Particle-covered droplet and a particle shell under compressive electric stress.

Understanding of the behavior of an individual droplet suspended in a liquid and subjected to a stress is important for studying and designing more complex systems, such as emulsions. Here, we present an experimental study of the behavior of a particle-covered droplet and its particle shell under compressive stress. The stress was induced by an application of a DC electric field. We studied how the particle coverage (φ), particle size (d), and the strength of an electric field (E) influence the magnitude of the droplet deformation (D). The experimental results indicate that adding electrically insulating particles to a droplet interface drastically changes the droplet deformation by increasing its magnitude. We also found that the magnitude of the deformation is not retraceable during the electric field sweeping, i.e., the strain-stress curves form a hysteresis loop due to the energy dissipation. The field-induced droplet deformation was accompanied by structural and morphological changes in the particle shell. We found that shells made of smaller particles were more prone to jamming and formation of arrested shells after removal of an electric stress.

Direction-Specific Release from Capsules with Homogeneous or Janus Shells Using an Ultrasound Approach.

A variety of approaches have been developed to release contents from capsules, including techniques that use electric or magnetic fields, light, or ultrasound as a stimulus. However, in the majority of the known approaches, capsules are disintegrated in violent way and the liberation of the encapsulated material is often in a random direction. Thus, the controllable and direction-specific release from microcapsules in a simple and effective way is still a great challenge. This greatly limits the use of microcapsules in applications where targeted and directional release is desirable. Here, we present a convenient ultrasonic method for controllable and unidirectional release of an encapsulated substance. The release is achieved by using MHz-frequency ultrasound that enables the inner liquid stretching, which imposes mechanical stress on the capsule's shell. This leads to the puncturing of the shell and enables smooth liberation of the liquid payload in one direction. We demonstrate that 1-4.3 MHz acoustic waves with the intensity of a few W/cm2 are capable of puncturing of particle capsules with diameters ranging from around 300 µm to 5 mm and the release of the encapsulated liquid in a controlled manner. Various aspects of our route, including the role of the capsule size, ultrasound wavelength, and intensity in the performance of the method, are studied in detail. We also show that the additional control of the release can be achieved by using capsules having patchy shells. The presented method can be used to facilitate chemical reactions in micro- and nanolitre droplets and various small-scale laboratory operations carried in bulk liquids in microenvironment. Our results may also serve as an entry point for testing other uses of the method and formulation of theoretical modeling of the presented ultrasound mechanism.

Opening and Closing of Particle Shells on Droplets via Electric Fields and Its Applications.

Active, tunable, and reversible opening and closing of particle shells on droplets may facilitate chemical reactions in droplets and enable various small-scale laboratory operations, including online detection, measurement, and adjustment of droplet liquid. Manipulating various types of particle shells in a controlled manner requires new routes. This work provides a new strategy for controlling the spatial arrangement of particle-covered oil droplets using electric fields that expands the application of responsive droplets beyond the abovementioned examples. The behavior of stimulated particle-covered droplets is exploited in multiple ways: to form an active smart device, fabricate Janus and patchy shells, create an online diagnostic tool, and produce a tool for fundamental studies. Electric fields are used here to manipulate particle films on oil droplets through the synergetic action of droplet deformation and electrohydrodynamic liquid flows. First, the effects of electric field strength and liquid viscosity on droplet deformation, surface particle arrangements, and dynamics are examined in detail. Then three examples of applications of responsive particle-covered droplets are demonstrated. Our results show that the reversible opening and closing of the droplet's shells, composed of various types of particles, can be conveniently achieved through electric fields, opening up a new possibility for applications in optics, clinical diagnostics, microfluidics, and material engineering.

Assembly of 1D Granular Structures from Sulfonated Polystyrene Microparticles.

Being able to systematically modify the electric properties of nano- and microparticles opens up new possibilities for the bottom-up fabrication of advanced materials such as the fabrication of one-dimensional (1D) colloidal and granular materials. Fabricating 1D structures from individual particles offers plenty of applications ranging from electronic sensors and photovoltaics to artificial flagella for hydrodynamic propulsion. In this work, we demonstrate the assembly of 1D structures composed of individual microparticles with modified electric properties, pulled out of a liquid environment into air. Polystyrene particles were modified by sulfonation for different reaction times and characterized by dielectric spectroscopy and dipolar force measurements. We found that by increasing the sulfonation time, the values of both electrical conductivity and dielectric constant of the particles increase, and that the relaxation frequency of particle electric polarization changes, causing the measured dielectric loss of the particles to shift towards higher frequencies. We attributed these results to water adsorbed at the surface of the particles. With sulfonated polystyrene particles exhibiting a range of electric properties, we showed how the electric properties of individual particles influence the formation of 1D structures. By tuning applied voltage and frequency, we were able to control the formation and dynamics of 1D structures, including chain bending and oscillation.

Electric Field-Driven Assembly of Sulfonated Polystyrene Microspheres.

A designed assembly of particles at liquid interfaces offers many advantages for development of materials, and can be performed by various means. Electric fields provide a flexible method for structuring particles on drops, utilizing electrohydrodynamic circulation flows, and dielectrophoretic and electrophoretic interactions. In addition to the properties of the applied electric field, the manipulation of particles often depends on the intrinsic properties of the particles to be assembled. Here, we present an easy approach for producing polystyrene microparticles with different electrical properties. These particles are used for investigations into electric field-guided particle assembly in the bulk and on surfaces of oil droplets. By sulfonating polystyrene particles, we produce a set of particles with a range of dielectric constants and electrical conductivities, related to the sulfonation reaction time. The paper presents diverse particle behavior driven by electric fields, including particle assembly at different droplet locations, particle chaining, and the formation of ribbon-like structures with anisotropic properties.

Mechanics of Pickering Drops Probed by Electric Field-Induced Stress.

Fluid drops coated with particles, so-called Pickering drops, play an important role in emulsion and capsule applications. In this context, knowledge of mechanical properties and stability of Pickering drops are essential. Here we prepare Pickering drops via electric field-driven self-assembly. We use direct current (DC) electric fields to induce mechanical stress on these drops, as a possible alternative to the use of, for example, fluid flow fields. Drop deformation is monitored as a function of the applied electric field strength. The deformation of pure silicone oil drops is enhanced when covered by insulating polyethylene (PE) particles, whereas drops covered by conductive clay particles can also change shape from oblate to prolate. We attribute these results to changes in the electric conductivity of the drop interface after adding particles, and have developed a fluid shell description to estimate the conductivity of Pickering particle layers that are assumed to be non-jammed and fluid-like. Retraction experiments in the absence of electric fields are also performed. Particle-covered drops retract slower than particle-free drops, caused by increased viscous dissipation due to the presence of the Pickering particle layer.

Continuous and discontinuous transitions between two types of capillary bridges on a beaded chain pulled out from a liquid.

Capillary bridges can be used for fabricating new materials and structures. Here, we describe theoretically and validate experimentally the mechanism of formation of capillary bridges during a process in which a beaded chain is being pulled out from a liquid with a planar surface. There are two types of capillary bridges present in this system, namely the sphere-planar liquid surface bridge initially formed between the spherical bead leaving the liquid bath and the initially planar liquid surface, and the sphere-sphere capillary bridge formed between neighbouring beads in the part of the chain above the liquid surface. During the process of pulling the chain out of the liquid, the sphere-planar liquid surface bridge transforms into the sphere-sphere bridge. We show that for monodisperse spherical beads comprising the chain, this transition can be either continuous or discontinuous. The transition is continuous when the diameter of the spherical beads is larger than the capillary length. Otherwise, the transition is discontinuous, likewise the capillary force acting on the chain.

Formation of printable granular and colloidal chains through capillary effects and dielectrophoresis.

One-dimensional conductive particle assembly holds promise for a variety of practical applications, in particular for a new generation of electronic devices. However, synthesis of such chains with programmable shapes outside a liquid environment has proven difficult. Here we report a route to simply 'pull' flexible granular and colloidal chains out of a dispersion by combining field-directed assembly and capillary effects. These chains are automatically stabilized by liquid bridges formed between adjacent particles, without the need for continuous energy input or special particle functionalization. They can further be deposited onto any surface and form desired conductive patterns, potentially applicable to the manufacturing of simple electronic circuits. Various aspects of our route, including the role of particle size and the voltages needed, are studied in detail. Looking towards practical applications, we also present the possibility of two-dimensional writing, rapid solidification of chains and methods to scale up chain production.

Assembly and Rearrangement of Particles Confined at a Surface of a Droplet, and Intruder Motion in Electro-Shaken Particle Films.

Manipulation of particles at the surface of a droplet can lead to the formation of structures with heterogeneous surfaces, including patchy colloidal capsules or patchy particles. Here, we study the assembly and rearrangement of microparticles confined at the surface of oil droplets. These processes are driven by electric-field-induced hydrodynamic flows and by 'electro-shaking' the colloidal particles. We also investigate the motion of an intruder particle in the particle film and present the possibility of segregating the surface particles. The results are expected to be relevant for understanding the mechanism for particle segregation and, eventually, lead to the formation of new patchy structures.

Electroformation of Janus and patchy capsules.

Janus and patchy particles have designed heterogeneous surfaces that consist of two or several patches with different materials properties. These particles are emerging as building blocks for a new class of soft matter and functional materials. Here we introduce a route for forming heterogeneous capsules by producing highly ordered jammed colloidal shells of various shapes with domains of controlled size and composition. These structures combine the functionalities offered by Janus or patchy particles, and those given by permeable shells such as colloidosomes. The simple assembly route involves the synergetic action of electro-hydrodynamic flow and electro-coalescence. We demonstrate that the method is robust and straightforwardly extendable to production of multi-patchy capsules. This forms a starting point for producing patchy colloidosomes with domains of anisotropic chemical surface properties, permeability or mixed liquid-solid phase domains, which could be exploited to produce functional emulsions, light and hollow supra-colloidosome structures, or scaffolds.

Active structuring of colloidal armour on liquid drops.

Adsorption and assembly of colloidal particles at the surface of liquid droplets are at the base of particle-stabilized emulsions and templating. Here we report that electrohydrodynamic and electro-rheological effects in leaky-dielectric liquid drops can be used to structure and dynamically control colloidal particle assemblies at drop surfaces, including electric-field-assisted convective assembly of jammed colloidal 'ribbons', electro-rheological colloidal chains confined to a two-dimensional surface and spinning colloidal domains on that surface. In addition, we demonstrate the size control of 'pupil'-like openings in colloidal shells. We anticipate that electric field manipulation of colloids in leaky dielectrics can lead to new routes of colloidosome assembly and design for 'smart armoured' droplets.

Oxygen-controlled phase segregation in poly(N-isopropylacrylamide)/laponite nanocomposite hydrogels.

The combination of nanoparticles and polymers into nanocomposite gels has been shown to be a promising route to creating soft materials with new or improved properties. In the present work, we have made use of Laponite nanoparticles in combination with a poly(N-isopropylacrylamide) (PNIPAAM) polymer and describe a phenomenon taking place during the polymerization and gelling of this system. The presence of small amounts of oxygen in the process induces two distinctly separated phases, one polymer-rich and one polymer-deficient water-clay phase. Complex interactions among clay, oxygen, and the polymer are found to govern the behavior of these phases. It is also observed that the initial clay concentration can be used to control the volume fraction of the polymer-deficient phase directly. The dynamics of the phase boundary is found to be dependent on water penetration and in general to exhibit non-Fickian behavior. An approach using video recording to monitor hydrogel swelling is also presented, and its advantages are addressed.

Guided self-assembly of nanostructured titanium oxide.

A series of nanostructured titanium oxide particles were synthesized by a simple wet chemical method and characterized by means of small-angle x-ray scattering (SAXS)/wide-angle x-ray scattering (WAXS), atomic force microscope (AFM), scanning electron microscope (SEM), transmission electron microscope (TEM), thermal analysis, and rheometry. Tetrabutyl titanate (TBT) and ethylene glycol (EG) can be combined to form either TiO(x) nanowires or smooth nanorods, and the molar ratio of TBT:EG determines which of these is obtained. Therefore, TiO(x) nanorods with a highly rough surface can be obtained by hydrolysis of TBT with the addition of cetyl-trimethyl-ammonium bromide (CTAB) as surfactant in an EG solution. Furthermore, TiO(x) nanorods with two sharp ends can be obtained by hydrolysis of TBT with the addition of salt (LiCl) in an EG solution. The AFM results show that the TiO(x) nanorods with rough surfaces are formed by the self-assembly of TiO(x) nanospheres. The electrorheological (ER) effect was investigated using a suspension of titanium oxide nanowires or nanorods dispersed in silicone oil. Oil suspensions of titanium oxide nanowires or nanorods exhibit a dramatic reorganization when submitted to a strong DC electric field and the particles aggregate to form chain-like structures along the direction of applied electric field. Two-dimensional SAXS images from chains of anisotropically shaped particles exhibit a marked asymmetry in the SAXS patterns, reflecting the preferential self-assembly of the particles in the field. The suspension of rough TiO(x) nanorods shows stronger ER properties than that of the other nanostructured TiO(x) particles. We find that the particle surface roughness plays an important role in modification of the dielectric properties and in the enhancement of the ER effect.

Minute synthesis of extremely stable gold nanoparticles.

We describe a rapid environmentally friendly wet-chemical approach to synthesize extremely stable non-toxic, biocompatible, water-soluble monodispersed gold nanoparticles (AuNPs) in one step at room temperature. The particles have been successfully achieved in just a few minutes by merely adding sodium hydroxide (NaOH) acting as an initiator for the reduction of HAuCl(4) in aqueous solution in the presence of polyvinylpyrrolidone (PVP) without the use of any reducing agent. It is also proved to be highly efficient for the preparation of AuNPs with controllable sizes. The AuNPs show remarkable stability in water media with high concentrations of salt, various buffer solutions and physiological conditions in biotechnology and biomedicine. Moreover, the AuNPs are also non-toxic at high concentration (100 microM). Therefore, it provides great opportunities to use these AuNPs for biotechnology and biomedicine. This new approach also involved several green chemistry concepts, such as the selection of environmentally benign reagents and solvents, without energy consumption, and less reaction time.