Showing papers in "Soft Matter in 2007"

Directional adhesion of superhydrophobic butterfly wings.

Abstract: We showed directional adhesion on the superhydrophobic wings of the butterfly Morpho aega. A droplet easily rolls off the surface of the wings along the radial outward (RO) direction of the central axis of the body, but is pinned tightly against the RO direction. Interestingly, these two distinct states can be tuned by controlling the posture of the wings (downward or upward) and the direction of airflow across the surface (along or against the RO direction), respectively. Research indicated that these special abilities resulted from the direction-dependent arrangement of flexible nano-tips on ridging nano-stripes and micro-scales overlapped on the wings at the one-dimensional level, where two distinct contact modes of a droplet with orientation-tuneable microstructures occur and thus produce different adhesive forces. We believe that this finding will help the design of smart, fluid-controllable interfaces that may be applied in novel microfluidic devices and directional, easy-cleaning coatings.

Soft biological materials and their impact on cell function

Abstract: Most organs and biological tissues are soft viscoelastic materials with elastic moduli ranging from on the order of 100 Pa for the brain to 100 000 Pa for soft cartilage. Biocompatible synthetic materials already have many applications, but combining chemical compatibility with physiologically appropriate mechanical properties will increase their potential for use both as implants and as substrates for tissue engineering. Understanding and controlling mechanical properties, specifically softness, is important for appropriate physiological function in numerous contexts. The mechanical properties of the substrate on which, or within which, cells are placed can have as large an impact as chemical stimuli on cell morphology, differentiation, motility, and commitment to live or die.

Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

Abstract: Wormlike micelles are elongated flexible self-assembly structures formed by the aggregation of amphiphiles. Above a threshold concentration, they entangle into a dynamic network, reminiscent of polymer solutions, and display remarkable visco-elastic properties, which have been exploited in numerous industrial and technological fields. Relating the microstructure of these intricate structures with their bulk properties is still an ongoing quest. In this review, we present a classification of wormlike micelles, with a focus on novel systems and applications. We describe the current state of understanding of their linear rheology and give a detailed account of recent progress in small-angle neutron scattering, a particularly powerful technique to elucidate their microstructure on a wide range of length-scales.

Flow boundary conditions from nano- to micro-scales

Abstract: The development of microfluidic devices has recently revived the interest in “old” problems associated with transport at, or across, interfaces. As the characteristic sizes are decreased, the use of pressure gradients to transport fluids becomes problematic, and new, interface driven, methods must be considered. This has lead to new investigations of flow near interfaces, and to the conception of interfaces engineered at various scales to reduce flow friction. In this review, we discuss the present theoretical understanding of flow past solid interfaces at different length scales. We also briefly discuss the corresponding phenomenon of heat transport, and the influence of surface slip on interface driven (e.g. electro-osmotic) flows.

Self-assembly of nanoparticles at interfaces

Abstract: Developments in the assembly of nanoparticles at liquid–liquid interfaces are reviewed where the assemblies can be controlled by tuning the size of the nanoparticles and the chemical characteristics of the ligands. Both synthetic and biological nanoparticles are discussed. By controlling the type of ligands, uniform and Janus-type nanoparticles can be produced where, at liquid–liquid interfaces, subsequent reactions of the ligands can be used to generate crosslinked sheets of nanoparticles at the interface that have applications including novel encapsulants, filtration devices with well-defined porosities, and controlled release materials. By controlling the size and volume fraction of the nanoparticles and the chemical nature of the ligands, nanoparticle–polymer composites can be generated where either enthalpy or entropy can be used to control the spatial distribution of the nanoparticles, thereby, producing auto-responsive materials that self-heal, self-corral assemblies of nanoparticles, or self-direct morphologies. Such systems hold great promise for generating novel optical, acoustic, electronic and magnetic materials.

Photo-mechanical effects in azobenzene-containing soft materials.

Abstract: The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. The most ubiquitous natural molecule for reversible shape change is the rhodopsin–retinal protein system that enables vision, and this is perhaps the quintessential reversible photo-switch. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. This review focuses on the study and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but non-mechanical photo-switching or photo-chemistry, nor any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere.

Microcontact printing: A tool to pattern

Abstract: Microcontact printing has proven to be a useful technique in the patterned functionalization of certain chemicals onto surfaces. It has been particularly valuable in the patterning of biological materials. In this review, we describe the basic principles of the technology as well as its use in several applications, with an emphasis on biological ones. We also discuss the limitations and future directions of this method.

Anisotropic wetting on tunable micro-wrinkled surfaces

Abstract: We examine the wettability of rough surfaces through a measurement approach that harnesses a wrinkling instability to produce model substrate topographies. Specifically, we probe the wetting of liquids on anisotropic micro-wrinkled features that exhibit well-defined aspect ratios (amplitude versus wavelength of the wrinkles) that can be actively tuned. Our study provides new insight into the wetting behavior on rough surfaces and into the interpretation of related liquid contact-angle measurements. In particular, we find that droplet wetting anisotropy is governed primarily by the roughness aspect ratio. In addition, comparison of our measurements to theoretical models demonstrates that droplet distortions and observed contact angles on surfaces with a strongly anisotropic texture can be quantitatively attributed to the difference in the energetic barriers to wetting along and perpendicular to substrate features.

Dielectric elastomers as next-generation polymeric actuators

Abstract: Due to their versatile properties, robust behavior, facile processability and low cost, organic polymers have become the material of choice for an increasing number of mature and cutting-edge technologies. In the last decade or so, a new class of polymers capable of responding to external electrical stimulation by displaying significant size or shape change has emerged. These responsive materials, collectively referred to as electroactive polymers (EAPs), are broadly classified as electronic or ionic according to their operational mechanism. Electronic EAPs generally exhibit superior performance relative to ionic EAPs in terms of actuation strain, reliability, durability and response time. Among electronic EAPs, dielectric elastomers exhibit the most promising properties that mimic natural muscle for use in advanced robotics and smart prosthetics, as well as in haptic and microfluidic devices. Elastomers derived from homopolymers such as acrylics and silicones have received considerable attention as dielectric EAPs, whereas novel dielectric EAPs based on selectively swollen nanostructured block copolymers with composition-tailorable properties have only recently been reported. Here, we provide an overview of various EAPs in terms of their operational mechanisms, uses and shortcomings, as well as a detailed account of dielectric elastomers as next-generation actuators.

Actively moving polymers

Abstract: The ability of polymers to move actively in response to an external stimulus such as heat or light is of high scientific and technological significance. In any instance stimuli-responsive effects on the molecular level are converted into macroscopic movement, whereby generally two different moving behaviors have to be differentiated for polymer-based materials: the shape-memory effect and the shape-changing capability. Basic concepts for the molecular design of suitable polymer architectures for shape-memory polymers as well as tailored programming processes are presented. The thermally-induced shape-memory effect of polymers is described as well as the extension of this concept to other stimuli than heat. Indirect actuation of the thermally-induced effect by IR-irradiation, electric current, humidity or alternating magnetic fields are outlined as well as recent work on light-induced shape-memory polymers. For shape-changing polymers, two basic concepts are presented: shape changes occurring during phase orientation of liquid crystal elastomers (LCE) and the photomechanical effect based on photoisomerization of moieties, such as azo-groups incorporated in suitable polymer systems.

Microrheology: a review of the method and applications

Abstract: A set of local mechanical probes has been developed over the last ten years, allowing a kind of dynamical mechanical testing known as microrheology. This paper provides a short introductory review of these methods of performing rheology, comparing them to conventional rheometry, and highlighting the major advantages. The authors also share their outlook on some of the most promising and fastest developing areas that are being studied though microrheology, in the areas of biophysics and soft matter.

Recent advances in the preparation of cyclodextrin-based polyrotaxanes and their applications to soft materials

Abstract: The present review article deals with recent novel studies on the preparation and application of polyrotaxanes comprised of cyclodextrins (CDs) and various linear polymers, especially poly(ethylene glycol) (PEG). First, a brief introduction of the historical background of the pioneering work on the preparation of an inclusion complex and polyrotaxane is provided. Subsequently, the authors have focused on the recently developed solvent systems for the polyrotaxane. These new solvents are interesting from two fundamental viewpoints: (1) from the perspective of the clarification of the hydrogen-bonding-based dissolution mechanism of polyrotaxanes; and (2) from the practical viewpoint of the preparation of modified polyrotaxanes or slide-ring gels containing ionic liquids. A wide variety of polyrotaxane derivatives, whose cyclodextrin moiety was modified to carry various functional groups, and their intriguing characteristics are introduced in this article. Finally, many instances of the application of the PEG–CD polyrotaxane to soft materials, such as gels, molecular tubes and multivalent ligand systems, are summarized.

Electrochemically enabled polyelectrolyte multilayer devices: from fuel cells to sensors

Abstract: With an ever-increasing need for thin, flexible and functional materials in electrochemical systems, the layer-by-layer (LbL) technique provides a simple and affordable route in creating new, active electrodes and electrolytes. The LbL technique, which is based upon the alternate adsorption of oppositely charged species from aqueous solution, possesses unprecedented control of materials selection (e.g. polyelectrolytes, clays, nanoparticles, proteins), materials properties (e.g. conductivity, glass-transition temperature) and architecture (e.g. blends, stratified-layers, pores). These advantages make LbL assemblies excellent candidates for use in proton-exchange membrane and direct methanol fuel-cells, batteries, electrochromic devices, solar cells, and sensors. This review addresses the design of LbL films for electrochemical systems and recent progress.

Microfluidic consecutive flow-focusing droplet generators.

Abstract: This article describes emulsification in a microfluidic double droplet generator (DDR) comprising two consecutive flow-focusing devices with locally modified surface chemistry. We generated W/O/W, O/O/W and O/W/O double emulsions with precisely controlled sizes and morphology of droplets. Secondly, by combining two mechanisms of droplet formation (the flow-focusing mechanism and the break up of liquid threads at T-junction) we produced multiple populations of droplets with varying size and/or composition. These droplets were used as the structural units for the formation of complex dynamic lattices.

Friction behavior of high-density poly(2-methacryloyloxyethyl phosphorylcholine) brush in aqueous media

Abstract: Super-hydrophilic polymer brushes were prepared by surface-initiated atom transfer radical polymerization of 2-methacryloyloxyethyl phosphorylcholine (MPC) on initiator-immobilized silicon wafers. The graft density was estimated to be ca. 0.22 chains nm−2 based on the linear relationship between Mn and the layer thickness. The contact angle against water was very low, and air bubbles in water hardly attached onto the brush surface, indicating a super-hydrophilic surface. Neutron reflectivity measurements of the poly(MPC) brush showed that the grafting polymer chains extended a fair amount in the vertical direction from the substrate in a good solvent such as water, while they shrunk in a poor solvent. Frictional properties of the poly(MPC) brushes were characterized by sliding a glass ball probe in air and various solvents under a load of 0.49 N at a sliding velocity of 90 mm min−1. An extremely low friction coefficient of the poly(MPC) brush was observed in humid atmosphere because water molecules adsorbed into the brush layer acted as a lubricant.

Bioinspired functional block copolymers

Abstract: The diversity and complexity of structures and functions in synthetic polymer systems can be increased through conjugation with biological segments or, in other words, through generation of “polymer-bioconjugates” or “macromolecular chimeras”. The present contribution highlights major synthetic approaches toward sophisticated functional hybrid block copolymers and analyses of structure–function relationships.

Tunable colloids: control of colloidal phase transitions with tunable interactions

Abstract: Systems of spherical colloidal particles mimic the thermodynamics of atomic crystals. Control of interparticle interactions in colloids, which has recently begun to be extensively exploited, gives rise to rich phase behaviours as well as crystal structures with nanoscale and micron-scale lattice spacings. This provides model systems in which to study fundamental problems in condensed matter physics, such as the dynamics of crystal nucleation and melting, and the nature of the glass transition, at experimentally accessible lengthscales and timescales. Tunable control of these interactions provides reversible control. This will enable quantitative studies of phase transition kinetics as well as the creation of advanced materials with switchability of function and properties.

Rheological fingerprinting of gastropod pedal mucus and synthetic complex fluids for biomimicking adhesive locomotion

Abstract: Nonlinear rheological properties are often relevant in understanding the response of a material to its intended environment. For example, many gastropods crawl on a thin layer of pedal mucus using a technique called adhesive locomotion, in which the gel structure is periodically ruptured and reformed. We present a mechanical model that captures the key features of this process and suggests that the most important properties for optimal inclined locomotion are a large, reversible yield stress, followed by a small shear viscosity and a short thixotropic restructuring time. We present detailed rheological measurements of native pedal mucus in both the linear and nonlinear viscoelastic regimes and compare this “rheological fingerprint” with corresponding observations of two bioinspired slime simulants, a polymer gel and a clay-based colloidal gel, that are selected on the basis of their macroscopic rheological similarities to gastropod mucin gels. Adhesive locomotion (of snails or mechanical crawlers) imposes a large-amplitude pulsatile simple shear flow onto the supporting complex fluid, motivating the characterization of nonlinear rheological properties with large amplitude oscillatory shear (LAOS). We represent our results in the form of Lissajous curves of oscillatory stress against time-varying strain. The native pedal mucus gel is found to exhibit a pronounced strain-stiffening response, which is not imitated by either simulant.

Non-covalent polymer assembly using arrays of hydrogen-bonds

Abstract: Hydrogen-bond-mediated non-covalent polymer synthesis is an emerging area that presents new opportunities for the design of stimuli-responsive materials. Central to this is the ready availability of appropriate building blocks with which to link monomers together. This review discusses recent developments in the design, synthesis and use of arrays of hydrogen-bonds as supramolecular linking units.

Nanoscale organization in room temperature ionic liquids: a coarse grained molecular dynamics simulation study.

Abstract: A model to perform coarse grained molecular dynamics simulations of room temperature ionic liquids of the family 1-n-alkyl-3-methylimidazolium hexafluorophosphate has been developed. Large scale simulations of ionic liquids with butyl, heptyl, and decyl side chains have been carried out. Calculated structure factors demonstrate intermediate range ordering in these liquids. The spatial correlations between anions are shown to dominate the neutron or X-ray scattering at low wave vectors. Ionic liquids with long side chains exhibit a bicontinuous morphology, one region consisting of polar moieties and the other of non-polar, alkyl tails.

Giant vesicles in electric fields

Abstract: This review is dedicated to electric field effects on giant unilamellar vesicles, a cell-size membrane system. We summarize various types of behavior observed when vesicles are subjected either to weak AC fields at various frequency, or to strong DC pulses. Different processes such as electro-deformation, -poration and -fusion of giant vesicles are considered. We describe some recent developments, which allowed us to detect the dynamics of the vesicle response with a resolution below milliseconds for all of these processes. Novel aspects on electric field effects on vesicles in the gel phase are introduced.

Rheophysics of pastes: a review of microscopic modelling approaches

Abstract: Pastes are materials intermediate between solids and liquids which are of great practical interest as they keep the shape they have been given. Despite their various internal structures it is possible to draw up a generic rheophysical scheme from which one qualitatively understands, from a physical point of view, their main mechanical characteristics, i.e. solid regime, solid–liquid transition, liquid regime, thixotropy and aging. Here we review in detail these different properties as they are generally observed with most pasty materials and the attempts to describe them using microscopic structure-based theoretical models. For real systems a unified, qualitative, conceptual description is provided. For some model systems (e.g., foams, colloidal gels…) there exist consistent microscopic approaches providing quantitative relationships between rheological parameters in the solid regime and physical parameters of the system. For the liquid regime and thixotropy the situation is more complex.

Elucidating the assembled structure of amphiphiles in solution via cryogenic transmission electron microscopy

Abstract: For the past twenty years, significant progress has been made in both developing cryogenic transmission electron microscopy (cryo-TEM) technology and understanding assembled behavior of amphiphilic molecules. Cryo-TEM can provide high-resolution images of complex fluids in a near state. Samples embedded in a thin layer of vitrified solvent do not exhibit artifacts that would normally occur when using chemical fixation or staining-and-drying techniques. Cryo-TEM has been useful in imaging biological molecules in aqueous solutions. Cryo-TEM has become a powerful tool in the study of -assembled structures of amphiphiles in solution as a complementary tool to small-angle X-ray and neutron scattering, light scattering, rheology measurements, and nuclear magnetic resonance. The application of cryo-TEM in the study of assembled behavior of amphiphilic block copolymers, hydrogels, and other complex soft systems continues to emerge. In this context, the usage of cryo-TEM in the field of amphiphilic complex fluids and self-assembled nano-materials is briefly reviewed, and its unique role in exploring the nature of assembled structure in liquid suspension is highlighted.

Curable, biodegradable elastomers: emerging biomaterials for drug delivery and tissue engineering

Abstract: Biodegradable elastomers have a number of potential applications in the biomedical area, especially in the emerging field of soft-tissue engineering where the mechanical properties of the polymer scaffold should match those of the tissue to be grown. An increasing number of synthesis strategies have been employed in order to prepare such elastomers. In this review, these synthesis strategies and the properties of these elastomers are outlined. The factors that influence the characteristics of these elastomers including mechanical properties, degradation rate, and mechanical property change during degradation, are discussed in terms of the design of the elastomer and their advantages and disadvantages for the biomedical applications considered.

Autonomous microfluidics with stimuli-responsive hydrogels

Abstract: There has been increasing interest in integrated microfluidic systems because performing biological and chemical laboratory tasks on a single chip is appealing. One straightforward approach to constructing these ‘lab on chips’ is to fabricate individual components and to assemble them for desired functionalities. As the functionalities of the microfluidic systems become increasingly complicated, more functional components and relevant controls need to be integrated on a miniaturized chip, especially when a closed loop is needed for autonomous functionality. Instead, an emerging approach is to incorporate stimuli-responsive hydrogels directly into microfluidics to reduce the system complexity. Due to the hydrogels' ability of transducing stimuli into mechanical actions in response to their surrounding aqueous environment, hydrogel-based microfluidic elements can act as both sensors and actuators simultaneously, alleviating the requirement of most controls and even power sources. This provides microfluidic systems with autonomous functionalities. In this article, we will focus on a few autonomous microfluidic devices including valves, flow sorters, pH regulators, pumps, mixers, drug-delivery devices, fluidic cooling devices, and liquid microlenses.

Cavitation rheology for soft materials

Abstract: To guide the development of tissue scaffolds and the characterization of naturally heterogeneous biological tissues, we have developed a method to determine the local modulus at an arbitrary point within a soft material. The method involves growing a cavity at the tip of a syringe needle and monitoring the pressure of the cavity at the onset of a mechanical instability. This critical pressure is directly related to the local modulus of the material. The results focus on the network development of poly(lactide)–poly(ethylene oxide)–poly(lactide) triblock copolymer and poly(vinyl alcohol) hydrogels. These materials serve as model materials for tissue scaffolds and soft biological tissues. This new method not only provides an easy, efficient, and economical method to guide the design and characterization of soft materials, but it also provides quantitative data of the local mechanical properties in naturally heterogeneous materials.

Complex dynamics of shear banded flows

Abstract: Many complex fluids undergo a flow induced transition to a state of coexisting bands of differing viscosities and internal structuring. This effect, which is called “shear banding”, is widely observed in wormlike micellar surfactants, onion surfactants, colloidal suspensions and polymer solutions. According to a rapidly accumulating body of experimental evidence, shear bands often exhibit complex dynamics, which can be either oscillatory or chaotic in nature. This can be seen in the unsteady response of the bulk rheological signals, and in the motion of the interface between the bands. After giving a brief overview of this experimental evidence, we review in some detail recent efforts to address it theoretically.

The preparation of regenerated silk fibroin microspheres

Abstract: The objective of the present study is to investigate the possibility of preparing pure protein microspheres from regenerated silk fibroin (RSF). It is found that RSF microspheres, with predictable and controllable sizes ranging from 0.2 to 1.5 µm, can be prepared via mild self-assembling of silk fibroin molecular chains. The merits of this novel method include a rather simple production apparatus and no potentially toxic agents, such as surfactants, initiators, cross-linking agents, etc. The results show that the particle size and size distribution of RSF microspheres are greatly affected by the amount of ethanol additive, the freezing temperature and the concentration of silk fibroin. Finally, the mechanism of RSF microspheres formation is also discussed based on our experimental results.

Segmental dynamics in miscible polymer blends: recent results and open questions

Abstract: In this short review we summarize the outcome of the large amount of effort made during the past decade from both the experimental and the theoretical point of view in order to understand the effect of blending on the segmental dynamics in polymers. Each of the two families of models proposed—one based on thermally activated concentration fluctuations, the other on chain connectivity effects—account for each of the two main experimental observations: the broadening of the component response with respect to that of the homopolymer and the dynamic heterogeneity, respectively. The complementarity of these approaches, their main achievements and failures, are critically revised. We also include recent results on blends of components with very different mobilities. In the neighbourhood of the glass-transition of the slow polymer, the dynamics of the other component seem to be confined within the frozen chains. We suggest possible ingredients and new routes to be considered in order to elaborate more predictive theoretical frameworks for all these phenomena.

A lithography-free method for directed colloidal crystal assembly based on wrinkling

Abstract: A simple, novel, and completely lithography-free assembly strategy is reported for directed colloidal crystal assembly on optically transparent substrates. The templates are stable, well-controlled relaxation-wrinkles of ultrathin multilayer films in a non-stretched state, which are fabricated by layer-by-layer self-assembly of polymeric films on soft elastomeric substrates followed by uniaxial plastic deformations. The results show that the wrinkles can be used efficiently to topographically direct colloidal crystal assembly in dip coating. Remarkably highly regular 1- and 2-dimensional patterned colloidal crystals with controlled structures have been obtained. Furthermore, the concept is rather universal and applicable to various particle types provided surface interactions between particles and template are suitable.