A macroporous smart gel based on a pH-sensitive polyacrylic polymer for the development of large size artificial muscles with linear contraction.

The physics of soft matter can contribute to the revolution in robotics and medical prostheses. These two fields require the development of artificial muscles with behavior close to biological muscles. Today, artificial muscles rely mostly on active materials, which can deform reversibly. Nevertheless transport kinetics is the major limit for all of these materials. These actuators are only made of a thin layer of active material and using a large thickness dramatically reduces the actuation time. In this article, we demonstrate that a porous material reduces the limit of transport and enables the use of a large volume of active material. We synthesize a new active material: a macroporous gel, which is based on polyacrylic acid. This gel shows very large swelling when we increase the pH and the macroporosity dramatically reduces the swelling time of centimetric samples from one day to 100 s. We characterize the mechanical properties and swelling kinetics of this new material. This material is well adapted for soft robotics because of its large swelling ratio (300%) and its capacity to apply a pressure of 150 mbar during swelling. We demonstrate finally that this material can be used in a McKibben muscle producing linear contraction, which is particularly adapted for robotics. The muscle contracts by 9% of its initial length within 100 s, which corresponds to the gel swelling time.

Effect of Ethylcellulose on the Rheology and Mechanical Heterogeneity of Asphaltene Films at the Oil-Water Interface.

Asphaltenes are surface-active molecules that exist naturally in crude oil. They adsorb at the water-oil interface and form viscoelastic interfacial films that stabilize emulsion droplets, making water-oil separation extremely challenging. There is, thus, a need for chemical demulsifiers to disrupt the interfacial asphaltene films, and, thereby, facilitate water-oil separation. Here, we examine ethylcellulose (EC) as a model demulsifier and measure its impact on the interfacial properties of asphaltene films using interfacial shear microrheology. When EC is mixed with an oil and asphaltene solution, it retards the interfacial stiffening that occurs between the oil phase in contact with a water phase. Moreover, EC introduces relatively weak regions within the film. When EC is introduced to a pre-existing asphaltene film, the stiffness of the films decreases abruptly and significantly. Direct visualization of interfacial dynamics further reveals that EC acts inhomogeneously, and that relatively soft regions in the initial film are seen to expand. This mechanism likely impacts emulsion destabilization and provides new insight to the process of demulsification.

Measuring Interfacial Polymerization Kinetics Using Microfluidic Interferometry.

A range of academic and industrial fields exploit interfacial polymerization in producing fibers, capsules, and films. Although widely used, measurements of reaction kinetics remain challenging and rarely reported, due to film thinness and reaction rapidity. Here, polyamide film formation is studied using microfluidic interferometry, measuring monomer concentration profiles near the interface during the reaction. Our results reveal that the reaction is initially controlled by a reaction-diffusion boundary layer within the organic phase, which allows the first measurements of the rate constant for this system.

Interfacial Rheology and Heterogeneity of Aging Asphaltene Layers at the Water-Oil Interface.

Surface-active asphaltene molecules are naturally found in crude oil, causing serious problems in the petroleum industry by stabilizing emulsion drops, thus hindering the separation of water and oil. Asphaltenes can adsorb at water-oil interfaces to form viscoelastic interfacial films that retard or prevent coalescence. Here, we measure the evolving interfacial shear rheology of water-oil interfaces as asphaltenes adsorb. Generally, interfaces stiffen with time, and the response crosses over from viscous-dominated to elastic-dominated. However, significant variations in the stiffness evolution are observed in putatively identical experiments. Direct visualization of the interfacial strain field reveals significant heterogeneities within each evolving film, which appear to be an inherent feature of the asphaltene interfaces. Our results reveal the adsorption process and aged interfacial structure to be more complex than that previously described. The complexities likely impact the coalescence of asphaltene-stabilized droplets, and suggest new challenges in destabilizing crude oil emulsions.

Surface shear inviscidity of soluble surfactants.

Foam and emulsion stability has long been believed to correlate with the surface shear viscosity of the surfactant used to stabilize them. Many subtleties arise in interpreting surface shear viscosity measurements, however, and correlations do not necessarily indicate causation. Using a sensitive technique designed to excite purely surface shear deformations, we make the most sensitive and precise measurements to date of the surface shear viscosity of a variety of soluble surfactants, focusing on SDS in particular. Our measurements reveal the surface shear viscosity of SDS to be below the sensitivity limit of our technique, giving an upper bound of order 0.01 µN·s/m. This conflicts directly with almost all previous studies, which reported values up to 10(3)-10(4) times higher. Multiple control and complementary measurements confirm this result, including direct visualization of monolayer deformation, for SDS and a wide variety of soluble polymeric, ionic, and nonionic surfactants of high- and low-foaming character. No soluble, small-molecule surfactant was found to have a measurable surface shear viscosity, which seriously undermines most support for any correlation between foam stability and surface shear rheology of soluble surfactants.

Collective Rayleigh-Plateau Instability: A Mimic of Droplet Breakup in High Internal Phase Emulsion.

Using a microfluidic multi-inlet coflow system, we show the Rayleigh-Plateau instability of adjacent, closely spaced fluid threads to be collective. Although droplet size distributions and breakup frequencies are unaffected by cooperativity when fluid threads are identical, breakup frequencies and wavelengths between mismatched fluid threads become locked due to this collective instability. Locking narrows the size distribution of drops that are produced from dissimilar threads, and thus the polydispersity of the emulsion. These observations motivate a hypothesized two-step mechanism for high internal phase emulsification, wherein coarse emulsion drops are elongated into close-packed fluid threads, which break into smaller droplets via a collective Rayleigh Plateau instability. Our results suggest that these elongated fluid threads break cooperatively, whereupon wavelength-locking reduces the ultimate droplet polydispersity of high-internal phase emulsions, consistent with experimental observations.

Boundary conditions for soft glassy flows: slippage and surface fluidization.

We explore the question of surface boundary conditions for the flow of a dense emulsion. We make use of microlithographic tools to create surfaces with well controlled roughness patterns and measure using dynamic confocal microscopy both the slip velocity and the shear rate close to the wall, which we relate to the notion of surface fluidization. Both slippage and wall fluidization depend non-monotonously on the roughness. We interpret this behavior within a simple model in terms of the building of a stratified layer and the activation of plastic events by the surface roughness.

Development of a Soft Actuator from Fast Swelling Macroporous PNIPAM Gels for Smart Braille Device Applications in Haptic Technology.

The development of a cost-efficient braille device is a crucial challenge in haptic technology to improve the integration of visually impaired people. Exclusion of any group threatens the proper functioning of society. Commercially available braille devices still utilize piezoelectric actuators, which are expensive and bulky. The challenge of a more adapted braille device lies in the integration of a high number of actuators─on a millimeter scale─in order to independently move a matrix of pins acting as tactile cues. Unfortunately, no actuation strategy has been adapted to tackle this challenge. In this study, we develop a soft actuator based on a thermosensitive poly(N-isopropylacrylamide) (PNIPAM) gel. We introduce macroporosity to the gel (pores of 10 to 100 µm). It overcomes the diffusion─which is the limiting kinetic factor─and accelerates the gel response time from hours for the bulk gel to seconds for the macroporous gel. We study the properties of porous gels with various porosities. We also compare a mechanically reinforced nanocomposite gel (made of PNIPAM and Laponite clay) to a "classic" gel. As a result, we develop a fast-actuating gel with high cyclic performance. We then develop a single-pin braille setup, where actuation is controlled thanks to a swift temperature control of a macroporous gel cylinder. This new strategy offers a very promising actuation technology. It offers a simple and cost-efficient alternative to the current braille devices.

Highly Stable Low-Strain Flexible Sensors Based on Gold Nanoparticles/Silica Nanohelices.

Flexible strain sensors based on nanoparticle (NP) arrays show great potential for future applications such as electronic skin, flexible touchscreens, healthcare sensors, and robotics. However, even though these sensors can exhibit high sensitivity, they are usually not very stable under mechanical cycling and often exhibit large hysteresis, making them unsuitable for practical applications. In this work, strain sensors based on silica nanohelix (NH) arrays grafted with gold nanoparticles (AuNPs) can overcome these critical aspects. These 10 nm AuNPs are functionalized with mercaptopropionic acid (MPA) and different ratios of thiol-polyethylene glycol-carboxylic acid (HS-PEG7-COOH) to optimize the colloidal stability of the resulting NH@AuNPs nanocomposite suspensions, control their aggregation state, and tune the thickness of the insulating layer. They are then grafted covalently onto the surface of the NHs by chemical coupling. These nanomaterials exhibit a well-defined arrangement of AuNPs, which follows the helicity of the silica template. The modified NHs are then aligned by dielectrophoresis (DEP) between interdigitated electrodes on a flexible substrate. The flexibility, stability, and especially sensitivity of these sensors are then characterized by electromechanical measurements and scanning electron microscopy observations. These strain sensors based on NH@AuNPs nanocomposites are much more stable than those containing only nanoparticles and exhibit significantly reduced hysteresis and high sensitivity at very slight strains. They can retain their sensitivity even after 2 million consecutive cycles with virtually unchanged responsiveness. These improved performances come from their mechanical stability and the use of nanohelices as stable mechanical templates.

Dynamical flow arrest in confined gravity driven flows of soft jammed particles.

Using numerical simulations, we study the gravity driven flow of jammed soft disks in confined channels. We demonstrate that confinement results in increasing the yield threshold for the Poiseuille flow, in contrast to the planar Couette flow. By solving a nonlocal flow model for such systems, we show that this effect is due to the correlated dynamics responsible for flow, coupled with the stress heterogeneity imposed for the Poiseuille flow. We also observe that with increasing confinement, the cooperative nature of the flow results in increasing intermittent behavior. Our studies indicate that such features are generic properties of a wide variety of jammed materials.

Microscale rheology of a soft glassy material close to yielding.

Using confocal microscopy, we study the flow of a model soft glassy material: a concentrated emulsion. We demonstrate the micro-macro link between in situ measured movements of droplets during the flow and the macroscopic rheological response of a concentrated emulsion, in the form of scaling relationships connecting the rheological "fluidity" with local standard deviation of the strain-rate tensor. Furthermore, we measure correlations between these local fluctuations, thereby extracting a correlation length which increases while approaching the yielding transition, in accordance with recent theoretical predictions.

Measuring concentration fields in microfluidic channels in situ with a Fabry-Perot interferometer.

Recent advancements in microfluidic technology have allowed for the generation and control of complex chemical gradients; however, few general techniques can measure these spatio-temporal concentration profiles without fluorescent labeling. Here we describe a Fabry-Perot interferometric technique, capable of measuring concentration profiles in situ, without any chemical label, by tracking Fringes of Equal Chromatic Order (FECO). The technique has a sensitivity of 10(-5) RIU, which can be used to track local solute changes of ~0.05% (w/w). The technique is spatially resolved (1 µm) and easily measures evolving concentration fields with ~20 Hz rate. Here, we demonstrate by measuring the binary diffusion coefficients of various solutes and solvents (and their concentration-dependence) in both free solution and in polyethylene glycol diacrylate (PEG-DA) hydrogels.