Janus Particle

Paunov VN, Cayre OJ. Supraparticles and Janus particles fabricated by replication of particle monolayers at liquid surfaces using a gel trapping technique. Adv Mater 2004 16 788-791. 

The control that can be exerted over the flow of immiscible fluids in micro-fluidic devices to formation of monodisperse droplets and bubbles can be extended to formation of more complicated objects and architectures of the droplets, such as multiple emulsions [27-32], Janus particles [33, 34] and other morphologies of liquid droplets, solidified particles and capsules [35-39], Figure 13 presents micrographs of the droplets, particles and capsules produced in exemplary techniques. 

T. Nisisako, T. Torii, T. Takahashi, and Y. TaMzawa, Synthesis of monodisperse bicolored janus particles with electrical anisotropy using a microfluidic coflow system. Advanced Materials, 18, 1152—i-, (2006). 

Keywords Block copolymer/nanoparticle mixtures Interfacial assembly Nanoparticles Janus particles... 

As discussed by Binks and Lumsdon, amphiphilic Janus particles can exhibit an interfacial activity several times higher than simple homogeneous particles [54], Janus particles combine the amphiphilic character of surfactants and the physical properties of nanoparticles, which opens new opportunities in emerging areas of nanotechnology and emulsion stabilization. 

Recently, Perro et al. [55] reviewed the developments in the field of Janus particles over the last 15 years, describing various strategies to obtain Janus-type particles using polymer precursors. One strategy is based on the self-assembly of ABC terpolymers in bulk [56, 57] or in solution [58], Another uses the electrostatic interactions of AB and CD diblock copolymers, which lead to inter-polyelectrolyte complexes [59], A different synthetic concept is to obtain Janus particles made of inorganic materials, e.g., acorn-like particles made of PdSx-CogSg [60] or... 

Fig. 9 Geometry of a Janus particle at the oil-water interface. The relative areas of the polar and apolar particle surface regions are parameterized by the angle a. J3 denotes the immersion angle of the particle at the oil-water interface. Reprinted with permission from Langmuir [54]. Copyright (2001) American Chemical Society...

Fig. 10 TEM images of the nanoparticles (a) Janus particles consisting of gold (darker spheres) and iron oxide (brighter spheres)-, (b) homogeneous iron oxide particles (c) gold particles. Scale bars 25 nm. Reprinted with permission from Langmuir [68], Copyright (2006) American Chemical Society...

A special case of nanoparticle self-assembly is the Janus particle. It was shown that Janus particles are considerably more active than homogeneous particles of comparable size and chemical nature and that the interfacial activity can be increased by increasing the amphiphilic character of the particles. Thus, the Janus particles show a significant advantage in the stabilization of emulsions and foams over homogeneous particles as they unify the Pickering concept and the amphiphilicity of a simple surfactant. 

Binks BP, Fletcher PDI (2001) Particles adsorbed at the oil-water interface a theoretical comparison between spheres of uniform wettability and Janus particles. Langmuir 17(16) 4708 1710... 

Walther A, Matussek K, Muller AHE (2008) Engineering nanostructured polymer blends with controlled nanoparticle location using Janus particles. Acs Nano 2(6) 1167-1178... 

Walther A, Muller AHE (2008) Janus particles. Soft Matter 4(4) 663-668... 

Glaser N et al (2006) Janus particles at liquid-liquid interfaces. Langmuir 22 5227... 

Fig. 10 Droplet microfluidics for microparticle synthesis, (a) Janus particle synthesis by coflowing two monomer streams. Reproduced with permission from [82]. (b) Channel geometry facilitated aspherical particles synthesis. Reproduced with permission from [111], (c) Multiple emulsion templated composite particles synthesis. Reproduced with permission from [112]...

The previous sections reviewed recent advancements in sequential electrostatic assembly to form NP-shelled structures. An alternate route to NP assembly arises from interfacial activity and stabilization of NPs. Colloidal particles with partial hydrophilic and hydrophobic character are known to behave like surface-active molecules (surfactants), particularly when adsorbed to a fluid-fluid interface. The assembly of small particles at interfaces is of relevance to advance fields that traditionally feature emulsions, foams, and flotation systems. It is also of pertinence to the development of new fields such as the synthesis of novel materials that include Janus particles, colloidosomes, porous solids, and anisotropic particles, all recently prepared by particle assembly at interfaces [36,38]. 

A graphic picture of possible configurations obtained using microfluidic devices is reported in Figure 20.13, including liquid and solid particles, and they can be extended to production of more complicated objects and architectures such as multiple emulsions or Janus particles. 

Along this line of using amphiphilic features of particles to drive assembly using a hydrophobic effect, there has been a recent surge of interest in the fabrication and behavior of anisotropic patchy or Janus-type colloidal particles as a promising route to innovative nanocomposite materials [40, 41]. Whereas a thorough review lies outside our scope, we would like to highlight a few examples. Muller and coworkers prepared disc-like polymer Janus particles from assem-... 

Muller and coworkers prepared disc-like polymer Janus particles from assembled films of the triblock copolymer SBM and, after hydrolysis of the ester groups into methacrylic acid units, used these as Pickering stabilizer in the soap-free emulsion polymerization of styrene and butyl acrylate [111]. Armes and coworkers described the synthesis of PMMA/siUca nanocomposite particles in aqueous alcoholic media using silica nanoparticles as stabilizer [112], extending this method to operate in water with a glycerol-modified silica sol [113, 114]. Sacanna showed that methacryloxypropyltrimethoxysilane [115] in the presence of nanosized silica led to spontaneous emulsification in water, which upon a two-step polymerization procedure afforded armored particles with an outer shell of PMMA [116]. Bon and coworkers demonstrated the preparation of armored hybrid polymer latex particles via emulsion polymerization of methyl methacrylate and ethyl methacrylate stabilized by unmodified silica nanoparticles (Ludox TM O) [117]. Performance of an additional conventional seeded emulsion polymerization step provided a straightforward route to more complex multilayered nanocomposite polymer colloids (see Fig. 14). 

Nie L, Liu S, Shen W, Chen D, Jiang M (2007) One-pot synthesis of amphiphilic polymeric Janus particles and their self-assembly into supermiceUes with a narrow size distribution. Angew Chem Int Ed 46 6321-6324... 

Walther A, Hoffmann M, Mueller AHE (2008) Emulsion polymerization using Janus particles as stabilizers. Angew Chem Int Ed 47(4) 711-714... 

Figure 5.17 Illustration of the use of a Pickering emulsion to prepare Janus particles.

Initially, a major advantage of the micro-reactor was its ability to achieve effective mixing this was due to the small axial dimensions and, hence, small diffusion distances [1]. Nonetheless, later versions of micro-reactors were often preceded by a micro-mixer, which mixed the reaction components very effectively and quickly. The exception to this was the preparation of so-called Janus particles in a micro-reactor [2, 3], when it was important to prevent any mixing. Although a simple T-junction can serve as a micro-mixer, more advanced mixing devices are now available on a commercial scale (Figure 14.1). 

The creation of Janus particles, with interesting asymmetric electrical properties, was reported by Nisisako et al. [2]. In this case, isobornyl acrylate was used as a hydrophobic, water-insoluble, monomer, and pigmented with carbon black and titanium dioxide as black and white colors, respectively (Figure 14.21). 

Applications of Induced-Charge Electrokinetic in Microfluidics, Fig. 1 (a) The computational domain of the proposed ICEK microvalve with three microchannels and one suspended Janus particle In it. i, and 2,... 

Daghighi Y, Li D (2011) Microvalve using induced-charge electrokinetic motion of Janus particle. Lab Chip 11 2929-2940... 

The canonical example is that of a Janus particle with one metallic and one insulating hemisphere [9], using the standard low-voltage model for electrokinetic motion of polarizable particles. In response to an applied electric field, the Janus particle rotates to align the interface between the two hemispheres with the field axis, due to both ICEP (electrohydrodynamics) and DEP (electrostatics). At the same time, for any orientation, the particle translates in the direction of its insulating end, propelled by ICEO flow on the metallic end, with a velocity... 

Electrokinetic Motion of Heterogeneous Particles, Fig. 3 Induced-charge electrophoresis of Janus particles, illustrated for the case of metal partially coated with insulating thin films, fiom [9]. (a) Stable orientation in a uniform field, showing induced-charge and slip... 

This example suggests how to design particles that spin continuously in a uniform field, as noted by Squires and Bazant [9]. Since a Janus particle always translates toward its less polarizable end, a set of three Janus particles connected by rigid rods can be set into continuous motion like a pinwheel, if... 

Transverse ICEP motion of metallo-dielectric Janus particles in a uniform AC field has recently been observed by Gangwal et al. [10]. Consistent with theoretical predictions in Fig. 3, the particles align and translate perpendicular to the field in the direction of the less polarizable (light) end, as shown in Fig. 4. Larger particles move faster than smaller ones, as expected from Eq. 2, and the velocity scales like the field squared in dilute NaCl solutions. The ICEP velocity decays at higher concentrations, extrapolating to zero around 10 mM. The same concentration dependence is also observed in AC electroosmotic flow and other nonlinear electrokinetic phenomena, which, although poorly understood, further reinforces that the motion is indeed due to ICEP. 

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