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Determination of the rheological properties and the interfacial structure–property relationships for complex fluid–fluid interfaces can be of crucial importance for the understanding of physiological and biomedical systems, designing and engineering industrial processes, and developing environmental remediation technologies. For the measurement of interfacial shear rheological material functions, it has been determined that the control of the surface pressure during the application of deformation is essential for obtaining reproducible data especially when measuring complex interfaces, such as particle- and polymer-laden interfaces. Moreover, the study of complex fluid interfaces is complicated by kinematically mixed interfacial flow fields, which include both shear and dilatation (shape and area changes), leading to a possible complex flow history. To address this, specialized rheometers have been developed to provide clear kinematic conditions. For instance, a radial trough has been introduced to enhance the study of dilatational interfacial rheology, effectively solving the challenges posed by the mixed flow fields typical in standard rectangular Langmuir–Pockels (LP) troughs or pendant drops. In the present work, we utilize a new trough instrument, the Quadrotrough (QT), capable of performing on the same device (and sample) independent dilatational and shear deformations at the air/liquid interface under strain and surface pressure control. Brewster angle microscopy (BAM) imaging is carried out in situ simultaneously with rheological measurements. Thus, the QT embodies the combined advantages of the circular trough and the controlled surface pressure shear interfacial rheometer. The interfacial rheology of poly(tert-butyl methacrylate) at the air/water interface was measured for both pure dilatation and pure shear in steady and small amplitude oscillatory (SAO) dilatation (D) and shear (S) modes on the same interface. BAM images were obtained during shear and compression. The results obtained by the QT were highly reproducible and in good agreement with measurements performed previously using the LP trough-mounted double wall ring rheometer and the radial trough. The Hencky strain model was employed to derive steady shear and dilatational interfacial moduli. Very good agreement was observed between the steady and complex shear moduli. However, the dilatational moduli measured under steady compression were markedly smaller than those measured by small amplitude oscillatory dilatational at fixed molecular areas, further highlighting the complicating factor of deformation history on material properties for complex interfaces. In summary, the QT has been shown to be a valuable tool for exploring interfacial rheology and providing insights into complex interfacial systems. [ABSTRACT FROM AUTHOR]

Film drainage, essential in droplet and bubble coalescence and surface wetting, is influenced strongly by the stress boundary condition, in particular, when interfacial stresses are present. These stresses, caused by ubiquitous surface-active components, significantly impact the dynamics of liquid films. Through dynamic thin film balance experiments, we compare the effects of Marangoni stresses, interfacial viscosity, and interfacial viscoelasticity on the drainage of free-standing thin liquid films. These data serve to demonstrate that film deformation intricately depends on the interplay between these stresses and capillarity, resulting in widely varied drainage times. Seemingly subtle changes, especially in the local stress-carrying capacity of the interface, can lead to significant differences in film dynamics. This makes it a promising area for research into interfacial-rheologically active materials for stabilizing potentially more sustainable multiphase materials. [ABSTRACT FROM AUTHOR]

Manufacturing intricately controlled, hierarchically distributed structures poses significant fabrication challenges, but is crucial for enhancing functionality in synthetic systems. A 3D printing technique combining advective assembly with rotational direct ink writing is developed and exploited to build topologically complex, multimaterial structures with high precision. A modular advective assembler printhead is fabricated and employed in the process. This flow‐structuring device is designed with a complex network of internal channels that patterns flowing hydrogel‐based inks, creating multi‐layered filaments whose structures go well beyond conventional nozzle shape and size limitations. The composite filaments are extruded into a rotating support bath of Polyacrylic acid microgels. The rheology of the inks and support bath are critical to maintain print fidelity and integrity, and are characterized by linear and nonlinear bulk rheometry. Optimization of the materials creates a platform where curvilinear, multimaterial architectures are constructed without being constrained to slicing across X, Y, and Z axes. The versatility of this manufacturing platform is demonstrated by printing helical structures that undergo swelling‐induced actuation. This processing method has the potential to significantly enhance additive manufacturing by enabling the production of intricate, multiscale composite structures with broad applicability in fields such as bioengineering, soft robotics, and functional composite materials. [ABSTRACT FROM AUTHOR]

Relatively recently, we advanced a route to create, in a controlled fashion, combined horizontal and vertical stratified structures by simple and energyefficient processing operations employing static mixing elements. While in state-of-the-art static mixing the focus is on layer multiplication, here the aim is to create hierarchical fractal structures. Therefore, the main question addressed in this article is how structures, rather than layers, can be multiplied. The key aspect is the addition of layers on the sides or in the midplane of the flow during the process; every addition step increases the hierarchy by one level. This article derives the general formalism for forming fractal structures with controlled hierarchy, and we develop the language required to design and construct the dies. The main part of the article addresses this main topic and is based on the splitting serpentine static mixer geometry that can be easily made on the parting surfaces of a mold on both the micro- and the macroscale. The second part of the article addresses the strategy to minimize the number of mirroring steps, eventually avoiding mirroring completely, and is based on the rotation-free multiflux static mixer geometry. With the design language derived, complex hierarchical fractal structures can be generated simply by changing the number and sequence of operators within extrusion dies or molds, providing a one-step solution to produce material structures for potential use in diverse applications ranging from advanced mechanical systems to photovoltaic devices, where controlled assembly of dissimilar materials, and the realization of huge interfaces and genuine cocontinuity throughout the cross section, is critical. KEYWORDS microlayers, microstructures, processing, static mixers, 1 | INTRODUCTION The last two decennia witnessed major efforts in the design of different routes to structure organic matter, such as polymers, down to the nanoscopic level for use [...]

Dielectric elastomer transducers (DET) are promising candidates for electrically-driven soft robotics. However, the high viscosity and low yield stress of DET formulations prohibit 3D printing, the most common manufacturing method for designer soft actuators. DET inks optimized for direct ink writing (DIW) produce elastomers with high stiffness and mechanical losses, diminishing the utility of DET actuators. To address the antagonistic nature of processing and performance constraints, principles of capillary suspensions are used to engineer DIW DET inks. By blending two immiscible polysiloxane liquids with a filler, a capillary ink suspension is obtained, in which the ink rheology can be tuned independently of the elastomer electromechanical properties. Rheometry is performed to measure and optimize processibility as a function of filler and secondary liquid fraction. Including polar polysiloxanes as the secondary liquid produces a printed elastomer exhibiting a four-fold permittivity increase over commercial polydimethylsiloxane. The characterization and multimaterial printing into layered DET devices demonstrates that the immiscible capillary suspension improves the processability of the inks and enhances the properties of the elastomers, enabling actuation of the devices at comparatively low voltages. It is anticipated that this formulation approach will allow soft robotics to harness the full potential of DETs. [ABSTRACT FROM AUTHOR]

Confinement of living microorganisms and self-propelled particles by an external trap provides a means of analysing the motion and behaviour of active systems. Developing a tweezer with a trapping radius large compared with the swimmers' size and run length has been an experimental challenge, as standard optical traps are too weak. Here we report the novel use of an acoustic tweezer to confine self-propelled particles in two dimensions over distances large compared with the swimmers' run length. We develop a near-harmonic trap to demonstrate the crossover from weak confinement, where the probability density is Boltzmann-like, to strong confinement, where the density is peaked along the perimeter. At high concentrations the swimmers crystallize into a close-packed structure, which subsequently 'explodes' as a travelling wave when the tweezer is turned off. The swimmers' confined motion provides a measurement of the swim pressure, a unique mechanical pressure exerted by self-propelled bodies.

When surface-active molecules or particles assemble at fluid–fluid interfaces, these interfaces acquire complex rheological properties that are of importance in processes that involve flow and deformation of interfaces. Although much progress has been made, interfacial rheology measurements and, in particular, the measurement of interfacial rheological properties of polymers at the air-water interface remain challenging. These are due to weak interactions with the water subphase, the polymer backbone conformation, the glass transition of the interfacial layer, and memory effects. In the present work, we describe systematic rheological measurements of polymer-laden interfaces. The measurements were performed with four different interfacial shear rheometers that can be classified into two types: rheometers in which the surface pressure can be controlled independently, and devices based on fixtures mounted on standard rotational rheometers and lacking control of the surface pressure. We use poly(tert-butyl methacrylate) and poly(methyl methacrylate), two high glass transition temperature, hydrophobic polymers anchored to the water subphase by means of the acrylate group. Using a Langmuir–Pockels (LP) trough, we identify the transition of the polymer monolayer from a viscous to a solid elastic or soft-glassy interface as the polymer surface concentration increases by compression. Then, we compare the linear viscoelastic properties of the interface as obtained by each rheometer. Our results show poor reproducibility and comparability of the rheological data as obtained by different rheometers for the same polymer. This is mainly due to differences in the method used to prepare the layers. For LP-based devices, spreading under dilute conditions and subsequent compression yields layers of compressed glassy blobs with reproducible results. On the other hand, for devices without surface pressure control, deposition of the amount needed to reach a desired concentration may lead to the formation of ill-defined layers resulting in irreproducible data. Furthermore, we find that only when spreading the polymer to form a dilute layer and then controlling the surface pressure by compression, we can clearly distinguish the fluidlike from solidlike interfaces, and a clear correlation is observed between the surface pressure (or interfacial polymer concentration) and the rheological properties of the interface. [ABSTRACT FROM AUTHOR]