Fourier Transform Rheology: A New Tool to Characterize Material Properties

Liquid multiphase systems such as polymer blends or emulsions are ubiquitous in many applications, including plastic production, food processing, pharmaceutical and cosmetic production. When the constituents of the multiphase system are incompatible the phases are immiscible, and, depending on their relative amount, the microstructure can consist of droplets in a matrix, elongated fibrils or a co-continuous structure (Utracki, 2003) as schematically shown in Figure 1. The morphology of the liquid multiphase system is important in the applications as it strongly affects processing properties, and the properties of the final products. With the term “morphology” we here indicate not only the overall form or shape of the physical structure of the system, but also the distribution and orientation of the phases, the interfacial area, and the volume of the interphase. Hence, a profound knowledge of the relation between processing parameters, material properties and morphology is essential to optimize the performances of the liquid multiphase systems.Substantial efforts were done in the last decades to set up experimental protocols aimed at evaluating the morphological properties of polymer blends and emulsions via rheological measurements. So far, the most reliable strategy for morphological characterization through rheological measurements is based on the dynamic small amplitude oscillatory shear (SAOS) experiment: the samples are subjected to small amplitude shearing oscillations, and the measured shear stress response is used to gain information on the blend properties (e.g. Palierne, 1990). Here, we present an alternative technique we have recently proposed to characterize the liquid two-phase system morphology. This methodology is based on Large Amplitude Oscillatory Shear (LAOS) flows. This kind of analysis is often referred in the literature as Fourier Transform Rheology (FTR) (Wilhelm et al., 1998), since the stress response is usually analyzed in the Fourier domain. It will be shown that Fourier Transform Rheology possesses a high sensitivity in the characterization of the morphology, thus allowing evaluation of properties that might otherwise be hardly appreciated with traditional linear methodologies

Microfoamed Strands by 3D Foam Printing

We report the design, production, and characterization of microfoamed strands by means of a green and sustainable technology that makes use of CO2 to create ad-hoc innovative bubble morphologies. 3D foam-printing technology has been recently developed; thus, the foaming mechanism in the printer nozzle is not yet fully understood and controlled. We study the effects of the operating parameters of the 3D foam-printing process to control and optimize CO2 utilization through a maximization of the foaming efficiency. The strands' mechanical properties were measured as a function of the foam density and explained by means of an innovative model that takes into consideration the polymer's crystallinity content. The innovative microfoamed morphologies were produced using a bio-based and compostable polymer as well as polylactic acid and were then blown with CO2. The results of the extensive experimental campaigns show insightful maps of the bubble size, density, and crystallinity as a function of the process parameters, i.e., the CO2 concentration and temperature. A CO2 content of 15 wt% enables the acquirement of an incredibly low foam density of 40 kg/m3 and porosities from the macro-scale (100-900 μm) to the micro-scale (1-10 μm), depending on the temperature. The foam crystallinity content varied from 5% (using a low concentration of CO2) to 45% (using a high concentration of CO2). Indeed, we determined that the crystallinity content changes linearly with the CO2 concentration. In turn, the foamed strand's elastic modulus is strongly affected by the crystallinity content. Hence, a corrected Egli's equation was proposed to fit the strand mechanical properties as a function of foam density

Retraction of thin films coated by insoluble surfactants

We investigate the retraction of a circular thin film coated with insoluble surfactants, which is punctured at its centre. We assume that the surface pressure of the liquid-gas interface is related to the number density of surfactants through a linear equation of state, which is characterized by a single parameter: the Gibbs dilation modulus. To solve the governing equations and track the deformation of the domain, we use the finite element method with an arbitrary Lagrangian-Eulerian approach where the film surface is sharp. Our simulations show that the surface elasticity introduced by the surfactants slows down the retraction and introduces oscillations at early times. In agreement with previous experiments and theoretical analysis, we find that the presence of surfactants introduces perturbations of the film thickness over progressively larger distances as the surface elasticity increases. The surface perturbations travel faster than the retracting edge of the film at a speed proportional to the Gibbs modulus. For large values of the Gibbs modulus, the interface behaviour approaches that of an incompressible two-dimensional solid. Our analysis sheds light on the effect of insoluble surfactants on the opening of a circular hole in a thin film and can be extended to investigate the onset of surface cracks and fractures. © 2022 Author

Full-Field and Quantitative Analysis of a Thin Liquid Film at the Nanoscale by Combining Digital Holography and White Light Interferometry

Visualizing and measuring thin-film thickness at the nanoscale during dynamic evolution has been an open challenge for long term. Here, a joint-imaging method and the thereof innovative procedure are presented for merging digital holography (DH) and white light colorimetric interferometry (WLCI) measurement data in a single intelligent tool. This approach allows a complete quantitative study of the dynamic evolution of freestanding thin films under high spatial resolution and full-field modality over a large area. By merging interferometric and holographic fringes, it is possible to overcome the lack of DH in thickness measurements of ultrathin layers, providing a reliable reference for full-field quantitative mapping of the whole film with interferometric accuracy. Thanks to the proposed approach, the time-related and concentration-related evolution of surfactant film thickness can be studied. The thickness distribution curves reveal the small changes in the film thickness with time and concentration. The reported tool opens a route for comprehending deeply the physics behind the behavior of freestanding thin liquid films as it provides an in situ, continuous monitoring of film formation and dynamic evolution without limits of thickness range and in full-field mode. This can be of fundamental importance to many fields of applications, such as fluids, polymers, biotechnology, bottom-up fabrication, etc

Quantitative imaging of the complexity in liquid bubbles' evolution reveals the dynamics of film retraction

The dynamics and stability of thin liquid films have fascinated scientists over many decades. Thin film flows are central to numerous areas of engineering, geophysics, and biophysics and occur over a wide range of length, velocity, and liquid properties scales. In spite of many significant developments in this area, we still lack appropriate quantitative experimental tools with the spatial and temporal resolution necessary for a comprehensive study of film evolution. We propose tackling this problem with a holographic technique that combines quantitative phase imaging with a custom setup designed to form and manipulate bubbles. The results, gathered on a model aqueous polymeric solution, provide an unparalleled insight into bubble dynamics through the combination of full-field thickness estimation, three-dimensional imaging, and fast acquisition time. The unprecedented level of detail offered by the proposed methodology will promote a deeper understanding of the underlying physics of thin film dynamics

A microcapillary rheometer for microliter sized polymer characterization

Abstract We report the design of a microcapillary rheometer (μCR) that allows to perform experiments rapidly and in a broad range of shear rates (i.e., from 0.1 to 1000 s−1), using small amounts of material (i.e., just few milligrams). Additionally, multiple measurement parallelization makes it suitable for High-Throughput Rheological Experimentation of polymer melts (HT-Rheo-E). The novel rheometer consists of a set of three cylindrical microcapillaries in which the fluid flows driven by a controlled pressure. A camera, placed at the capillary exit, records the fluid motion to measure its flow rate, from which the fluid viscosity can be determined. The optimization of the setup allowed for reliable and fast viscosity measurements using ca. 10 mg of material. The current work reports the design of the rheometer and validation measurements on several model fluids. The microfabricated μCR is of potential interest for applications in quality control and research where rapid and repeated measurements using limited milligrams of polymer are required, as well as for High-Throughput-Experimentation of complex fluids (e.g., biological systems)

Design Of An Optofluidic Device For The Measurement Of The Elastic Modulus Of Deformable Particles

Suspensions carrying deformable inclusions are ubiquitous in nature and applications. Hence, high-throughput characterization of the mechanical properties of soft particles is of great interest. Recently, a non-invasive optofluidic technique has been developed for the measurement of the interfacial tension between two immiscible liquids [8]. We have adapted such technique to the case of soft solid beads, thus designing a non-invasive optofluidic device for the measurement of the mechanical properties of deformable particles from real-time optical imaging of their deformation. The device consists of a cylindrical microfluidic channel with a cross-section reduction in which we make initially spherical soft beads flow suspended in a Newtonian carrier. By imaging the deformation of a particle in real time while it goes through the constriction, it is possible to get a measure of its elastic modulus through a theoretically derived-correlation. We provide both experimental and numerical validation of our device

Quantitative imaging of the complexity in liquid bubbles’ evolution reveals the dynamics of film retraction

Thin liquid films: Seeing bubbles in a better light A procedure for imaging the complex fluid dynamics in bubbles could greatly assist efforts to understand and exploit thin liquid films in applications ranging through medicine, industrial chemistry and engineering. Thin liquid films are ubiquitous in nature, found in such varied systems as soap bubbles, biological membranes, detergents, oils, insulation, foods and geological magma. Researchers in Italy led by Biagio Mandracchia at the Institute of Applied Science and Intelligent Systems in Naples, devised a novel holographic phase imaging technique to watch bubbles as they form, develop, burst and retract. The researchers built customized apparatus to create and manipulate the bubbles. The unprecedented level of detail being revealed offers deeper understanding of the physics underlying thin film behavior. Insights into the complex fluid dynamics within bubbles could advance thin film technology for many applications

Correction: Rheometry-on-a-chip: measuring the relaxation time of a viscoelastic liquid through particle migration in microchannel flows

n/

Fluid Viscoelasticity Drives Self-Assembly of Particle Trains in a Straight Microfluidic Channel

Strings of equally spaced particles (particle train) are tremendously important in a variety of microfluidic applications. By using inertial microfluidics, particle trains can be formed near the channel walls. However, the high particle rotation and large local shear gradient near the microchannel walls can lead to blurred images and cell damage, thus negatively affecting applications related to flow cytometry. To address this challenge, we demonstrate that adding a tiny amount of hyaluronic acid biopolymer to an aqueous suspension drives self-assembly of a particle train on the centerline of a square-shaped straight microchannel, with a throughput up to approximately 2400 particles/s. The fraction of equally spaced particles increases by increasing the volumetric flow rate and the distance from the channel inlet. Numerical simulations corroborate the experimental observations and, together with a simple qualitative argument on the particle train stability, shed insights on the underlying mechanism leading to particle ordering