Two-Dimensional Interlayer Space Induced Horizontal Transformation of Metal-Organic Framework Nanosheets for Highly Permeable Nanofiltration Membranes.

Laminar membranes comprising graphene oxide (GO) and metal-organic framework (MOF) nanosheets benefit from the regular in-plane pores of MOF nanosheets and thus can support rapid water transport. However, the restacking and agglomeration of MOF nanosheets during typical vacuum filtration disturb the stacking of GO sheets, thus deteriorating the membrane selectivity. Therefore, to fabricate highly permeable MOF nanosheets/reduced GO (rGO) membranes, a two-step method is applied. First, using a facile solvothermal method, ZnO nanoparticles are introduced into the rGO laminate to stabilize and enlarge the interlayer spacing. Subsequently, the ZnO/rGO membrane is immersed in a solution of tetrakis(4-carboxyphenyl)porphyrin (H2 TCPP) to realize in situ transformation of ZnO into Zn-TCPP in the confined interlayer space of rGO. By optimizing the transformation time and mass loading of ZnO, the obtained Zn-TCPP/rGO laminar membrane exhibits preferential orientation of Zn-TCPP, which reduces the pathway tortuosity for small molecules. As a result, the composite membrane achieves a high water permeance of 19.0 L m-2  h-1  bar-1 and high anionic dye rejection (>99% for methyl blue).

Star-Shaped Peptide-Polymer Hybrids as Fast pH-Responsive Supramolecular Hydrogels.

Significant challenges have gone into the design of smart hydrogels, with numerous potential applications in the industrial, cosmetic, and biomedical fields. Herein, we report the synthesis of novel 4-arm self-assembling peptide-polyethylene glycol (PEG) hybrid star-shaped polymers and their comprehensive hydrogel properties. ß-sheet-forming oligopeptides with alternating hydrophobic Leu/ionizable Glu repeats and Cys residues were successfully conjugated to 4-arm PEG via a thiol-maleimide click reaction. The hybrid star-shaped polymers demonstrated good cytocompatibility and reversible ß-sheet (lightly acidic pH)-to-random coil (neutral and basic pH) transition in dilute aqueous solutions. At increasing polymer concentrations up to 0.5 wt %, the star-shaped polymers formed transparent hydrogels with shear-thinning and self-healing behaviors via ß-sheet self-assembly, as well as a conformation-dependent gel-sol transition. Interestingly, the star-shaped polymers responded rapidly to pH changes, causing gelation to occur rapidly within a few seconds from the change in pH. Hydrogel characteristics could be modulated by manipulating the length and net charge of the peptide blocks. Furthermore, these star-shaped polymers served as satisfactory network scaffolds that could respond to dynamic environmental changes in the pH-oscillation system, owing to their excellent gelation capability and pH sensitivity. As such, they are highly favorable for diverse applications, such as pH-responsive controlled release.

Autonomous Movement System Induced by Synergy between pH Oscillation and a pH-Responsive Oil Droplet.

A sustainable droplet motion that is driven by pH oscillation was obtained. The pH oscillation is only of a single pulse in a batch reactor. However, it shows continuous oscillation around the moving droplet, as the motion itself controls the diffusion flux in an asymmetric manner. Various types of motions that are spontaneous in nature may be obtained by a single-pulse oscillation coupled with mass transport.

Droplet duos on water display pairing, autonomous motion, and periodic eruption.

Under non-equilibrium conditions, liquid droplets dynamically couple with their milieu through the continuous flux of matter and energy, forming active systems capable of self-organizing functions reminiscent of those of living organisms. Among the various dynamic behaviors demonstrated by cells, the pairing of heterogeneous cell units is necessary to enable collective activity and cell fusion (to reprogram somatic cells). Furthermore, the cyclic occurrence of eruptive events such as necroptosis or explosive cell lysis is necessary to maintain cell functions. However, unlike the self-propulsion behavior of cells, cyclic cellular behavior involving pairing and eruption has not been successfully modeled using artificial systems. Here, we show that a simple droplet system based on quasi-immiscible hydrophobic oils (perfluorodecalin and decane) deposited on water, mimics such complex cellular dynamics. Perfluorodecalin and decane droplet duos form autonomously moving Janus or coaxial structures, depending on their volumes. Notably, the system with a coaxial structure demonstrates cyclic behavior, alternating between autonomous motion and eruption. Despite their complexity, the dynamic behaviors of the system are consistently explained in terms of the spreading properties of perfluorodecalin/decane duplex interfacial films.

Reversed spin of a ratchet motor on a vibrating water bed.

A ratchet gear on a vibrating water bed exhibits a one-way spin. However, the spinning direction is opposite to that of the gear placed on the granular bed. The one-way spin is caused by the surface waves of water. Surface deformation causes transportation of the water element to rotate the gear. The spatial symmetry of the surface wave and gear geometry regulates the rotational torque. In this study, the same ratchet shows reversed motion between the granular and water beds, and the direction is not determined only by the ratchet geometry. The self-organization of the fluid medium caused by small agitation induces a nontrivial inversion of the spinning direction.

Horizontal spin of ratchet motor by vertical agitation.

The horizontal spin of a ratchet motor by vertical vibration is reported. A macroscopic ratchet gear is placed on a granular bed, where nearly half of the gear is penetrated in the bed. The gear and granular bed are mechanically vibrated. The gear shows a random motion or one-way spin that depend on the diameter of the granules, vibration frequency, and degree of vertical motion allowed for the gear. Even when one-way spin is observed, the spin direction depends on the abovementioned factors. Although the dependency is complicated, it is deterministic because the motion or flows of granular matter determines it. The characteristics observed in the experiments are explained by a simple model that accounts for the statistical variance in the motion of the granular matter. Extraction of systematic motion from small and non-useful motions such as mechanical agitation will be developed into energy harvest technology and may facilitate the science of a spontaneously moving system in a uniform potential field.

Mode Bifurcation on Contact Line Dynamics at Oil/Water Interface Depending on the Contact Line Length.

The motion of the contact line at the oil/water interface caused by chemical reactions is well known as a typical example of artificial active matter in the field of nonlinear science. When water (containing trimethylstearylammonium chloride) and nitrobenzene (containing iodide anion) phases are in contact, the regulated traveling-wave patterns appear along the inner wall of the glass container. In this study, we demonstrate a new dynamical mode of the contact line, an up-and-down motion, which becomes dominant with the decrease in the size of a glass tube, and the probability of occurrence is extremely high when the diameter of the glass tube is below 1 mm. A physicochemical model of the contact line motion that incorporates the spatiotemporal variation of the surfactant concentration on a glass surface is proposed, and its effect on the wettability of oil/water phases on the walls of the glass tubes is studied. The present model can reproduce the mode bifurcation of the dynamical motion depending on the inner diameter of the glass tubes.