Magnetic nanoparticles-induced anisotropic shrinkage of polymer emulsion droplets

Abstract

We here report magnetic nanoparticles (NPs)-induced buckling instability and anisotropic shrinkage behavior of polymer emulsion droplets. The oil-in-water emulsion is stabilized by the surfactant, and NPs are dispersed into the oil phase. The surface ligands (oleic acid and oleylamine) number of the NPs is an important factor to affect the shrinkage process. When a part of the ligands of the NPs is removed, the NPs show good interface attachment at the oilwater interface even with the presence of a large amount of surfactant. The increase of the interfacial viscoelasticity resulting from the attachment of NPs induced the occurrence of a buckling process. The mechanism is explored and the effect of the concentration of polystyrene and NPs is investigated in detail. The results could be helpful to understand and solve problems related with coating techniques and elastic instabilities in nature.The drying or shrinkage of colloidal suspensions has attracted noticeable attention for fundamental and practical reasons.1-3 Liquid droplets even only containing tiny colloidal particles show different and more complicated rheological behaviour. For example, the shell buckling phenomena was observed when a latex suspension drop was drying on the superhydrophobic planar substrate, or when a free liquid drop was shrinking on a hot plate with a temperature above 150 C (Leidenfrost effect). 4,5 Normally, when a colloid suspension droplet is shrinking, the evaporation of the solvent results in the rise of colloidal particle concentration and the reduction of the interparticle distance accordingly with the receding of the liquid/gas interface. 2,5,6 If the solvent loss is fast enough, these particles do not have enough time to diffuse to the bulk phase to homogenize the whole liquid drop, and they will be jammed at the interface.2 This process can transform the interface layer from a viscous liquid state to a glass-like solid state. The further loss of solvent decreases the pressure of the inner phase. Under the external pressure, the elasticity of the shell results in an inversion of curvature. However, the onset of buckling is dependent on particle interaction.5 Before the occurrence of buckling, the liquid droplet behavior is dominated by its viscosity, thus the shrinkage process is isotropic.7 However, the buckling of the shell results in an anisotropic shrinkage.Similar buckling phenomena have been intensively studied in the shrinkage of the interface of Pickering emulsion. Generally, simple liquid drops have isotropic interface shrinkage in a continuous bulk phase with the solvent evaporation.8 However, colloidal particles are considered to irreversibly attach to an oil-water interface by a selfdriving of minimum energy. This attaching behavior could affect the interface properties.9-11 The corresponding reduction of the interfacial energy is expressed as E ¼ pr 2 g ow (1 + cos q) 2 , where r is particle radius, g ow is the tension of the oil-water interface, and q is the contact angle measured through the water phase.9 Tho...

Encapsulation of hydrophobic or hydrophilic iron oxide nanoparticles into poly(lactic acid) micro/nanoparticles via adaptable emulsion setup

“…The inner particle surface was rougher than the outer, with the appearance of cracks. It has been reported that the magnetic NPs at the interface could induce the buckling instability of emulsion droplets during the solvent removal, causing the shrinkage of particles . This is consistent with our proposed W/O/W emulsion formation, namely, hydrophilic IONPs have been captured into the inner water phase (Figure ).…”

“…This led to the formation of the closed‐hollow microparticles after drying [Figure (c)]. IONPs‐induced anisotropic shrinkage caused cracks on the surface or collapse of polymer particles as shown in a higher magnification [Figure (b)]. To encapsulate hydrophobic IONPs into PLA nanoparticles, the one‐step W/O/W emulsion was not necessary and RT was enough to capture hydrophobic IONPs [Figure (b), RT], corresponding to sample F in Table [Figure (d)].…”

Synthesis of Polystyrene-Coated Superparamagnetic and Ferromagnetic Cobalt Nanoparticles

“…Generally, thermogravimetric analysis (TGA) is a widely used technique to evaluate the organic content on the surface of NPs [62,63,64,65,66]. The density of polystyrene on the nanoparticle surface was calculated according to TGA.…”

Sono‐Assembly of Highly Biocompatible Polysaccharide Capsules for Hydrophobic Drug Delivery

“…With the loss of the toluene, the surface area of the capsule decreases. The onset of collapse takes place when the critical pressure inside the spherical shell cannot resist the external pressure . As seen in Figure , three capsules were imaged to record the collapse of their shells in the shrinking process.…”