Dynamic Oscillation and Motion of Oil-in-Water Emulsion Droplets

The interplay of interfacial tensions on droplets results in a range of self-powered motions that mimic those of living systems and serve as a tunable model to understand their complex non-equilibrium behavior. Spontaneous shape deformations and oscillations are crucial features observed in nature but difficult to incorporate in synthetic artificial systems. Here, we report sessile oil-in-water emulsions that exhibit rapid oscillating behavior. The oscillations depend on the nature and concentration of the surfactant, the chemical composition of the oil, and the wettability of the solid substrate. The rapid changes in the contact angle per oscillation are observed using side-view optical microscopy. We propose that the changes in the interfacial tension of the oil droplets is due to the partitioning of the surfactant into the oil phase and the movement of self-emulsified oil out of the parent droplets giving rise to the rhythmic variation in droplet contact-line. The ability to control and understand droplet oscillation can help model similar oscillations in out-of-equilibrium systems in nature and reproduce biomimetic behavior in artificial systems for various applications, such as microfluidic lab-on-a-chip and adaptive materials.

Crescent-shaped micromotor sorbents with sulfonic acid functionalized convex surface: The synthesis by A Janus emulsion strategy and adsorption for Li. JOURNAL OF HAZARDOUS MATERIALS 2022;422:126870. [PMID: 34425430 DOI: 10.1016/j.jhazmat.2021.126870]

Self-propelled micromotor (SPM) plays a vital role in recycling of lithium (Li⁺) from wastewater in battery industry. In this work, a crescent-shaped micromotor sorbent (CSMSs) with sulfonic group on convex surface was prepared by Janus emulsion to extract Li⁺. Using sodium p-styrene sulfonate as a functional monomer, well-designed CSMSs was prepared by UV-induced monomer interfacial polymerization, and their pit size can be controlled by adjusting the ratio of two incompatible oils (ethoxylated trimethylolpropane triacrylate and liquid paraffin). In addition, MnO₂ nanoparticles, which embedded into concave interface, generated O₂ bubbles in the presence of H₂O₂, and constant circular or line motion of CSMSs was observed. Zeta potential of CSMSs was -51.66 eV at pH = 10, and strong electrostatic attraction between sulfonate groups and Li⁺ endowed the maximum monolayer adsorption capacity of 31 mg g^-1 at 25 °C. Self-propelled effect further enhanced kinetic performance for Li⁺, and equilibrium time can be reduced from original 10-6.0 h, suggesting autonomous movement achieves rapid mixing and mass transportation. After three adsorption/desorption cycles, the adsorption capacity of the material remains above 90%. This simple and large-scale preparation strategy provided a synthetic method for functional and Janus SPM, as well as sorbents for Li⁺ enrichment.

Photothermally responsive Pickering emulsions stabilised by polydopamine nanobowls

Pickering emulsions with stimuli responsive properties have attracted mounting research attention owing to their potential for on-demand destabilisation of emulsions. However, a combination of biocompatibility and long-term stability are essential to efficiently apply such systems in biomedical applications, and this remains a significant challenge. To address current limitations, here we report the formation of photothermally responsive oil-in-water (o/w) Pickering emulsions fabricated using biocompatible stabilisers and showing prolonged stability. For the first time, we explore polydopamine (PDA) bowl-shaped mesoporous nanoparticles (PDA nanobowls) as a Pickering stabiliser without any surface modification or other stabiliser present. As-prepared PDA nanobowl-stabilised Pickering emulsions are shown to be pH responsive, and more significantly show high photothermal efficiency under near-infrared illumination due the incorporation of PDA into the system, which has remarkable photothermal response. These biocompatible, photothermally responsive o/w Pickering emulsion systems show potential in controlled drug release applications stimulated by NIR illumination.

Exploring the transition of polydopamine-shelled perfluorohexane emulsion droplets into microbubbles using small- and ultra-small-angle neutron scattering

Perfluorocarbon emulsion droplets are interesting colloidal systems with applications, ranging from diagnostics and theranostics to drug delivery, due to their controllable phase transition into microbubbles via heat application or acoustic droplet vapourisation. This work highlights the application of small- and ultra-small-angle neutron scattering (SANS and USANS, respectively), in combination with contrast variation techniques, in observing the in situ phase transition of polydopamine-stabilised perfluorohexane (PDA/PFH) emulsion droplets into microbubbles during heating. Results show peak USANS intensities at temperatures around 90 °C, which indicates that the phase transition of PDA/PFH emulsion droplets occurs at significantly higher temperatures than the bulk boiling point of pure liquid PFH (56 °C). Analysis and model fitting of the SANS and USANS data allowed us to estimate droplet sizes and interfacial properties at different temperatures (20 °C, 90 °C, and 20 °C after cooling), giving valuable insights about the transformation of these polydisperse emulsion droplet systems.

Nanomechanical Insights into Versatile Polydopamine Wet Adhesive Interacting with Liquid-Infused and Solid Slippery Surfaces

Mussel-inspired polydopamine (PDA) can be readily deposited on almost all kinds of substrates and possesses versatile wet adhesion. Meanwhile, slippery surfaces have attracted much attention for their self-cleaning capabilities. It remains unclear how the versatile PDA adhesive would interact with slippery surfaces. In this work, both liquid-infused poly(tetrafluoroethylene) (PTFE) (LI-PTFE) and solid slippery surfaces (i.e., self-assembly of small thiol-terminated organosilane, polysiloxane covalently attached to substrates) were fabricated to investigate their capability to prevent PDA deposition. It was found that PDA particles could be easily deposited on a PTFE membrane and the two types of solid slippery surfaces, which resulted in the alternation of their surface wettability and slippery behavior of water droplets. Adhesion was detected between a PDA-coated silica colloidal probe and the PTFE membrane or solid slippery surfaces through quantitative force measurements using an atomic force microscope (AFM), mainly due to van der Waals (vdW) and hydrophobic interactions, which led to the PDA deposition phenomenon. In contrast, LI-PTFE with a thin liquid lubricant film could effectively prevent PDA deposition, with negligible changes in surface morphology, wettability, and slippery characteristics. Although PDA particles could be loosely attached to the lubricant/water interface for LI-PTFE based on the capillary adhesion measured by AFM, they could be readily removed by gentle rinsing with water, as demonstrated by the ultralow friction over LI-PTFE as compared to PTFE using lateral force microscopy (LFM). Our results indicate that LI-PTFE possesses excellent antifouling and self-cleaning properties even when interacting with the versatile PDA wet adhesives. This work provides new insights into the deposition of PDA on slippery surfaces and their interaction mechanism at the nanoscale, with useful implications for the design and development of novel slippery surfaces.

Electrostatic Interaction-Controlled Formation of Pickering Emulsion for Continuous Flow Catalysis

Although noble metal or non-noble metal-catalyzed reactions are widely used, it is still difficult to apply these reactions in the large-scale synthesis of chemicals because most of the reactions are carried out by the inefficient batch reaction strategy. Herein, Pickering emulsion-based continuous flow catalysis was utilized to address this problem. Cellulose nanofibers with aldehyde groups (ACNF) were generated through oxidizing C2 and C3 hydroxyl groups of cellulose nanofibers into aldehyde groups by NaIO₄, followed by in situ depositing Ag nanoparticles on ACNF to produce Ag-decorated ACNF (ACNF@Ag) via a facile aldehyde-induced reduction method. ACNF@Ag with ∼2 wt % Ag (ACNF@Ag2) has been used to prepare the Pickering emulsion by controlling the electrostatic interaction between ACNF@Ag2 and the oil-water interface via adjusting the pH. It was found that the Pickering emulsion could be generated at a pH around 3.29 and was determined to be the oil-in-water emulsion. The reduction of organic molecules (4-nitrophenol (4-NP), methylene blue (MB), and methyl orange (MO)) was selected as a model reaction to test the reliability of the Pickering emulsion in continuous flow catalysis, which demonstrated very high conversion rates for 4-NP (>98%, 50 h), MB (>99%, 30 h), and MO (>96%, 40 h).

Effect of NaCl concentration on stability of a polymer–Ag nanocomposite based Pickering emulsion: validation via rheological analysis with varying temperature

Schematic for the impact of NaCl on droplet stabilization in Pickering emulsions.

Particle-stabilized Janus emulsions that exhibit pH-tunable stability

A simple and large-scale fabrication approach to create Janus emulsions that can be controlled in size, geometry and stability.

Fabrication of Hybrid Capsules via CaCO3 Crystallization on Degradable Coacervate Droplets

Organic-inorganic CaCO₃ capsules were prepared by crystallization of CaCO₃ on Pickering emulsion prepared using coacervate droplets made from thermoresponsive and degradable poly(2-methylene-1,3-dioxepane- co-2-hydroxyethyl acrylate) (poly(MDO- co-HEA)) in sole aqueous medium. The diameters of CaCO₃-based Pickering emulsion could be controlled by varying several parameters: diameter of CaCO₃ powders, initial polymer concentration, and copolymer composition. The CaCO₃ Pickering emulsion was able to load low-molecular-weight hydrophobic substances at temperatures above the lower critical solution temperature (LCST) due to formation of polymer-concentrated phases, i.e., coacervate droplets. The diameter of CaCO₃ capsules prepared by crystallization also depended on the diameter of the CaCO₃ Pickering emulsion. The CaCO₃ shell was composed of calcite-type crystals, the most stable polymorph among known CaCO₃ crystals. The facially prepared CaCO₃ capsules are valuable for use in functional biomaterials, such as drug delivery carriers and cell culture scaffolds for noninvasive bone-regenerative medicine.

Stimuli-Responsive Polymeric Nanoparticles

There is increasing evidence that stimuli-responsive nanomaterials have become significantly critical components of modern materials design and technological developments. Recent advances in synthesis and fabrication of stimuli-responsive polymeric nanoparticles with built-in stimuli-responsive components (Part A) and surface modifications of functional nanoparticles that facilitate responsiveness (Part B) are outlined here. The synthesis and construction of stimuli-responsive spherical, core-shell, concentric, hollow, Janus, gibbous/inverse gibbous, and cocklebur morphologies are discussed in Part A, with the focus on shape, color, or size changes resulting from external stimuli. Although inorganic/metallic nanoparticles exhibit many useful properties, including thermal or electrical conductivity, catalytic activity, or magnetic properties, their assemblies and formation of higher order constructs are often enhanced by surface modifications. Section B focuses on selected surface reactions that lead to responsiveness achieved by decorating nanoparticles with stimuli-responsive polymers. Although grafting-to and grafting-from dominate these synthetic efforts, there are opportunities for developing novel synthetic approaches facilitating controllable recognition, signaling, or sequential responses. Many nanotechnologies utilize a combination of organic and inorganic phases to produce ceramic or metallic nanoparticles. One can envision the development of new properties by combining inorganic (metals, metal oxides) and organic (polymer) phases into one nanoparticle designated as "ceramers" (inorganics) and "metamers" (metallic).

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion

The aim of this paper is to review the key findings about how particle-stabilised (or Pickering) emulsions respond to stress and break down. Over the last ten years, new insights have been gained into how particles attached to droplet (and bubble) surfaces alter the destabilisation mechanisms in emulsions. The conditions under which chemical demulsifiers displace, or detach, particles from the interface were established. Mass transfer between drops and the continuous phase was shown to disrupt the layers of particles attached to drop surfaces. The criteria for causing coalescence by applying physical stress (shear or compression) to Pickering emulsions were characterised. These findings are being used to design the structures of materials formed by breaking Pickering emulsions.

The significant impact of polydopamine on the catalytic performance of the carried Au nanoparticles

Mussel-inspired polydopamine catalyst carriers dramatically enhance the catalytic performance (∼450%) of Au nanoparticles in methylene blue reduction, which is attributed to the local enrichment mechanism caused by the favourable attractive interaction between the polydopamine and reactants.

Mussel-inspired gold hollow superparticles for photothermal therapy

Gold hollow superparticles are prepared taking advantage of the dopamine chemistry. The plasmon coupling of the gold nanoparticles makes the superparticles an effective photothermal conversion agent in the photothermal therapy of cancer. Moreover, the mussel-inspired assembly approach could be extremely useful for the transfer of nanomaterial science to realistic technologies.

Preparation of polydopamine nanocapsules in a miscible tetrahydrofuran–buffer mixture

A non-emulsion soft template method based on a miscible tetrahydrofuran–tris buffer mixture has been used to fabricate polydopamine nanocapsules.