Order-to-disorder transition in colored microgel monolayers
12 Jul 2019-Vol. 2115, Iss: 1, pp 030048
TL;DR: In this paper, loosely packed homogeneous monolayers of soft colloidal particles can be fabricated via simple evaporative drying of aqueous dispersion containing soft microgels, resulting in a uniform deposition of the particles throughout the fluid-substrate contact area.
Abstract: We show that loosely packed homogeneous monolayers of soft colloidal particles can be fabricated via simple evaporative drying of aqueous dispersion containing colloidal particles. Owing to the inherent amphiphilicity of the soft microgels, they are preferentially adsorbed onto the water-vapor interface of the drying droplet, resulting in a uniform deposition of the particles throughout the fluid-substrate contact area. Distinct order-to-disorder transition is also identified within the dried deposits on changing spatial locations from edge to center of the circular stain. Such monolayer deposits exhibit vibrant colors on irradiating with white light due to interference.
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TL;DR: In this article, the authors ascribe the characteristic pattern of the deposition to a form of capillary flow in which pinning of the contact line of the drying drop ensures that liquid evaporating from the edge is replenished by liquid from the interior.
Abstract: When a spilled drop of coffee dries on a solid surface, it leaves a dense, ring-like deposit along the perimeter (Fig 1a) The coffee—initially dispersed over the entire drop—becomes concentrated into a tiny fraction of it Such ring deposits are common wherever drops containing dispersed solids evaporate on a surface, and they influence processes such as printing, washing and coating1,2,3,4,5 Ring deposits also provide a potential means to write or deposit a fine pattern onto a surface Here we ascribe the characteristic pattern of the deposition to a form of capillary flow in which pinning of the contact line of the drying drop ensures that liquid evaporating from the edge is replenished by liquid from the interior The resulting outward flow can carry virtually all the dispersed material to the edge This mechanism predicts a distinctive power-law growth of the ring mass with time—a law independent of the particular substrate, carrier fluid or deposited solids We have verified this law by microscopic observations of colloidal fluids
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TL;DR: In this paper, the authors synthesized soft uncharged microgels made of poly(N-isopropylacrylamide) of variable cross-linking degrees and probed their efficiency as stabilisers to fabricate oil-in-water emulsions.
Abstract: We synthesised soft uncharged microgels made of poly(N-isopropylacrylamide) of variable cross-linking degrees and we probed their efficiency as stabilisers to fabricate oil-in-water emulsions. Interestingly, such particles undergo a swollen-to-collapsed transition above a critical temperature. By combining several microscopy methods and by exploiting the limited coalescence process, we were able to determine both the particle concentration and structure at the interface, as a function of the cross-linking density. Being deformable, the initially spherical microgels adopt a “fried egg-like” structure when adsorbed at the oil/water interface. As expected, the interfacial deformation is increasingly pronounced as the cross-linking degree decreases. The most deformable microgels tend to form 2D connected networks characterised by significant overlapping of the peripheral parts. When the deformability is lost, by increasing the cross-linking density or the temperature, the stabilisation efficiency is considerably reduced. Our results strongly suggest that emulsion stability is mainly determined by the microgels' deformability and we discuss the origin of that empirical link in terms of lateral overlapping and interfacial elasticity.
315 citations
TL;DR: In this article, ordered 2D structures composed of poly(N-isopropylacrylamide) (PNIPAM) microgel particles that had regularity on a sub-micrometer length scale were prepared by using sterically stabilized PNIPAM microgel particle as components, the ordered array was formed by a self-assembly process.
Abstract: Ordered 2-D structures composed of poly(N-isopropylacrylamide) (PNIPAM) microgel particles that had regularity on a sub-micrometer length scale were prepared By using sterically stabilized PNIPAM microgel particles as components, the ordered array was formed by a self-assembly process The particle array was prepared by depositing a droplet of the microgel dispersion on a substrate Atomic force microscopy observation of the resulting thin films revealed that they comprised a monolayer particle array The periodic structure of the array produced iridescent colors due to optical diffraction Since a homogeneous particle array can be prepared simply by drying the dispersion, this particle dispersion may be considered as a new ink whose color is generated from the microstructure in the films produced
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TL;DR: The relationship between the drying mechanism and structure of the resultant microgel thin film has been clarified by changing the microgel concentration of the dispersions.
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