Interfacial viscoelasticity controls buckling, wrinkling and arrest in emulsion drops undergoing mass transfer

Contrary to the notion that ‘oil and water do not mix’, many oils possess a residual diffusive mobility through water, causing the drop sizes in oil-in-water emulsions to slowly evolve with time. Liquid interfaces are therefore typically stabilized with polymeric or particulate emulsifiers. Upon adsorption, these may induce strong, localized viscoelasticity in the interfacial region. Here, we show that shrinkage of oil drops due to bulk mass transfer may render such adsorption layers mechanically unstable, causing them to buckle, crumple and, finally, to attain a stationary shape and size. We demonstrate using two types of model interfaces that this only occurs if the adsorption layer has a high interfacial shear elasticity. This is typically the case for adsorbed layers that are cross-linked or ‘jammed’. Conversely, interfacial compression elasticity alone is a poor predictor of interface buckling or arrest. These results provide a new perspective on the role of interfacial rheology for compositional ripening in emulsions. Moreover, they directly affect a variety of applications, including the rapid screening of amphiphilic biopolymers such as the Acacia gum or the octenyl succinic anhydride modified starch used here, the interpretation of light scattering data for size measurements of emulsion drops, or the formulation of delivery systems for encapsulation and release of drugs and volatiles.

Towards Textile Energy Storage from Cotton T-Shirts

A simple chemical activation route is developed to convert insulating cotton T-shirt textiles into highly conductive and flexible activated carbon textiles (ACTs) for energy-storage applications. Such conversion gives these ACTs an ideal electrical double-layer capacitive behavior. The constructed asymmetric supercapacitors based on the ACTs and MnO2/ACT composite show superior electrochemical performances.

State-of-the-Art Graphene High-Frequency Electronics

High-performance graphene transistors for radio frequency applications have received much attention and significant progress has been achieved. However, devices based on large-area synthetic graphene, which have direct technological relevance, are still typically outperformed by those based on mechanically exfoliated graphene. Here, we report devices with intrinsic cutoff frequency above 300 GHz, based on both wafer-scale CVD grown graphene and epitaxial graphene on SiC, thus surpassing previous records on any graphene material. We also demonstrate devices with optimized architecture exhibiting voltage and power gains reaching 20 dB and a wafer-scale integrated graphene amplifier circuit with voltage amplification.

Printable Superhydrophilic-Superhydrophobic Micropatterns Based on Supported Lipid Layers

The ability to create superhydrophilic-superhydrophobic micropatterns and arrays is essential for a variety of applications ranging from microfluidics to cell microarrays and high-throughput screenings. Here we report a novel facile method for printing superhydrophilic patterns on a superhydrophobic surface using a simple microcontact printer. The formation of superhydrophilic areas is based on printing an ethanol solution containing a phospholipid onto a superhydrophobic porous polymer surface. This creates a supported lipid layer on the polymer surface, thereby switching from superhydrophobicity to superhydrophilicity. Therefore, the amphiphilic lipid functions as an ink that can be printed to create superhydrophilic patterns on the superhydrophobic surface.

Hydrodynamically directed multiscale assembly of shaped polymer fibers

A long-sought goal of material science is the development of fabrication processes by which synthetic materials can be made to mimic the multiscale organization many natural materials utilize to achieve unique functional and material properties. Here we demonstrate how the microfluidic fabrication of polymer fibers can take advantage of hydrodynamic forces to simultaneously direct assembly at the molecular and micron scales. The microfluidic device generates long fibers by initiating polymerization of a continuously flowing fluid via UV irradiation within the microfluidic channel. Prior to polymerization, hydrodynamic shear forces direct molecular scale assembly and a combination of hydrodynamic focusing and advection driven by grooves in the channel walls manipulate the cross-sectional shape of the pre-polymer stream. Polymerization subsequently locks in both molecular scale alignment and micron-scale fiber shape. This simple method for generating structures with multiscale organization could be useful for fabricating materials with multifunctionality or enhanced mechanical properties.

Walking with coffee: Why does it spill?

 

In our busy lives, almost all of us have to walk with a cup of coffee. While often we spill the drink, this familiar phenomenon has never been explored systematically. Here we report on the results of an experimental study of the conditions under which coffee spills for various walking speeds and initial liquid levels in the cup. These observations are analyzed from the dynamical systems and fluid mechanics viewpoints as well as with the help of a model developed here. Particularities of the common cup sizes, the coffee properties, and the biomechanics of walking proved to be responsible for the spilling phenomenon. The studied problem represents an example of the interplay between the complex motion of a cup, due to the biomechanics of a walking individual, and the low-viscosity-liquid dynamics in it.

Effects of chemical bonding on heat transport across interfaces

Interfaces often dictate heat flow in micro- and nanostructured systems^{1, 2, 3}. However, despite the growing importance of thermal management in micro- and nanoscale devices^{4, 5, 6}, a unified understanding of the atomic-scale structural features contributing to interfacial heat transport does not exist. Herein, we experimentally demonstrate a link between interfacial bonding character and thermal conductance at the atomic level. Our experimental system consists of a gold film transfer-printed to a self-assembled monolayer (SAM) with systematically varied termination chemistries. Using a combination of ultrafast pump–probe techniques (time-domain thermoreflectance, TDTR, and picosecond acoustics) and laser spallation experiments, we independently measure and correlate changes in bonding strength and heat flow at the gold–SAM interface. For example, we experimentally demonstrate that varying the density of covalent bonds within this single bonding layer modulates both interfacial stiffness and interfacial thermal conductance. We believe that this experimental system will enable future quantification of other interfacial phenomena and will be a critical tool to stimulate and validate new theories describing the mechanisms of interfacial heat transport. Ultimately, these findings will impact applications, including thermoelectric energy harvesting, microelectronics cooling, and spatial targeting for hyperthermal therapeutics.

Spatial-temporal dynamics of collective chemosensing

Although the process of chemosensing by individual cells is intrisically stochastic, multicellular organisms exhibit highly regulated responses to external stimulations. Two key elements to understand the deterministic features of chemosensing are intercellular communications and the role of pacemaker cells. To characterize the collective behavior induced by these two factors, we study the spatial-temporal calcium dynamics of fibroblast cells in response to ATP stimulation. We find that closely packed cell colonies exhibit faster, more synchronized, and highly correlated responses compared to isolated cells. In addition, we demonstrate for chemosensing the existence of pacemaker cells and how the presence of gap junctions impact the first step of the collective response. By further comparing these results with the calcium dynamics of cells embedded in thin hydrogel films, where intercellular communication is only possible via diffusing molecules, we conclude that gap junctions are required for synchronized and highly correlated responses among cells in high density colonies. In addition, in high density cell colonies, both communication channels lead to calcium oscillations following the stimulation by external ATP. While the calcium oscillations associated with cells directly exposed to external flows were transient, the oscillations of hydrogel trapped cells can persist with a fundamental frequency and higher harmonics. Our observations and measurements highlight the crucial role of intercellular signaling for generating regulated spatial and temporal dynamics in cell colonies and tissues.

Microfluidic synthesis of monodisperse porous microspheres with size-tunable pores

We use a perfluorinated-dendrimer–dye complex that stabilizes microbubbles as a novel pore-forming agent. We use microfluidics to produce monodisperse emulsions containing a polymer matrix material, a model active, and the perfluorinated complex; upon drying, the emulsions form porous microspheres. This porosity causes the encapsulated model active to be released faster than from non-porous microspheres. Moreover, because of the fluorous features of the pores, we can also attach an additional guest molecule to the pores which is released with a profile that is distinct from that of the encapsulated active. These porous microspheres can encapsulate and controllably release multiple actives; this makes them valuable for applications such as drug delivery and imaging.

Self-Assembly of Colloidal Cubes via Vertical Deposition

The vertical deposition technique for creating crystalline microstructures is applied for the first time to nonspherical colloids in the form of hollow silica cubes. Controlled deposition of the cubes results in large crystalline films with variable symmetry. The microstructures are characterized in detail with scanning electron microscopy and small-angle X-ray scattering. In single layers of cubes, distorted square to hexagonal ordered arrays are formed. For multilayered crystals, the intralayer ordering is predominantly hexagonal with a hollow site stacking, similar to that of the face centered cubic lattice for spheres. Additionally, a distorted square arrangement in the layers is also found to form under certain conditions. These crystalline films are promising for various applications such as photonic materials.

Transparent Conducting Silver Nanowire Networks

We present a transparent conducting electrode composed of a periodic two-dimensional network of silver nanowires. Networks of Ag nanowires are made with wire diameters of 45–110 nm and a pitch of 500, 700, and 1000 nm. Anomalous optical transmission is observed, with an averaged transmission up to 91% for the best transmitting network and sheet resistances as low as 6.5 Ω/sq for the best conducting network. Our most dilute networks show lower sheet resistance and higher optical transmittance than an 80 nm thick layer of ITO sputtered on glass. By comparing measurements and simulations, we identify four distinct physical phenomena that govern the transmission of light through the networks: all related to the excitation of localized surface plasmons and surface plasmon polaritons on the wires. The insights given in this paper provide the key guidelines for designing high-transmittance and low-resistance nanowire electrodes for optoelectronic devices, including thin-film solar cells. For the latter, we discuss the general design principles to use the nanowire electrodes also as a light trapping scheme.

Tunable metallic-like conductivity in microbial nanowire networks

Electronic nanostructures made from natural amino acids are attractive because of their relatively low cost, facile processing and absence of toxicity(1-3). However, most materials derived from natural amino acids are electronically insulating(1-6). Here, we report metallic-like conductivity in films of the bacterium Geobacter sulfurreducens(7) and also in pilin nanofilaments (known as microbial nanowires(8,9)) extracted from these bacteria. These materials have electronic conductivities of similar to 5 mS cm(-1), which are comparable to those of synthetic metallic nanostructures(2). They can also conduct over distances on the centimetre scale, which is thousands of times the size of a bacterium. Moreover, the conductivity of the biofilm can be tuned by regulating gene expression, and also by varying the gate voltage in a transistor configuration. The conductivity of the nanofilaments has a temperature dependence similar to that of a disordered metal, and the conductivity could be increased by processing.
 

Control of the length of microfibers

Uniform polymeric microfibers of prescribed lengths were synthesized in microfluidic devices using two different approaches—valve actuation and pulses of ultraviolet (UV) light. The more versatile valve approach was employed to demonstrate control of the length of the microfiber as a function of the frequency of valve actuation.

Magnetic field responsive silicone elastomer loaded with short steel wires having orientation distribution

We fabricated a magnetic field responsive silicone rubber–steel wire composite that exhibits large bending deformation under a magnetic field as weak as 0.2 T. The starting materials are commonly available, and the fabrication method is simple. The deformation is the result of the magnetic torque produced by the chosen orientation of short steel wires embedded in the elastomer matrix. This mechanism is confirmed by theoretical modelling. A strip of the composite exhibits a flexible motion of inchworm-like walking when it is subjected to repeated increases and decreases in the applied magnetic field.

Unexpected β-sheets and molecular orientation in flagelliform spider silk as revealed by Raman spectromicroscopy

Silk Materials – A Road to Sustainable High Technology

This review addresses the use of silk protein as a sustainable material in optics and photonics, electronics and optoelectronic applications. These options represent additional developments for this technology platform that compound the broad utility and impact of this material for medical needs that have been recently described in the literature. The favorable properties of the material certainly make a favorable case for the use of silk, yet serve as a broad inspiration to further develop biological foundries for both the synthesis and processing of Nature's materials for technological applications.

Molecular Logic with a Saccharide Probe on the Few-Molecules Level

In this Communication we describe a two-component saccharide probe with logic capability. The combination of a boronic acid-appended viologen and perylene diimide was able to perform a complementary implication/not implication logic function. Fluorescence quenching and recovery with fructose was analyzed with fluorescence correlation spectroscopy on the level of a few molecules of the reporting dye.

Nanotextured Silica Surfaces with Robust Superhydrophobicity and Omnidirectional Broadband Supertransmissivity

Designing multifunctional surfaces that have user-specified interactions with impacting liquids and with incident light is a topic of both fundamental and practical significance. Taking cues from nature, we use tapered conical nanotextures to fabricate the multifunctional surfaces; the slender conical features result in large topographic roughness, while the axial gradient in the effective refractive index minimizes reflection through adiabatic index-matching between air and the substrate. Precise geometric control of the conical shape and slenderness of the features as well as periodicity at the nanoscale are all keys to optimizing the multifunctionality of the textured surface, but at the same time, these demands pose the toughest fabrication challenges. Here we report a systematic approach to concurrent design of optimal structures in the fluidic and optical domains and a fabrication procedure that achieves the desired aspect ratios and periodicities with few defects and large pattern area. Our fabricated nanostructures demonstrate structural superhydrophilicity or, in combination with a suitable chemical coating, robust superhydrophobicity. Enhanced polarization-independent optical transmission exceeding 98% has also been achieved over a broad range of bandwidth and incident angles. These nanotextured surfaces are also robustly antifogging or self-cleaning, offering potential benefits for applications such as photovoltaic solar cells.

Crystallization Mechanisms in Convective Particle Assembly

Colloidal particles are continuously assembled into crystalline particle coatings using convective fluid flows. Assembly takes place inside a meniscus on a wetting reservoir. The shape of the meniscus defines the profile of the convective flow and the motion of the particles. We use optical interference microscopy, particle image velocimetry and particle tracking to analyze the particles’ trajectory from the liquid reservoir to the film growth front and inside the deposited film as a function of temperature. Our results indicate a transition from assembly at a static film growth front at high deposition temperatures to assembly in a precursor film with high particle mobility at low deposition temperatures. A simple model that compares the convective drag on the particles to the thermal agitation explains this behavior. Convective assembly mechanisms exhibit a pronounced temperature dependency and require a temperature that provides sufficient evaporation. Capillary mechanisms are nearly temperature independent and govern assembly at lower temperatures. The model fits the experimental data with temperature and particle size as variable parameters and allows prediction of the transition temperatures. While the two mechanisms are markedly different, dried particle films from both assembly regimes exhibit hexagonal particle packings. We show that films assembled by convective mechanisms exhibit greater regularity than those assembled by capillary mechanisms.

Novel 3D Wollastonite-Based Scaffolds from Preceramic Polymers Containing Micro- and Nano-Sized Reactive Particles

Wollastonite (CaSiO3) ceramics are well known biomaterials which can be produced using many different techniques. The present paper illustrates an innovative processing method employing preceramic polymers (silicone resins) containing CaCO3 micro- and nano-sized particles, which act as reactive fillers. Silica from the decomposition of the silicone resins reacts at low temperature with the CaO deriving from the fillers, yielding wollastonite ceramics. Hydroxyapatite powders can also be added, to modify the biological response of the material. This approach enables the fabrication of 3D scaffolds via fused deposition or via conventional hot extrusion.

Photoinduced Deformation of Crosslinked Liquid-Crystalline Polymer Film Oriented by a Highly Aligned Carbon Nanotube Sheet

Bending over backwards: A highly aligned carbon nanotube sheet orients crosslinked liquid-crystalline polymer through a simple melting process (see picture). The resulting composite film can be rapidly bent and unbent by alternate irradiation with UV and visible light. The film also exhibits excellent mechanical and electrical properties due to the incorporation of aligned CNTs.

A Strong Bio-Inspired Layered PNIPAM–Clay Nanocomposite Hydrogel

Inspired by nacre, a layered poly(N-isopropylacrylamide)–clay nanocomposite hydrogel was successfully fabricated by combination of vacuum-filtration self-assembly and photo-initiated in situ polymerization. This bio-inspired layered nanocomposite hydrogel shows excellent mechanical properties, which can rival some biological soft tissues (see picture).

Force–Reactivity Property of a Single Monomer Is Sufficient To Predict the Micromechanical Behavior of Its Polymer

We demonstrate an accurate prediction of the micromechanical behavior of a single chain of cyclopropanated polybutadiene, which is governed by rapid isomerization of the cyclopropane moieties at 1.2 nN, from the force–rate correlation of this reaction measured in a small series of increasingly strained macrocycles. The data demonstrate that a single physical quantity, force, uniquely defines the dynamics across length scales from >100 to <1 nm and that strain imposed through molecular design and that imposed by micromanipulation techniques have equivalent effects on the kinetics of a chemical reaction. This represents a new method of screening potential monomers for applications in stress-responsive materials that could also facilitate atomistic interpretations of single-molecule force experiments.

Carbon Nanotubes in the Liquid Phase: Addressing the Issue of Dispersion

The inherent size and hollow geometry with extraordinary electronic and optical properties make carbon nanotubes (CNTs) promising building blocks for molecular or nanoscale devices. Unfortunately, their hydrophobic nature and their existence in the form of agglomerated and parallel bundles make this interesting material inadequately soluble or dispersible in most of the common solvents, which is crucial to their processing. Therefore, various ingenious techniques have been reported to disperse the CNTs in various solvents with different experimental conditions. However, by analyzing the published scientific research articles, it is evident that there is an important issue or misunderstanding between the term “dispersion” and “solubilization”. As a result many researchers use the terms interchangeably, particularly when stating the interaction of CNTs with liquids, which causes confusion among the readers, students, and researchers. In this article, this fundamental issue is addressed in order to give basic insight to the researchers who are working with CNTs, as well as to the scientists who deal with nano-related research domains.

Stoichiometric Self-Assembly of Isomeric, Shape-Persistent, Supramacromolecular Bowtie and Butterfly Structures

 Two novel macromolecular constitutional isomers have been self-assembled from previously unreported terpyridine ligands in a three-component system. The terpyridine ligands were synthesized in high yields via a key Suzuki coupling. Restrictions of the possible outcomes for self-assembly ultimately provided optimum conditions for isolation of either a molecular bowtie or its isomeric butterfly motif. These isomers have been characterized by ESI-MS, TWIM-MS, ¹H NMR, and ¹³C NMR. Notably, these structural isomers have remarkably different drift times in ion mobility separation, corresponding to different sizes and shapes at high charge states.