Nonmonotonic polymer translocation kinetics through nanopores under changing surface-polymer interactions.

Understanding the dynamics of polymers in confined environments is pivotal for diverse applications ranging from polymer upcycling to bioseparations. In this study, we develop an entropic barrier model using self-consistent field theory that considers the effect of attractive surface interactions, solvation, and confinement on polymer kinetics. In this model, we consider the translocation of a polymer from one cavity into a second cavity through a single-segment-width nanopore. We find that, for a polymer in a good solvent (i.e., excluded volume, u0 > 0), there is a nonmonotonic dependence of mean translocation time (τ) on surface interaction strength, ɛ. At low ɛ, excluded volume interactions lead to an energetic penalty and longer translocation times. As ɛ increases, the surface interactions counteract the energetic penalty imposed by excluded volume and the polymer translocates faster through the nanopore. However, as ɛ continues to increase, an adsorption transition occurs, which leads to significantly slower kinetics due to the penalty of desorption from the first cavity. The ɛ at which this adsorption transition occurs is a function of the excluded volume, with higher u0 leading to an adsorption transition at higher ɛ. Finally, we consider the effect of translocation across different size cavities. We find that the kinetics for translocation into a smaller cavity speeds up while translocation to a larger cavity slows down with increasing ɛ due to higher surface contact under stronger confinement.

Computational Design of Peptides for Biomaterials Applications.

Computer-aided molecular design and protein engineering emerge as promising and active subjects in bioengineering and biotechnological applications. On one hand, due to the advancing computing power in the past decade, modeling toolkits and force fields have been put to use for accurate multiscale modeling of biomolecules including lipid, protein, carbohydrate, and nucleic acids. On the other hand, machine learning emerges as a revolutionary data analysis tool that promises to leverage physicochemical properties and structural information obtained from modeling in order to build quantitative protein structure-function relationships. We review recent computational works that utilize state-of-the-art computational methods to engineer peptides and proteins for various emerging biomedical, antimicrobial, and antifreeze applications. We also discuss challenges and possible future directions toward developing a roadmap for efficient biomolecular design and engineering.

Bioinspired Switchable Passive Daytime Radiative Cooling Coatings.

Passive daytime radiative cooling (PDRC) relies on simultaneous reflection of sunlight and radiation toward cold outer space. Current designs of PDRC coatings have demonstrated potential as eco-friendly alternatives to traditional electrical air conditioning (AC). While many features of PDRC have been individually optimized in different studies, for practical impact, it is essential for a system to demonstrate excellence in all essential aspects, like the materials that nature has created. We propose a bioinspired PDRC structure templated by bicontinuous interfacially jammed emulsion gels (bijels) that possesses excellent cooling, thinness, tunability, scalability, and mechanical robustness. The unique bicontinuous disordered structure captures key features of Cyphochilus beetle scales, enabling a thin (130 µm) bijel PDRC coating to achieve high solar reflectance (≳0.97) and high longwave-infrared (LWIR) emissivity (≳0.93), resulting in a subambient temperature drop of ∼5.6 °C under direct sunlight. We further demonstrate switchable cooling inspired by the exoskeleton of the Hercules beetle and mechanical robustness in analogy to spongy bone structures.

Bicontinuous interfacially jammed emulsion gels with nearly uniform sub-micrometer domains via regulated co-solvent removal.

Porous materials possess numerous useful functions because of their high surface area and ability to modulate the transport of heat, mass, fluids, and electromagnetic waves. Unlike highly ordered structures, disordered porous structures offer the advantages of ease of fabrication and high fault tolerance. Bicontinuous interfacially jammed emulsion gels (bijels) are kinetically trapped disordered biphasic materials that can be converted to porous materials with tunable features. Current methods of bijel fabrication result in domains that are micrometers or larger, and non-uniform in size, limiting their surface area, mechanical strength, and interaction with electromagnetic waves. In this work, scalable synthesis of bijels with uniform and sub-micrometer domains is achieved via a two-step solvent removal process. The resulting bijels are characterized quantitatively to verify the uniformity and sub-micrometer scale of the domains. Moreover, these bijels have structures that resemble the microstructure of the scale of the white beetle Cyphochilus. We find that such bijel films with relatively small thicknesses (<150 µm) exhibit strong solar reflectance as well as high brightness and whiteness in the visible range. Considering their scalability in manufacturing, we believe that VIPS-STRIPS bijels have great potential in large-scale applications including passive cooling, solar cells, and light emitting diodes (LEDs).

Ionic Strength-Dependent Assembly of Polyelectrolyte-Nanoparticle Membranes via Interfacial Complexation at a Water-Water Interface.

Complexation between oppositely charged nanoparticles (NPs) and polyelectrolytes (PEs) is a scalable approach to assemble functional, stimuli-responsive membranes. Complexation at interfaces of aqueous two-phase systems (ATPSs) has emerged as a powerful method to assemble these functional structures. Membranes formed at these interfaces can grow continuously to thicknesses approaching several millimeters and display a high degree of tunability via modification of solution properties such as ionic strength. To identify the membrane assembly mechanism, we study interfacial assembly in a prototypical dextran/PEG ATPS, in which silica (SiO2) NPs suspended in the PEG phase undergo interfacial complexation with poly(diallyldimethylammonium chloride) (PDADMAC) supplied in the dextran phase. Using a microfluidic device that facilitates sequential insertion of fluorescent and nonfluorescent PDADMAC, we observe a transition in the membrane growth mechanism with ionic strength. In the absence of added salt ([NaCl] = 0 mM) PDADMAC chains permeate through the existing membrane to complex with NPs on the PEG side of the membrane, leading to the formation of well-stratified structures. At elevated ionic strength ([NaCl] = 500 mM), this permeation mechanism is lost. Rather, the complexing species incorporate uniformly across the membrane. We attribute this transition to a rapid exchange of PE-counterion, NP-counterion, and PE/NP binding sites facilitated by an increase in extrinsically compensated charged groups on the NPs and PEs at high salinity. These PDADMAC/SiO2 NP membranes have tremendous potential for the formation of functional membranes, offering control over the internal structure and serving as an ideal system for the generation of targeted release systems.

Topological defect-propelled swimming of nematic colloids.

Topological defects on colloids rotating in nematic liquid crystals form far-from-equilibrium structures that perform complex swim strokes in which the defects periodically extend, depin, and contract. These defect dynamics propel the colloid, generating translation from rotation. The swimmer's speed and direction are determined by the topological defect's polarity and extent of elongation. Defect elongation is controlled by a rotating external magnetic field, allowing control over particle trajectories. The swimmers' translational motion relies on broken symmetries associated with lubrication forces between the colloid and the bounding surfaces, line tensions associated with the elongated defect, and anisotropic viscosities associated with the defect elongation adjacent to the colloid. The scattering or effective pair interaction of these swimmers is highly anisotropic, with polarization-dependent dimer stability and motion that depend strongly on entanglement and sharing of their extended defect structures. This research introduces transient, far-from-equilibrium topological defects as a class of virtual functional structures that generate modalities of motion and interaction.

Surface Topography-Adaptive Robotic Superstructures for Biofilm Removal and Pathogen Detection on Human Teeth.

The eradication of biofilms remains an unresolved challenge across disciplines. Furthermore, in biomedicine, the sampling of spatially heterogeneous biofilms is crucial for accurate pathogen detection and precise treatment of infection. However, current approaches are incapable of removing highly adhesive biostructures from topographically complex surfaces. To meet these needs, we demonstrate magnetic field-directed assembly of nanoparticles into surface topography-adaptive robotic superstructures (STARS) for precision-guided biofilm removal and diagnostic sampling. These structures extend or retract at multilength scales (micro-to-centimeter) to operate on opposing surfaces and rapidly adjust their shape, length, and stiffness to adapt and apply high-shear stress. STARS conform to complex surface topographies by entering angled grooves or extending into narrow crevices and "scrub" adherent biofilm with multiaxis motion while producing antibacterial reagents on-site. Furthermore, as the superstructure disrupts the biofilm, it captures bacterial, fungal, viral, and matrix components, allowing sample retrieval for multiplexed diagnostic analysis. We apply STARS using automated motion patterns to target complex three-dimensional geometries of ex vivo human teeth to retrieve biofilm samples with microscale precision, while providing "toothbrushing-like" and "flossing-like" action with antibacterial activity in real-time to achieve mechanochemical removal and multikingdom pathogen detection. This approach could lead to autonomous, multifunctional antibiofilm platforms to advance current oral care modalities and other fields contending with harmful biofilms on hard-to-reach surfaces.

Dynamic and mechanical evolution of an oil-water interface during bacterial biofilm formation.

We present an experimental study combining particle tracking, active microrheology, and differential dynamic microscopy (DDM) to investigate the dynamics and rheology of an oil-water interface during biofilm formation by the bacteria Pseudomonas Aeruginosa PA14. The interface transitions from an active fluid dominated by the swimming motion of adsorbed bacteria at early age to an active viscoelastic system at late ages when the biofilm is established. The microrheology measurements using microscale magnetic rods indicate that the biofilm behaves as a viscoelastic solid at late age. The bacteria motility at the interface during the biofilm formation, which is characterized in the DDM measurements, evolves from diffusive motion at early age to constrained, quasi-localized motion at later age. Similarly, the mobility of passively moving colloidal spheres at the interface decreases significantly with increasing interface age and shows a dependence on sphere size after biofilm formation that is orders-of-magnitude larger than that expected in a homogeneous system in equilibrium. We attribute this anomalous size dependence to either length-scale-dependent rheology of the biofilm or widely differing effects of the bacteria activity on the motion of spheres of different sizes.

Polymer-Infiltrated Nanoparticle Films Using Capillarity-Based Techniques: Toward Multifunctional Coatings and Membranes.

Polymer-infiltrated nanoparticle films (PINFs) are a new class of nanocomposites that offer synergistic properties and functionality derived from unusually high fractions of nanomaterials. Recently, two versatile techniques,capillary rise infiltration (CaRI) and solvent-driven infiltration of polymer (SIP), have been introduced that exploit capillary forces in films of densely packed nanoparticles. In CaRI, a highly loaded PINF is produced by thermally induced wicking of polymer melt into the nanoparticle packing pores. In SIP, exposure of a polymer-nanoparticle bilayer to solvent vapor atmosphere induces capillary condensation of solvent in the pores of nanoparticle packing, leading to infiltration of polymer into the solvent-filled pores. CaRI/SIP PINFs show superior properties compared with polymer nanocomposite films made using traditional methods, including superb mechanical properties, thermal stability, heat transfer, and optical properties. This review discusses fundamental aspects of the infiltration process and highlights potential applications in separations, structural coatings, and polymer upcycling-a process to convert polymer wastes into useful chemicals.

Interfacial Flow around Brownian Colloids.

Understanding the flow created by particle motion at interfaces is a critical step toward understanding hydrodynamic interactions and colloidal self organization. We have developed correlated displacement velocimetry to measure flow fields around interfacially trapped Brownian particles. These flow fields can be decomposed into interfacial hydrodynamic multipoles, including force monopole and dipole flows. These structures provide key insights essential to understanding the interface's mechanical response. Importantly, the flow structure shows that the interface is incompressible for scant surfactant near the ideal gaseous state and contains information about interfacial properties and hydrodynamic coupling with the bulk fluid. The same dataset can be used to predict the response of the interface to applied, complex forces, enabling virtual experiments that produce higher order interfacial multipoles.

Dimerization and structure formation of colloids via capillarity at curved fluid interfaces.

Capillary interactions are ubiquitous between colloids trapped at fluid interfaces. Generally, colloids in fluid interfaces have pinned, undulated contact lines that distort the interface around them. To minimize the area, and therefore the energy of these distortions, colloids interact and assemble in a manner that depends on the shape of the host interface. On curved interfaces, capillary interactions direct isolated colloid motion along deviatoric curvature gradients. This directed motion relies on the leading order, long-ranged quadrupolar distortions made by the colloids' undulated pinned contact lines. Here we study pair interactions and dimer formation of colloids on non-uniformly curved fluid interfaces. Pair interaction energies are inferred to be order of 104kBT, and interacting forces are of order 10-1 pN for 10 micron particles adsorbed on interfaces formed around a 250 micron micropost. We compare experiments to analysis for the pair interaction energy, and identify criteria for dimers to form. We also study the formation of trapped structures by multiple particles to discern the influence of the underlying interface shape and the contact line undulations. By comparison to Monte Carlo simulations with potentials of interactions based on analysis, we find that higher order terms in the distortion fields generated by the particles play a major role in the structure formation on the curved interface. These interactions are determined by the particle's contact line and the host interface shape, and can be used to assemble particles independent of their material properties.

Fabrication of solvent transfer-induced phase separation bijels with mixtures of hydrophilic and hydrophobic nanoparticles.

Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are co-continuous throughout the structure, have potential for applications in separation, catalysis, tissue engineering and energy devices. Among the possible fabrication paths, the solvent transfer-induced phase separation (STRIPS) method has proven to be a powerful approach to produce bijels in a continuous fashion with a broad selection of liquids and nanoparticles. The successful formation of bicontinuous domains requires the use of neutrally wetting particles which was achieved by in situ modification of silica nanoparticles with an oppositely charged surfactant. This approach, however, is not ideal for applications that are adversely affected by the presence of surfactant. In this work, we use a pair of nanoparticles, one hydrophilic, and the other hydrophobic, to stabilize STRIPS bijels without any surfactants and show that the ratio of the hydrophilic to hydrophobic nanoparticles required to form stable bijels changes with the polarity of the oil phase. Highly non-polar oils require a smaller ratio than moderately polar oils. Furthermore, if a sufficiently polar oil is selected, STRIPS bijels can be stabilized using only the hydrophilic nanoparticle. Our results demonstrate the potential to imbue the interface of biphasic liquid mixtures such as bijels with multifunctionality by using two functional nanoparticles of opposite polarity.

Motile Bacteria at Oil-Water Interfaces: Pseudomonas aeruginosa.

Bacteria are important examples of active or self-propelled colloids. Because of their directed motion, they accumulate near interfaces. There, they can become trapped and swim adjacent to the interface via hydrodynamic interactions, or they can adsorb directly and swim in an adhered state with complex trajectories that differ from those in bulk in both form and spatiotemporal implications. We have adopted the monotrichous bacterium Pseudomonas aeruginosa PA01 as a model species and have studied its motion at oil-aqueous interfaces. We have identified conditions in which bacteria swim persistently without restructuring the interface, allowing detailed and prolonged study of their motion. In addition to characterizing the ensemble behavior of the bacteria, we have observed a gallery of distinct trajectories of individual swimmers on and near fluid interfaces. We attribute these diverse swimming behaviors to differing trapped states for the bacteria in the fluid interface. These trajectory types include Brownian diffusive paths for passive adsorbed bacteria, curvilinear trajectories including curly paths with radii of curvature larger than the cell body length, and rapid pirouette motions with radii of curvature comparable to the cell body length. Finally, we see interfacial visitors that come and go from the interfacial plane. We characterize these individual swimmer motions. This work may impact nutrient cycles for bacteria on or near interfaces in nature. This work will also have implications in microrobotics, as active colloids in general and bacteria in particular are used to carry cargo in this burgeoning field. Finally, these results have implications in engineering of active surfaces that exploit interfacially trapped self-propelled colloids.

Scalable Manufacturing of Bending-Induced Surface Wrinkles.

The wide range of textures that can be generated via wrinkling can imbue surfaces with functionalities useful for a variety of applications including tunable optics, stretchable electronics, and coatings with controlled wettability and adhesion. Conventional methods of wrinkle fabrication rely on batch processes in piece-by-piece fashion, not amenable for scale-up to enable commercialization of surface wrinkle-related technologies. In this work, a scalable manufacturing method for surface wrinkles is demonstrated on a cylindrical support using bending-induced strains. A bending strain is introduced to a thin layer of ultraviolet-curable poly(dimethylsiloxane) (UV-PDMS) coated on top of a soft PDMS substrate by wrapping the bilayer around a cylindrical roller. After curing the UV-PDMS and subsequently releasing the bending strain, one-dimensional or checkerboard surface wrinkles are produced. Based on experimental and computational analyses, we show that these patterns form as a result of the interplay between swelling and bending strains. The feasibility of continuous manufacturing of surface wrinkles is demonstrated by using a two-roller roll-to-roll prototype, which paves the way for scalable roll-to-roll processing. To demonstrate the utility of these textures, we show that surface wrinkles produced in this manner enhance the light harvesting and thus efficiency of a solar cell at oblique angles of illumination due to their strong light scattering properties.

Author Correction: Films of Bacteria at Interfaces (FBI): Remodeling of Fluid Interfaces by Pseudomonas aeruginosa.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Cellular sensing of micron-scale curvature: a frontier in understanding the microenvironment.

The vast majority of cell biological studies examine function and molecular mechanisms using cells on flat surfaces: glass, plastic and more recently elastomeric polymers. While these studies have provided a wealth of valuable insight, they fail to consider that most biologically occurring surfaces are curved, with a radius of curvature roughly corresponding to the length scale of cells themselves. Here, we review recent studies showing that cells detect and respond to these curvature cues by adjusting and re-orienting their cell bodies, actin fibres and nuclei as well as by changing their transcriptional programme. Modelling substratum curvature has the potential to provide fundamental new insight into cell behaviour and function in vivo.

Deck the Walls with Anisotropic Colloids in Nematic Liquid Crystals.

Nematic liquid crystals (NLCs) offer remarkable opportunities to direct colloids to form complex structures. The elastic energy field that dictates colloid interactions is determined by the NLC director field, which is sensitive to and can be controlled by boundaries including vessel walls and colloid surfaces. By molding the director field via liquid-crystal alignment on these surfaces, elastic energy landscapes can be defined to drive structure formation. We focus on colloids in otherwise defect-free director fields formed near undulating walls. Colloids can be driven along prescribed paths and directed to well-defined docking sites on such wavy boundaries. Colloids that impose strong alignment generate topologically required companion defects. Configurations for homeotropic colloids include a dipolar structure formed by the colloid and its companion hedgehog defect or a quadrupolar structure formed by the colloid and its companion Saturn ring. Adjacent to wavy walls with wavelengths larger than the colloid diameter, spherical particles are attracted to locations along the wall with distortions in the nematic director field that complement those from the colloid. This is the basis of lock-and-key interactions. Here, we study ellipsoidal colloids with homeotropic anchoring near complex undulating walls. The walls impose distortions that decay with distance from the wall to a uniform director in the far field. Ellipsoids form dipolar defect configurations with the colloid's major axis aligned with the far field director. Two distinct quadrupolar defect structures also form, stabilized by confinement; these include the Saturn I configuration with the ellipsoid's major axis aligned with the far field director and the Saturn II configuration with the major axis perpendicular to the far field director. The ellipsoid orientation varies only weakly in bulk and near undulating walls. All configurations are attracted to walls with long, shallow waves. However, for walls with wavelengths that are small compared to the colloid length, Saturn II is repelled, allowing selective docking of aligned objects. Deep, narrow wells prompt the insertion of a vertical ellipsoid. By introducing an opening at the bottom of such a deep well, we study colloids within pores that connect two domains. Ellipsoids with different aspect ratios find different equilibrium positions. An ellipsoid of the right dimension and aspect ratio can plug the pore, creating a class of 2D selective membranes.

Colloids in confined liquid crystals: a plot twist in the lock-and-key mechanism.

By confining soft materials within tailored boundaries it is possible to design energy landscapes to address and control colloidal dynamics. This provides unique opportunities to create reconfigurable, hierarchically organized structures, a leading challenge in materials science. Example soft matter systems include liquid crystals. For instance, when nematic liquid crystals (NLCs) are confined in a vessel with undulated boundaries, bend and splay distortions can be used to position particles. Here we confine this system in a twist cell. We also study cholesteric liquid crystals, which have an "intrinsic" twist distortion which adds to the ones imposed by the solid boundaries. The cholesteric pitch competes with the other length scales in the system (colloid radius, vessel thickness, wavelength of boundary undulations), enriching the possible configurations. Depending on the pitch-to-radius and pitch-to-thickness ratios the interaction can be attractive or repulsive. By tuning the pitch (i.e. changing the concentration of the chiral dopant), it is possible to selectively promote or inhibit particle trapping at the docking sites.