Toward vanishing droplet friction on repellent surfaces.

Superhydrophobic surfaces are often seen as frictionless materials, on which water is highly mobile. Understanding the nature of friction for such water-repellent systems is central to further minimize resistance to motion and energy loss in applications. For slowly moving drops, contact-line friction has been generally considered dominant on slippery superhydrophobic surfaces. Here, we show that this general rule applies only at very low speed. Using a micropipette force sensor in an oscillating mode, we measure the friction of water drops approaching or even equaling zero contact-line friction. We evidence that dissipation then mainly stems from the viscous shearing of the air film (plastron) trapped under the liquid. Because this force is velocity dependent, it can become a serious drag on surfaces that look highly slippery from quasi-static tests. The plastron thickness is found to be the key parameter that enables the control of this special friction, which is useful information for designing the next generation of ultraslippery water-repellent coatings.

Compressibility and porosity modulate the mechanical properties of giant gas vesicles.

Gas vesicles used as contrast agents for noninvasive ultrasound imaging must be formulated to be stable, and their mechanical properties must be assessed. We report here the formation of perfluoro-n-butane microbubbles coated with surface-active proteins that are produced by filamentous fungi (hydrophobin HFBI from Trichoderma reesei). Using pendant drop and pipette aspiration techniques, we show that these giant gas vesicles behave like glassy polymersomes, and we discover novel gas extraction regimes. We develop a model to analyze the micropipette aspiration of these compressible gas vesicles and compare them to incompressible liquid-filled vesicles. We introduce a sealing parameter to characterize the leakage of gas under aspiration through the pores of the protein coating. Utilizing this model, we can determine the elastic dilatation modulus, surface viscosity, and porosity of the membrane. These results demonstrate the engineering potential of protein-coated bubbles for echogenic and therapeutic applications and extend the use of the pipette aspiration technique to compressible and porous systems.

Droplet Friction on Superhydrophobic Surfaces Scales With Liquid-Solid Contact Fraction.

It is generally assumed that contact angle hysteresis of superhydrophobic surfaces scales with liquid-solid contact fraction, however, its experimental verification has been problematic due to the limited accuracy of contact angle and sliding angle goniometry. Advances in cantilever-based friction probes enable accurate droplet friction measurements down to the nanonewton regime, thus suiting much better for characterizing the wetting of superhydrophobic surfaces than contact angle hysteresis measurements. This work quantifies the relationship between droplet friction and liquid-solid contact fraction, through theory and experimental validation. Well-defined micropillar and microcone structures are used as model surfaces to provide a wide range of different liquid-solid contact fractions. Micropillars are known to be able to hold the water on top of them, and a theoretical analysis together with confocal laser scanning microscopy shows that despite the spiky nature of the microcones droplets do not sink into the conical structure either, rendering a diminishingly small liquid-solid contact fraction. Droplet friction characterization with a micropipette force sensor technique reveals a strong dependence of the droplet friction on the contact fraction, and the dependency is described with a simple physical equation, despite the nearly three-orders-of-magnitude difference in liquid-solid contact fraction between the sparsest cone surface and the densest pillar surface.

Long-term stability of aerophilic metallic surfaces underwater.

Aerophilic surfaces immersed underwater trap films of air known as plastrons. Plastrons have typically been considered impractical for underwater engineering applications due to their metastable performance. Here, we describe aerophilic titanium alloy (Ti) surfaces with extended plastron lifetimes that are conserved for months underwater. Long-term stability is achieved by the formation of highly rough hierarchically structured surfaces via electrochemical anodization combined with a low-surface-energy coating produced by a fluorinated surfactant. Aerophilic Ti surfaces drastically reduce blood adhesion and, when submerged in water, prevent adhesion of bacteria and marine organisms such as barnacles and mussels. Overall, we demonstrate a general strategy to achieve the long-term stability of plastrons on aerophilic surfaces for previously unattainable underwater applications.

Multifunctional ferrofluid-infused surfaces with reconfigurable multiscale topography.

Developing adaptive materials with geometries that change in response to external stimuli provides fundamental insights into the links between the physical forces involved and the resultant morphologies and creates a foundation for technologically relevant dynamic systems1,2. In particular, reconfigurable surface topography as a means to control interfacial properties3 has recently been explored using responsive gels4, shape-memory polymers5, liquid crystals6-8 and hybrid composites9-14, including magnetically active slippery surfaces12-14. However, these designs exhibit a limited range of topographical changes and thus a restricted scope of function. Here we introduce a hierarchical magneto-responsive composite surface, made by infiltrating a ferrofluid into a microstructured matrix (termed ferrofluid-containing liquid-infused porous surfaces, or FLIPS). We demonstrate various topographical reconfigurations at multiple length scales and a broad range of associated emergent behaviours. An applied magnetic-field gradient induces the movement of magnetic nanoparticles suspended in the ferrofluid, which leads to microscale flow of the ferrofluid first above and then within the microstructured surface. This redistribution changes the initially smooth surface of the ferrofluid (which is immobilized by the porous matrix through capillary forces) into various multiscale hierarchical topographies shaped by the size, arrangement and orientation of the confining microstructures in the magnetic field. We analyse the spatial and temporal dynamics of these reconfigurations theoretically and experimentally as a function of the balance between capillary and magnetic pressures15-19 and of the geometric anisotropy of the FLIPS system. Several interesting functions at three different length scales are demonstrated: self-assembly of colloidal particles at the micrometre scale; regulated flow of liquid droplets at the millimetre scale; and switchable adhesion and friction, liquid pumping and removal of biofilms at the centimetre scale. We envision that FLIPS could be used as part of integrated control systems for the manipulation and transport of matter, thermal management, microfluidics and fouling-release materials.

In-situ Monitoring of Photocontrollable Wrinkle Erasure in Azobenzene-based Supramolecular Systems.

In this contribution, dynamic photoinduced wrinkle erasure enabled by photomechanical changes in supramolecular polymer-azo complexes was characterized via confocal microscopy. Different photoactive molecules, disperse yellow 7 (DY7) and 4,4'-dihydroxyazobenzene (DHAB), were compared to 4-hydroxy-4'-dimethylaminoazobenzene (OH-azo-DMA). The characteristic erasure times of wrinkles were quickly assessed by using an image processing algorithm. The results confirm that the photoinduced movement on the topmost layer can be successfully transferred to the substrate. Furthermore, the chosen supramolecular strategy allows decoupling the effect of molecular weight of the polymer and photochemistry of the chromophore, allowing quantitative comparison of wrinkling erasure efficiency of different materials and providing a facile way to optimize the system for specific applications.

pH-Responsive Near-Infrared Emitting Gold Nanoclusters.

Near-infrared (NIR) fluorophores with pH-responsive properties suggest merits in biological analyses. This work establishes a general and effective method to obtain pH-responsive NIR emissive gold nanoclusters by introducing aliphatic tertiary amine (TA) groups into the ligands. Computational study suggests that the pH-responsive NIR emission is associated with electronic structure change upon protonation and deprotonation of TA groups. Photo-induced electron transfer between deprotonated TA groups and the surface Au-S motifs of gold nanoclusters can disrupt the radiative transitions and thereby decrease the photoluminescence intensity in basic environments (pH=7-11). By contrast, protonated TA groups curb the electron transfer and restore the photoluminescence intensity in acidic environments (pH=4-7). The pH-responsive NIR-emitting gold nanoclusters serve as a specific and sensitive probe for the lysosomes in the cells, offering non-invasive emissions without interferences from intracellular autofluorescence.

Fusion Dynamics of Hybrid Cell-Microparticle Aggregates: A Jelly Pearl Model.

We study the fusion of homogeneous cell aggregates and of hybrid aggregates combining cells and microparticles. In all cases, we find that the contact area does not vary linearly over time, as observed for liquid drops, but rather it follows a power law in t2/3. This result is interpreted by generalizing the fusion model of soft viscoelastic solid balls to viscoelastic liquid balls, akin to jelly pearls. We also explore the asymmetric fusion between a homogeneous aggregate and a hybrid aggregate. This latter experiment allows the determination of the self-diffusion coefficient of the cells in a tissue by following the spatial distribution of internalized particles in the cells.

Correction: Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams.

Correction for 'Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams' by Oskar Tainio et al., Soft Matter, 2021, 17, 145-152, DOI: 10.1039/D0SM01206H.

Chlamydomonas reinhardtii swimming in the Plateau borders of 2D foams.

Unicellular Chlamydomonas reinhardtii micro-algae cells were inserted into a quasi-2D Hele-Shaw chamber filled with saponin foam. The movement of the algae along the bubble borders was then manipulated and tracked. These self-propelled particles generate flow and stresses in their surrounding matter. In addition, the algae possess the capability of exerting forces that alter bubble boundaries while maintaining an imminent phototactic movement. We find that by controlling the gas fraction of the foam we can change the interaction of the algae and bubbles. Specifically, our data expose three distinct swimming regimes for the algae with respect to the level of confinement due to the Plateau border cross-section: unlimited bulk, transition, and overdamped regimes. At the transition regime we find the speed of the algae to be modeled by a simple force balance equation emerging from the shear inside the Plateau border. Thus, we have shown that it is possible to create an algae-friendly foam while controlling the algae motion. This opens doors to multiple applications where the flow of nutrients, oxygen and recirculation of living organisms is essential.

Origins of Extreme Liquid Repellency on Structured, Flat, and Lubricated Hydrophobic Surfaces.

There are currently three main classes of liquid-repellent surfaces: micro- or nanostructured superhydrophobic surfaces, flat surfaces grafted with "liquidlike" polymer brushes, and lubricated surfaces. Despite recent progress, the mechanistic explanation for the differences in droplet behavior on such surfaces is still under debate. Here, we measure the dissipative force acting on a droplet moving on representatives of these surfaces at different velocities U=0.01-1 mm/s using a cantilever force sensor with submicronewton accuracy and correlate it to the contact line dynamics observed using optical interferometry at high spatial (micron) and temporal (<0.1 s) resolutions. We find that the dissipative force-due to very different physical mechanisms at the contact line-is independent of velocity on superhydrophobic surfaces but depends nonlinearly on velocity for flat and lubricated surfaces. The techniques and insights presented here will inform future work on liquid-repellent surfaces and enable their rational design.

Reversible Supracolloidal Self-Assembly of Cobalt Nanoparticles to Hollow Capsids and Their Superstructures.

The synthesis and spontaneous, reversible supracolloidal hydrogen bond-driven self-assembly of cobalt nanoparticles (CoNPs) into hollow shell-like capsids and their directed assembly to higher order superstructures is presented. CoNPs and capsids form in one step upon mixing dicobalt octacarbonyl (Co2 CO8 ) and p-aminobenzoic acid (pABA) in 1,2-dichlorobenzene using heating-up synthesis without additional catalysts or stabilizers. This leads to pABA capped CoNPs (core ca. 5 nm) with a narrow size distribution. They spontaneously assemble into tunable spherical capsids (d≈50-200 nm) with a few-layered shells, as driven by inter-nanoparticle hydrogen bonds thus warranting supracolloidal self-assembly. The capsids can be reversibly disassembled and reassembled by controlling the hydrogen bonds upon heating or solvent exchanges. The superparamagnetic nature of CoNPs allows magnetic-field-directed self-assembly of capsids to capsid chains due to an interplay of induced dipoles and inter-capsid hydrogen bonds. Finally, self-assembly on air-water interface furnishes lightweight colloidal framework films.

Synthesis of toroidal gold nanoparticles assisted by soft templates.

A three-component system comprising surfactant molecules and molecularly cross-linked metal centers assembles into nanoring structures. The thickness of the nanorings is determined by the dimensions of the surfactant bilayer while the dimensions of the ring opening depend on and can be regulated by the concentrations of the participating species. Once formed, these organic-inorganic hybrids can be transformed, by air plasma treatment, into all-metal nanorings exhibiting strong adsorption in the near IR.

Ferromagnetic resonance in ϵ-Co magnetic composites.

We investigate the electromagnetic properties of assemblies of nanoscale ϵ-cobalt crystals with size range between 5 to 35 nm, embedded in a polystyrene matrix, at microwave (1-12 GHz) frequencies. We investigate the samples by transmission electron microscopy imaging, demonstrating that the particles aggregate and form chains and clusters. By using a broadband coaxial-line method, we extract the magnetic permeability in the frequency range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance (FMR) with respect to an externally applied magnetic field. We find that the zero-magnetic field ferromagnetic resonant peak shifts towards higher frequencies at finite magnetic fields, and the magnitude of complex permeability is reduced. At fields larger than 2.5 kOe the resonant frequency changes linearly with the applied magnetic field, demonstrating the transition to a state in which the nanoparticles become dynamically decoupled. In this regime, the particles inside clusters can be treated as non-interacting, and the peak position can be predicted from Kittel's FMR theory for non-interacting uniaxial spherical particles combined with the Landau-Lifshitz-Gilbert equation. In contrast, at low magnetic fields this magnetic order breaks down and the resonant frequency in zero magnetic field reaches a saturation value reflecting the interparticle interactions as resulting from aggregation. Our results show that the electromagnetic properties of these composite materials can be tuned by external magnetic fields and by changes in the aggregation structure.

Protein Cage Directed Assembly of Binary Nanoparticle Superlattices.

Inorganic nanoparticles can be assembled into superlattices with unique optical and magnetic properties arising from collective behavior. Protein cages can be utilized to guide this assembly by encapsulating nanoparticles and promoting their assembly into ordered structures. However, creating ordered multi-component structures with different protein cage types and sizes remains a challenge. Here, the co-crystallization of two different protein cages (cowpea chlorotic mottle virus and ferritin) characterized by opposing surface charges and unequal diameter is shown. Precise tuning of the electrostatic attraction between the cages enabled the preparation of binary crystals with dimensions up to several tens of micrometers. Additionally, binary metal nanoparticle superlattices are achieved by loading gold and iron oxide nanoparticles inside the cavities of the protein cages. The resulting structure adopts an AB2 FCC configuration that also impacts the dipolar coupling between the particles and hence the optical properties of the crystals, providing key insight for the future preparation of plasmonic and magnetic nanoparticle metamaterials.

Magnetic Quincke rollers with tunable single-particle dynamics and collective states.

Electrohydrodynamically driven active particles based on Quincke rotation have quickly become an important model system for emergent collective behavior in nonequilibrium colloidal systems. Like most active particles, Quincke rollers are intrinsically nonmagnetic, preventing the use of magnetic fields to control their complex dynamics on the fly. Here, we report on magnetic Quincke rollers based on silica particles doped with superparamagnetic iron oxide nanoparticles. We show that their magnetic nature enables the application of both externally controllable forces and torques at high spatial and temporal precision, leading to several versatile control mechanisms for their single-particle dynamics and collective states. These include tunable interparticle interactions, potential energy landscapes, and advanced programmable and teleoperated behaviors, allowing us to discover and probe active chaining, anisotropic active sedimentation-diffusion equilibria, and collective states in various geometries and dimensionalities.

Thermodynamically controlled multiphase separation of heterogeneous liquid crystal colloids.

Phase separation is a universal physical transition process whereby a homogeneous mixture splits into two distinct compartments that are driven by the component activity, elasticity, or compositions. In the current work, we develop a series of heterogeneous colloidal suspensions that exhibit both liquid-liquid phase separation of semiflexible binary polymers and liquid crystal phase separation of rigid, rod-like nanocellulose particles. The phase behavior of the multicomponent mixture is controlled by the trade-off between thermodynamics and kinetics during the two transition processes, displaying cholesteric self-assembly of nanocellulose within or across the compartmented aqueous phases. Upon thermodynamic control, two-, three-, and four-phase coexistence behaviors with rich liquid crystal stackings are realized. Among which, each relevant multiphase separation kinetics shows fundamentally different paths governed by nucleation and growth of polymer droplets and nanocellulose tactoids. Furthermore, a coupled multiphase transition can be realized by tuning the composition and the equilibrium temperature, which results in thermotropic behavior of polymers within a lyotropic liquid crystal matrix. Finally, upon drying, the multicomponent mixture undergoes a hierarchical self-assembly of nanocellulose and polymers into stratified cholesteric films, exhibiting compartmentalized polymer distribution and anisotropic microporous structure.

Supercharged Fluorescent Protein-Apoferritin Cocrystals for Lighting Applications.

The application of fluorescent proteins (FPs) in optoelectronics is hindered by the need for effective protocols to stabilize them under device preparation and operational conditions. Factors such as high temperatures, irradiation, and organic solvent exposure contribute to the denaturation of FPs, resulting in a low device performance. Herein, we focus on addressing the photoinduced heat generation associated with FP motion and rapid heat transfer. This leads to device temperatures of approximately 65 °C, causing FP-denaturation and a subsequent loss of device functionality. We present a FP stabilization strategy involving the integration of electrostatically self-assembled FP-apoferritin cocrystals within a silicone-based color down-converting filter. Three key achievements characterize this approach: (i) an engineering strategy to design positively supercharged FPs (+22) without compromising photoluminescence and thermal stability compared to their native form, (ii) a carefully developed crystallization protocol resulting in highly emissive cocrystals that retain the essential photoluminescence features of the FPs, and (iii) a strong reduction of the device's working temperature to 40 °C, leading to a 40-fold increase in Bio-HLEDs stability compared to reference devices.

Dispersing swimming microalgae in self-assembled nanocellulose suspension: Unveiling living colloid dynamics in cholesteric liquid crystals.

Active matter comprises individual energy-consuming components that convert locally stored energy into mechanical motion. Among these, liquid crystal dispersed self-propelled colloids have displayed fascinating dynamic effects and nonequilibrium behaviors. In this work, we introduce a new type of active soft matter based on swimming microalgae and lyotropic nanocellulose liquid crystal. Cellulose is a kind of biocompatible polysaccharide that nontoxic to living biological colloids. In contrast to microalgae locomotion in isotropic and low viscosity media, we demonstrate that the propulsion force of swimming microalgae can overcome the stabilizing elastic force in cholesteric nanocellulose liquid crystal, with the displacement dynamics (gait, direction, frequency, and speed) be altered by the surrounding medium. Simultaneously, the active stress and shear flow exerted by swimming microalgae can introduce local perturbation in surrounding liquid crystal orientation order. The latter effect yields hydrodynamic fluctuations in bulk phase as well as layer undulations, helicoidal axis splay deformation and director bending in the cholesteric assembly, which finally followed by a recovery according to the inherent viscoelasticity of liquid crystal matrix. Our results point to an unorthodox design concept to generate a new type of hybrid soft matter that combines nontoxic cholesteric liquid crystal and active particles, which are expected to open opportunities in biosensing and biomechanical applications.

ß-1,3-Glucan synthesis, novel supramolecular self-assembly, characterization and application.

ß-1,3-Glucans are ubiquitously observed in various biological systems with diverse physio-ecological functions, yet their underlying assembly mechanism and multiscale complexation in vitro remains poorly understood. Here, we provide for the first-time evidence of unidentified ß-1,3-glucan supramolecular complexation into intricate hierarchical architectures over several length scales. We mediated these unique assemblies using a recombinantly produced ß-1,3-glucan phosphorylase (Ta1,3BGP) by fine-tuning solution conditions during particle nucleation and growth. We report a synthesis of interconnected parallel hexagonal lamellae composed of 8 nm thick sheets of highly expanded paracrystals. The architecture consists of ß-1,3-glucan triple-helices with considerable inter-intra hydrogen bonding within, as well as in between adjacent triple-helices. The results extend our understanding of ß-1,3-glucan molecular organization and shed light on different aspects of the crystallization processes of biomolecules into structures unseen by nature. The presented versatile synthesis yields new materials for diverse medical and industrial applications.