of the particles. Thus, colloidal crystals made from this method will have different wavelengths for their optical stop band requires colloidal particles with different size or refractive index.
In this paper, the authors demonstrate a new method to fabricate colloidal crystal spheres through the combination of microfluidics and photopolymerization. The colloidal crystal spheres made from this novel process are differ from traditional ones in two aspects: (1) the wavelength of their photonic band activity is not uniquely determined by the size of the colloidal particles; instead, it can be adjusted by simply varying the particle concentration before polymerization. (2) The wavelength of the photonic band activity is also controlled by external stimuli, such as temperature. Thus the wavelength can be tuned even after the colloidal crystal spheres are formed.
The process for fabricating the gel-immobilized colloidal crystal spheres is illustrated in the following figure.
The majority of the process is carried out in the microfluidic device. The device has a flow-focusing geometry to generate monodisperse water-in-oil emulsions. (The water phase is pumped from one end of the outer square capillary while the oil phase flows from the opposite end into the orifice of the inner tapered collection tube. The oil phase hydrodynamically focuses the aqueous phase, which breaks up at the orifice of the collection tube to form monodisperse W/O droplets.) In addition, the water phase consists of (1) charged polystyrene particles with a diameter of 198 nm, (2) crosslinker, and (3) polymerization initiator for photopolymerization. The oil phase consist of PDMS oil with a surfactant Dow Corning 749 to stabilize the oil–water interfaces. After water-in-oil emulsions droplets formation, the emulsion droplets were irradiated with UV light to photopolymerize the aqueous droplets, thereby immobilizing the colloidal crystals in the polyacrylamide hydrogel.
By varying the conditions such as the surfactant concentration, the gel-reagent concentration, and the particle concentration, stable monodisperse emulsions can be formed (Figure 2a). After UV irradiation, the colloidal crystals in the water droplets is then immobilized in hydrogel spheres, without losing the crystalline structure of charged colloids (Figure 2b). The size of the gel spheres is determined by that of the pre-gel droplets, which can be controlled by adjusting the relative flow rates of the fluids or the size of the capillary orifice. Since the gel-immobilized colloidal crystal spheres are soft, they are deformable under compression between glass slides (Figure 2c). Upon illumination by visible light, the gel-immobilized colloidal crystal spheres show the characteristic colors of diffraction patterns (Figure 2d–g). The PNIPAm gel is a thermosensitive polymer that undergoes a volume transition at a lower critical solution temperature of 32 °C. Upon heating from 22 to 40 °C, the PNIPAm-immobilized colloidal crystal sphere begins to shrink rapidly at around 32 °C, resulting in a change in the diffraction color and Bragg diffraction wavelength (Figure 2g and 3).
Soft Matter Connection
The new tunable spherical colloidal crystals fabrication technique was based on many principles of soft condense matter. The formation of water-in-oil emulsions droplet is closely depended on the hydrophilic-hydrophobic interaction as well as interfacial surface tension. For the individual water droplet to form from a continuos stream of water, not only the flow rate is an important factor, the Laplace pressure arise from the surface tension is a important factor as well. One can imagine when the oil is replace by a different hydrophobic liquid (with different surface tension), the flow rate needs to be readjusted as well.