A new type of armored droplets, so-called polyhedral liquid marbles are introduced in this work. These armored liquid marbles consist of liquid droplets stabilized by hydrophobic hexagonal plates made of poly(ethylene terephthalate), which adsorb to the liquid-air interface. Depending on the specific combination of plate size and droplet diameter, the plates self-assemble into highly ordered hexagonally arranged domains. Even tetrahedral-, pentahedral-, and cube-shaped liquid marbles composed of only 4 to 6 plates are demonstrated. During evaporation of the internal liquid, due to the high adsorption energy of the plates at the liquid-air interface, the overall surface area stayed constant resulting in strongly deformed polyhedral liquid marbles. In line with this, highly asymmetric polyhedral liquid marbles and letters are obtained due to the strong interfacial jamming exerted by the rigid hexagonal plates. This is particularly pronounced for larger plate sizes leading to liquid marbles with unusually sharp edges (for example, rectangular edges). The polyhedral liquid marbles exhibit various stimuli-responsive behaviors simultaneously being exposed to water,
Larger particles are more readily extracted from an advancing bed of charged particles owing to decreased interparticle cohesion.
Liquid marbles are water droplets coated with solid particles that prevent coalescence and allow storage, transport, and handling of liquids in the form of a powder. Here, we report on the formation of liquid marbles that are stabilized entirely by a single monolayer of solid particles and thus minimize the amount of required solid material. As stabilizing particles, we synthesize relatively monodisperse, 80 μm-sized polystyrene (PS) particles coated with heptadecafluorooctanesulfonic acid-doped polypyrrole (PPy-C 8 F) shell (PS/PPy-C 8 F particles) by aqueous chemical oxidative seeded polymerization of pyrrole using FeCl 3 as an oxidant and heptadecafluorooctanesulfonic acid as a hydrophobic dopant. We characterize the physicochemical properties of the particles as a function of the PPy-C 8 F loading. Laser diffraction particle size analyses of dilute aqueous suspensions indicate that the polymer particles disperse stably in water medium before and after coating with the PPy-C 8 F shell. X-ray photoelectron spectroscopy studies indicate the formation of a PPy-C 8 F shell around the PS seed particles, which was further supported by deflated morphologies observed by scanning electron microscopy after extraction of the PS component from the PS/PPy-C 8 F particles. We investigate the performance of the dried PS/PPy-C 8 F particles to stabilize liquid marbles. Stereo-and laser microscope observations, as well as gravimetric studies, confirm that the PS/PPy-C 8 F particles adsorb to the water droplet surface in the form of a particle monolayer with the characteristic hexagonal close-packed structure expected for spherical (colloidal) particles. Mechanical integrity of the liquid marble increases with an increase of PPy-C 8 F loading of the PS/PPy-C 8 F particles. The water contact angle of the PS/PPy-C 8 F particles at air−water interface increases from 82 ± 12°to 102 ± 17°with an increase of PPy-C 8 F loading. This increase in water contact angle directly correlates with the shape of the LM, with higher contact angles giving more spherical LMs. Furthermore, the broadband light absorption properties of the PPy coating was used to control evaporation rate of the enclosed water phase on demand by irradiation with a near-infrared laser. The evaporation rate could be finely controlled by the thickness of the PPy-C 8 F shell of the particles stabilizing the liquid marbles.
A liquid marble (LM) describes a liquid droplet that is wrapped by nonwetting micro-or nanoparticles and therefore obtains characteristics of a solid powder particle. Here, we investigate the effect of the stabilizing particle size on the resulting structure and properties of the LM. We synthesize a series of polystyrene particles with ultrathin coatings of heptadecafluorooctanesulfonic acid-doped polypyrrole with diameters ranging between 1 and 1000 μm by an aqueous chemical oxidative seeded polymerization of pyrrole. The methodology produced a set of hydrophobic particles with similar surface characteristics to allow the formation of LMs and to probe size effects in the LM formation and stabilization efficiency. We found that particles with a size above 20 μm adsorb as a particle monolayer to the surface of the LM, while smaller particles are adsorbed as ill-defined, multilayered aggregates. These results indicate that the balance between particle−particle interaction and gravity is an important parameter to control the surface structure of the LMs. The assembly behavior and size of the particles also correlated with the mechanical integrity of the LM against fall impact. The mechanical resistance was affected by the gap distance between the inner liquid of the LM and supporting substrate, the capillary forces acting between the particles at the LM surface, and the potential energy that depended on the particle size. Last, we demonstrate that the broadband light-absorbing properties of the polypyrrole shell also allow manipulating the evaporation rate of the inner liquid.
Specific particle material properties such as conductivity, cohesion, and density have been neither directly nor thoroughly studied regarding particle behavior in an electrostatic field and the follow-on impact this has on the electrostatic formation of liquid marbles. In this method, an applied electric field drives the extraction of particles from a bed and their transport to a pendent, earthed water droplet. Herein, prior studies of electrostatic formation of particle-stabilized droplets and liquid marbles have been expanded to compare the impact of density using the spherical polystyrene (PS) latex and glass particles of similar shape and size. The addition of thin polymer shells to both samples, which increases the conductivity and cohesion, allows the interplay of these three properties to be examined systematically. Separation distances between the particle bed and the droplet from which particles can initially be extracted increase as the negative applied potential increases. Initial extraction distances of both core particles were found to be similar, ∼1.5 mm at 2.0 kV applied potential, despite the greater density, and thus mass of the glass particles. It is demonstrated that this is a result of competitive interactions between particle density, conductivity, and cohesion; PS is less conductive and more cohesive than glass. Introducing a polypyrrole shell increases the separation distance for extraction to approximately 4 mm for PS core particles but has little impact on glass core particles, demonstrating that for particles with constant conductivity and cohesion reducing the density facilitates extraction. Modeling and quantification of extraction threshold forces for each particle type were undertaken, utilizing the measurement of a radially symmetric area of the particle bed from which particles were observed in the initial extraction stages. This measurement highlighted that it is significantly easier to extract PS compared to glass, with particles extracted from a region in the bed up to 5 times the width in the PS case. Particle density is hypothesized to not be the determining factor in the stabilization of the coated liquid droplets; therefore, the interplay of a multitude of physical properties must be considered when determining the suitability of particulate materials for this electrostatic method.
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