Directed self-propulsion of droplets on surfaces absent of gradients for cargo transport.

HYPOTHESIS: Manipulating droplet transportation without inputting work is desired and important in microfluidic systems. Although the creation of wettability gradient on surfaces has been employed to achieve this goal, the transport distance is very limited, hindering its applications in long-term operations. EXPERIMENTS: Here, we show that programming long-ranged transport of droplets on surfaces can be achieved by the addition of trisiloxane surfactants and the creation of deep grooves. The former provides Marangoni stress to actuate the droplet motion and also reduces the inherent contact line pinning. The latter acts as a railing to guide the motion of surfactant-laden droplets to follow various layouts with geometric features of roads. FINDINGS: It is found that the droplets with microliters can move over 20 cm. This work-free method is applicable to a variety of substrate materials and liquids. By using self-running shuttles, a convenient platform for liquid cargos transport is developed and demonstrated. Moreover, the coalescence of cargos carried by different shuttles is accomplished in a three-branch layout, revealing new droplet microreactors.

UV-Resistant Self-Healing Emulsion Glass as a New Liquid-like Solid Material for 3D Printing.

Directly writing 3D structures into supporting mediums is a relatively new developing technology in additive manufacturing. In this work, durable and recyclable liquid-like solid (LLS) materials are developed as supporting mediums that are stable for both UV and thermal solidification. Our LLS material is comprised of densely packed oil droplets in a continuous aqueous medium, known as emulsion glass. Its elastic nature emerges from the jammed structure of oil droplets, which offers this LLS material rapidly self-healing ability. Moreover, the yield stress of the glass is relatively low and can be tuned by the viscosity and weight percentage of oil. The capability of the emulsion glass as supporting mediums is successfully demonstrated by directly writing and then curing designed structures. The emulsion glass has been repeatedly used at least 6 times upon exposure to UV irradiation and heat, implying it can expand the applications of supporting medium to the writing process involving UV- and thermal-curable inks simultaneously.

Strong competition between adsorption and aggregation of surfactant in nanoscale systems.

HYPOTHESIS: The size-dependent behavior including surface tension, surface density (Γ), and critical micelle concentration (CMC) of a nanoscale liquid film containing surfactant has not been investigated until now. EXPERIMENTS: Strong competition between surface adsorption and bulk aggregation of surfactant in nanoscale systems was explored by Many-body Dissipative Particle Dynamics simulations. FINDINGS: In nanoscale systems, as the surfactant concentration increases, Γ continues rising even after CMC is exceeded. The saturation level of Γ is achieved only when the surfactant bulk concentration is over ten times of CMC. Moreover, both surface micelles formed by adsorbed surfactant and the sublayer below the adsorbed layer are clearly identified. The former can reduce the contacts of adsorbed surfactant with water, while the latter has the surfactant concentration significantly higher than that in bulk. The strong coupling between adsorption and micellization is attributed to large surface-to-volume ratio compared to macroscopic systems, and can be simply realized by the fact that the ratio of the numbers of surfactant distributed in bulk (nbulk) and at interface (nads) is always less than unity (nbulk/nads < 1) in nanoscale systems.

Hybrid membranes of lipids and diblock copolymers: From homogeneity to rafts to phase separation.

Hybrid lipid-polymer vesicles can integrate benefits of liposomes and polymersomes. In this work, the phase behavior of hybrid membranes containing lipids and diblock copolymers is explored by dissipative particle dynamics simulations. The influences of lipid unsaturation and thickness mismatch between lipids and polymers are considered. The transition from the mixing state (homogeneous distribution) to the demixing state (formation of bilayered lipid-rich domains) is always observed as the lipid concentration (φ_{l}) exceeds a critical value, which increases with the degree of unsaturation. It is found that phase separation is driven by weak energy incompatibility between the hydrophobic segments of lipids and polymers. When the effect of thickness mismatch becomes significant, the occurrence of the demixing state is retarded, and monolayer lipid rafts emerge before phase separation. Lipid fluidity associated with the physical state of a hybrid membrane can be characterized by lateral lipid diffusivity (D_{l}). In the polymer-rich membrane, D_{l} is higher in the mixing state, but decreases generally with φ_{l} due to lipid-lipid interactions and interdigitation.

Zeolite-Based Antifogging Coating via Direct Wet Deposition.

Zeolites are strongly hydrophilic materials that are widely used as water adsorbents. They are also promising candidates for antifogging coatings; however, researchers have yet to devise a suitable method for coating glass substrates with zeolite-based films. Here, we report on a direct wet deposition technique that is capable of casting zeolite films on glass substrates without exposing the glass to highly basic solutions or the vapors used in zeolite synthesis. We began by preparing cast solutions of pure silica zeolite MFI synthesized in hydrothermal reactions of various durations. The solutions were then applied to glass substrates via spin-on deposition to form zeolite films. The resulting zeolite MFI thin films were characterized in terms of transmittance to visible light, surface topography, thin film morphology, and crystallinity. Wetting and antifogging properties were also probed. We found that hydrophilicity and antifogging capability increased with the degree of thin film crystallinity. We also determined that the presence of the amorphous silica in the thin films is critical to transparency. Fabricating high-performance zeolite-based antifogging coatings requires an appropriate composition of zeolite crystals and amorphous silica.

Phase diagram of solvophilic nanodiscs in a polymer solution: depletion attraction.

Entropic attraction between anisotropic nanoparticles in nonadsorbing polymer solutions is significantly greater than that between nanospheres. In this study, depletion attraction between two solvophilic nanodiscs and the aggregation behavior of a nanodisc suspension are explored by dissipative particle dynamics. The depletion force due to polymer addition is proportional to the product of the osmotic pressure and the area of the nanodisc, even for concentrated polymer solutions. For a suspension of nanodiscs, three possible equilibrium states are observed upon polymer addition: dispersion, pretransition, and phase separation. By varying nanodisc concentration (φD) and polymer concentration (φP), the phase diagram is obtained. Dispersion exists for small φD and φP. As φD exceeds critical aggregation concentration, finite sized clusters are mainly formed with columnar structure in the pretransition regime, similar to micellization associated with typical surfactants. The mean size of columnar clusters seems to grow with φD(1/2) and φP. For high enough φD and φP, finite sized clusters vanish and phase separation appears. The nanodisc-rich phase is formed by a bundle of columns or a pack of columns owing to face-to-side and side-to-side depletion attractions.

Polymer stretch in two-phase microfluidics: Effect of wall wettability.

Polymer stretching in two-phase microfluidics is investigated by dissipative particle dynamics. The flow patterns can be controlled by wall wettability, flowrate ratio between two phases, and Reynolds number (Re). For neutral and partially wettable walls, segmented flows are formed and polymer stretching can be controlled by Re and segment length. At high Re, stratified flows are observed and the extension ratio can be tuned by the flowrate ratio. For nonwettable walls, slug flows are formed and polymer stretching can be controlled by Re and slug length. At high Re or flowrate ratio, annular flows are observed and high extension ratio can be easily attained.