An "inside-out" microfluidic approach to monodisperse emulsions stabilized by solid particles. Nie, Z., Jai, I., P., Li, W., Bon, S., A., F., & Kumacheva, E. Journal of the American Chemical Society, 130(49):16508-16509, American Chemical Society, 12, 2008.
An "inside-out" microfluidic approach to monodisperse emulsions stabilized by solid particles [link]Website  doi  abstract   bibtex   1 download  
Particle-stabilized emulsions (Pickering emulsions) have recently seen a surge in interest, owing to their extremely high stability against coalescence and a broad range of applications in the fabrication of functional materials, for example, hollow permeable structures, 1 foams, 2,3 and hybrid supracolloidal assemblies. 4-6 Adsorption of colloidal particles to liquid-liquid interfaces occurs when they are not completely wetted by any of the phases. The attachment of a particle of radius a p to a water-oil interface is governed by a reduction of surface energy, E) πa p 2 γ(1 -|cos θ|) 2 , where γ is the interfacial tension between the two liquid phases and θ is the contact angle of the particle at the fluid interface. 7-10 Pickering emulsions are generated by injection methods or by the shearing of a mixture of two immiscible fluids with the solid particles present in excess, that is, in an amount that is significantly larger than is required for the complete coverage and stabilization of the droplets. Lower particle concentrations result in droplet coalescence 11 whereas an excess of particles in the system leads to their undesired loss and in principle, can affect the properties of the material derived from the Pickering emulsions. For a particular system, the concentration of particles required for the efficient stabilization of droplets depends on many factors, including the ratio between the dimensions of droplets and particles, and the number of particles. 9 Current methods for producing particle-coated emulsions generate droplets with a broad distribution of sizes, which complicates the rationalization of the amount of particles introduced in the system. Recently, microfluidic emulsi-fication has provided a means for the formation of highly mono-disperse droplets. 12,13 Furthermore, Subramaniam et al. 14 have shown that the deposition of microbeads from the continuous phase to the bubble-liquid interface is greatly assisted by hydrodynamic flow. Herein, we describe a microfluidic " inside-out " approach to the generation of monodisperse water-in-oil and oil-in-water Pickering emulsions, as well as the supracolloidal polymer microspheres. Our strategy has the following new features: the microfluidic emulsifica-tion of a dispersion of colloidal particles in the particle-free continuous phase and a rationalized, based on geometric consid-erations, approach to controlling the coverage of the droplets with a layer of solid particles. Emulsification was conducted in a microfluidic flow-focusing droplet generator. 13 A dispersion of 3.5 µm-diameter poly(divinyl-benzene-methacrylic acid) (poly(DVB-MAA)) particles in the water-ethanol (85/15 v/v) mixture was emulsified in hexadecane. The value of θ between the polymer film derived from the particles and the water-ethanol mixture was 82.2 (2.1°. Upon the formation of droplets with polydispersity below 5%, the microbeads rapidly migrated from the interior of the droplets to the droplet surface. Since the diameter of the particles used in the present work was significantly smaller than the diameter of the droplets, the surface coverage of the droplets, δ, was estimated as δ) A p /A d) C p F d a d / (4F p a p) (eq 1), where A d and A p are the surface area of the droplet and the area of the droplet coated with particles, respectively; F d is the density of the droplet phase, and C p and F p are the concentration and the density of particles, respectively. In Figure 1 the broken line shows the estimated variation in the concentration of particles required to achieve complete coverage of the droplets with varying size with a colloidal monolayer, with the assumptions that (i) all particles migrate from the droplet interior to the droplet surface and (ii) at the interface the particles form a hexagonal lattice with a packing density of 0.906. Below the line, the amount of particles is not sufficient for the complete coverage of the droplets; whereas above the line, the microbeads present in excess form a multilayer shell or remain in the droplet interior. Since the number of particles is proportional to the droplet volume (∼a d 3) and A d ≈ a d 2 , complete coverage of droplets with larger radii requires a lower concentration of the particles. In the experiments, we controlled the surface coverage of the droplets by varying independently the value of C p from 4 to 16 wt % and the values of a d from 40 to 100 µm (by tuning the ratio of flow rates of the droplet-to-continuous phases. 14 Filled symbols in Figure 1 show the experimental results. Droplets with δ g 0.7 were stable to coalescence, whereas droplets with δ < 0.7 were prone to coalescence when collected at the exit of the microfluidic droplet generator. The attachment of particles from the droplet phase to the liquid-liquid interface and the formation of the close-packed crystalline shell occurred within several seconds and was assisted by the hydrodynamic flow. 14 Owing to the very rapid particle jamming at the fluid interface, at high values of C p , we were able
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 title = {An "inside-out" microfluidic approach to monodisperse emulsions stabilized by solid particles},
 type = {article},
 year = {2008},
 pages = {16508-16509},
 volume = {130},
 websites = {http://dx.doi.org/10.1021/ja807764m},
 month = {12},
 publisher = {American Chemical Society},
 day = {10},
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 abstract = {Particle-stabilized emulsions (Pickering emulsions) have recently seen a surge in interest, owing to their extremely high stability against coalescence and a broad range of applications in the fabrication of functional materials, for example, hollow permeable structures, 1 foams, 2,3 and hybrid supracolloidal assemblies. 4-6 Adsorption of colloidal particles to liquid-liquid interfaces occurs when they are not completely wetted by any of the phases. The attachment of a particle of radius a p to a water-oil interface is governed by a reduction of surface energy, E) πa p 2 γ(1 -|cos θ|) 2 , where γ is the interfacial tension between the two liquid phases and θ is the contact angle of the particle at the fluid interface. 7-10 Pickering emulsions are generated by injection methods or by the shearing of a mixture of two immiscible fluids with the solid particles present in excess, that is, in an amount that is significantly larger than is required for the complete coverage and stabilization of the droplets. Lower particle concentrations result in droplet coalescence 11 whereas an excess of particles in the system leads to their undesired loss and in principle, can affect the properties of the material derived from the Pickering emulsions. For a particular system, the concentration of particles required for the efficient stabilization of droplets depends on many factors, including the ratio between the dimensions of droplets and particles, and the number of particles. 9 Current methods for producing particle-coated emulsions generate droplets with a broad distribution of sizes, which complicates the rationalization of the amount of particles introduced in the system. Recently, microfluidic emulsi-fication has provided a means for the formation of highly mono-disperse droplets. 12,13 Furthermore, Subramaniam et al. 14 have shown that the deposition of microbeads from the continuous phase to the bubble-liquid interface is greatly assisted by hydrodynamic flow. Herein, we describe a microfluidic " inside-out " approach to the generation of monodisperse water-in-oil and oil-in-water Pickering emulsions, as well as the supracolloidal polymer microspheres. Our strategy has the following new features: the microfluidic emulsifica-tion of a dispersion of colloidal particles in the particle-free continuous phase and a rationalized, based on geometric consid-erations, approach to controlling the coverage of the droplets with a layer of solid particles. Emulsification was conducted in a microfluidic flow-focusing droplet generator. 13 A dispersion of 3.5 µm-diameter poly(divinyl-benzene-methacrylic acid) (poly(DVB-MAA)) particles in the water-ethanol (85/15 v/v) mixture was emulsified in hexadecane. The value of θ between the polymer film derived from the particles and the water-ethanol mixture was 82.2 (2.1°. Upon the formation of droplets with polydispersity below 5%, the microbeads rapidly migrated from the interior of the droplets to the droplet surface. Since the diameter of the particles used in the present work was significantly smaller than the diameter of the droplets, the surface coverage of the droplets, δ, was estimated as δ) A p /A d) C p F d a d / (4F p a p) (eq 1), where A d and A p are the surface area of the droplet and the area of the droplet coated with particles, respectively; F d is the density of the droplet phase, and C p and F p are the concentration and the density of particles, respectively. In Figure 1 the broken line shows the estimated variation in the concentration of particles required to achieve complete coverage of the droplets with varying size with a colloidal monolayer, with the assumptions that (i) all particles migrate from the droplet interior to the droplet surface and (ii) at the interface the particles form a hexagonal lattice with a packing density of 0.906. Below the line, the amount of particles is not sufficient for the complete coverage of the droplets; whereas above the line, the microbeads present in excess form a multilayer shell or remain in the droplet interior. Since the number of particles is proportional to the droplet volume (∼a d 3) and A d ≈ a d 2 , complete coverage of droplets with larger radii requires a lower concentration of the particles. In the experiments, we controlled the surface coverage of the droplets by varying independently the value of C p from 4 to 16 wt % and the values of a d from 40 to 100 µm (by tuning the ratio of flow rates of the droplet-to-continuous phases. 14 Filled symbols in Figure 1 show the experimental results. Droplets with δ g 0.7 were stable to coalescence, whereas droplets with δ < 0.7 were prone to coalescence when collected at the exit of the microfluidic droplet generator. The attachment of particles from the droplet phase to the liquid-liquid interface and the formation of the close-packed crystalline shell occurred within several seconds and was assisted by the hydrodynamic flow. 14 Owing to the very rapid particle jamming at the fluid interface, at high values of C p , we were able},
 bibtype = {article},
 author = {Nie, Zhihong and Jai, Il Park and Li, Wei and Bon, Stefan A F and Kumacheva, Eugenia},
 doi = {10.1021/ja807764m},
 journal = {Journal of the American Chemical Society},
 number = {49}
}
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