Publications

High-pressure Effects on Gelatin Sol-Gel Transition

Nikolaos A. Burger, Gerhard Meier, Dimitris Vlassopoulos, Benoit Loppinet

https://doi.org/10.1021/acs.iecr.4c04861 

We investigated the effects of high hydrostatic pressure on the sol–gel transition of gelatin dispersions. We used dynamic light scattering (DLS) and DLS-based passive microrheology to monitor the evolution of the viscoelasticity during isothermal gelation. It provided easy identification of the sol–gel transition and the isothermal critical gelation time (tc) and values of viscosities of sols and shear modulus of gels. At a given temperature, tc decreased with increasing pressure. Up to 100 MPa, the temperature dependence of tc followed the established empirical rule 𝑡c∼(1−𝑇𝑇C)𝑛 and the critical temperature Tc increased with pressure by ∼0.04 K/MPa. The critical gelation time scaled with the quench depth T–Tc or equivalently with the distance from the pressure-dependent collagen denaturation temperature (∼314 K, at 0.1 MPa), which also increases by ∼0.04 K/MPa in the first 100 MPa. The pressure dependence also reflected on the time evolution of the intrinsic viscosity, ηi, or elastic modulus, Gp, in the sol or gel state, respectively, are reported. Both ηi or GP evolution speeds up with pressure. Finally, using a reverse quenching approach, we observed a slowing of the gel melting when the pressure increases. Our results confirmed that the rheological evolution reflects the helix formation process and that pressure stabilizes the helices.

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Orthogonal superposition rheometry of soft core-shell microgels

Bogri Panagiota, Pagani Gabriele, Vermant Jan, Sprakel Joris, Petekidis George

https://doi.org/10.1007/s00397-024-01469-5

The mechanisms of flow in suspensions of soft particles above the glass-transition volume fraction and in the jammed state were probed using orthogonal superposition rheometry (OSR). A small amplitude oscillatory shear flow is superimposed orthogonally onto a steady shear flow, which allows monitoring the viscoelastic spectra of sheared jammed core–shell microgels during flow. The characteristic crossover frequency ωc, deduced from the viscoelastic spectrum, provides information about the shear-induced structural relaxation time, which is connected to the microscopic yielding mechanism of cage breaking. The shear rate evolution of the crossover frequency is used to achieve a superposition of all spectra and get a better insight of the flow mechanism. Despite their inherent softness, the hybrid core–shell microgels exhibit similarities with hard sphere-like flow behavior, with the main difference that for the microgels, the transition from a glassy to a jammed state introduces a volume fraction dependence of the scaling of ωc with shear rate. We further check the application of the Kramers–Kronig relations on the experimental low strain amplitude OSR data finding a good agreement. Finally, the low frequency response at high strain rates was investigated with open bottom cell geometry, and instrumental limits were identified. Based on these limits, we discuss previous OSR data and findings in repulsive and attractive colloidal glasses and compare them with the current soft particle gels.

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Dextran stabilized hematite: a sustainable anode in aqueuous electrolytes

Sofia Panagiota, Evangelia Vasilaki, Nikos Katsarakis, Dimitra Vernardou, Maria Vamvakaki

https://doi.org/10.1039/D4NR04897K

During the last decades, the use of innovative hybrid materials in energy storage devices has led to notable advances in the field. However, further enhancement of their electrochemical performance faces significant challenges nowadays, imposed by the materials used in the electrodes and the electrolyte. Such problems include the high solubility of both the organic and the inorganic anode components in the electrolyte as well as the limited intrinsic electronic conductivity and substantial volume variation of the materials during cycling. The present work focuses on the fabrication of novel and sustainable anode electrodes for use in energy storage devices, utilizing cross-linked oxidized dextran (Ox-Dex) as the binder and hematite (α-Fe2O3) cubes as the active component. The ion diffusion mechanism within the anode electrode materials, as well as their cycling stability, were studied via cyclic voltammetry measurements, using Li+, Zn2+ and Al3+ aqueous electrolytes. The hybrid iron oxide electrodes exhibited the highest electrochemical performance in the Al2(SO4)3 electrolyte (3000 mA g−1), followed by ZnSO4 (2000 mA g−1) and Li2SO4 (800 mA g−1). The differences in the performance of the anodes for the three investigated electrolytes were attributed to the ionic radii of Li+, Zn2+ and Al3+, which affect the rate of ion diffusion within the material lattice exhibiting the highest diffusion coefficient of 4.64 × 10−9 cm2 s−1 in Al3+. Notably, the hybrid anodes demonstrated superior cycling performance (with the lowest variance percentage of 1.3% for hybrid compared to 38.1% for the bare in the presence of Zn2+), underlining the pivotal role of the natural binder. This was attributed to hydrogen bonding interactions, which increase the contact points between the inorganic and polymeric components, resulting in a more uniform network structure. Additionally, the cross-linking of Ox-Dex promotes stability and tolerance to the volume expansion of the electrodes. These results underscore the immense potential of the proposed hybrid electrodes in the field of energy storage

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Probing cage dynamics in concentrated hard-sphere suspensions and glasses with frequency rheometry

Thanasis Athanasiou, Baicheng Mei, Kenneth S. Schweizer, George Petekidis

https://doi.org/10.1039/D4SM01428F

The cage concept, a central microscopic mechanism for glassy dynamics, has been utilized in concentrated colloidal suspensions to describe a number of phenomena. Here, we probe the evolution of cage formation and shear elasticity with increasing volume fraction in hard sphere suspensions, with emphasis on the short-time dynamics. To this end, we utilize linear viscoelastic (LVE) measurements, by means of conventional rotational rheometers and a home-made HF piezo-rheometer, to probe the dynamic response over a broad range of volume fractions up to the very dense glassy regime in proximity to random close packing. We focus on the LVE spectra and times shorter than those corresponding to the dynamic shear modulus G′ plateau, where the system approaches transient localization and cage confinement. At these short times (higher frequencies), a dynamic cage has not yet fully developed and particles are not (strictly) transiently localized. This corresponds to an effective solid-to-liquid transition in the LVE spectrum (dynamic moduli) marked by a high frequency (HF) crossover. On the other hand, as the volume fraction increases caging becomes tighter, particles become more localized, and the onset of the localization time scale becomes shorter. This onset of transient localization to shorter times shifts the HF crossover to higher values. Therefore, the study of the dependence of the HF crossover properties (frequency and moduli) on volume fractions provides direct insights concerning the onset of particle in-cage motion and allows direct comparison with current theoretical models. We compare the experimental data with predictions of a microscopic statistical mechanical theory where qualitative and quantitative agreements are found. Findings include the discovery of microscopic mechanisms for the crossover between the two exponential dependences of the onset of the localization time scale and the elastic shear modulus at high volume fractions as a consequence of emergent many body structural correlations and their consequences on dynamic constraints. Moreover, an analytic derivation of the relationship between the high frequency localized short-time scale and the elastic shear modulus is provided which offers new physical insights and explains why these two variables are experimentally observed to exhibit nearly-identical behaviors.

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Nanofilament-Coated Membranes with Enhanced Scaling and Biofouling Resistance for Membrane Distillation

Mariana D. Sosa, Ivana K. Levy, Hans-Jürgen Butt, Michael Kappl

https://doi.org/10.1021/acsami.5c01758

Membrane distillation (MD) for water treatment can be applied in high salinity conditions and for treatment of wastewater. Current commercial membranes are made of fluorinated polymers such as polytetrafluoroethylene (PTFE). Here, porous membranes were coated with a silicone nanofilament layer to obtain a superhydrophobic and fluorine-free material. The classical coating procedure involves the use of toluene as a solvent. In this work, n-heptane was tested as a less toxic alternative. Different porous membranes were tested as the substrates of the nanofilament coating. The effect of acids, scaling solutions, and biofilm formation was analyzed in comparison to standard PTFE membranes. We demonstrate that superhydrophobic nanofilament-coated poly(ether sulfone) membranes (NF-PES) possess the required antiwetting properties for MD. Moreover, NF-PES membranes have static contact angles between 10 and 20° higher than PTFE standard membranes after immersion tests in solutions containing scaling substances, and biofilm grows from 20 to 50%, less in NF-PES than in PTFE

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How Preparation Protocols Control the Rheology of Organoclay Gels

Nikolaos A. Burger, Benoit Loppinet, Andrew Clarke, George Petekidis

https://doi.org/10.1021/acs.langmuir.5c02052

We elucidate the effect of preparation conditions on the rheological properties of organophilic clays consisting of platelet-like primary particles, VG69 (trademark of SLB) dispersed in oil, by varying the homogenization rate, homogenization temperature, and amount of added water. We establish that stable, nonsedimenting gel formation requires homogenization temperatures higher than 45 °C and the addition of a small amount of water during the homogenization stage. Dried organoclay dispersions, on the other hand, do not form stable gels, independent of the homogenization rate and temperature, suggesting the existence of only weak attractions in the absence of water molecules. Water-induced attraction is necessary to form gels, probably through hydrogen bonding between the silanol group of clay particles and water molecules. Moreover, the effect of homogenization temperature is related to the extent of exfoliation during the homogenization stage as confirmed by X-ray scattering. The gel plateau modulus, Gp, is found to increase with clay concentration as GP ∼ cclay3.9, typical of fractal gel networks. More interestingly, a linear increase in the elastic modulus with water concentration is observed over a wide range of water concentrations, while analyzing the effective yield strain deduced from the yield stress and elastic modulus reveals the existence of three regimes. We finally present dynamic state diagrams that clearly indicate the required conditions for the creation of stable gels and demonstrate the importance of controlling the preparation protocols in the formulation of clay dispersions and gels with desirable structural and mechanical properties.

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Polymer Thermophoresis by Mesoscale Simulations

Lisa Sappl, Christos N. Likos and Andreas Zöttl

https://doi.org/10.1021/acs.macromol.4c01656

We employ mesoscopic simulations to study the thermophoretic motion of polymers in a solvent via multiparticle collision dynamics (MPCD). As the usual solvent–monomer collision rules employed in MPCD involving polymers fail to cause thermophoresis, we extend the technique by introducing explicit solvent–monomer interactions, while the solvent molecules remain ideal with respect to one another. We find that with purely repulsive polymer–solvent interaction, the polymer exhibits thermophilic behavior, whereas to display thermophobic behavior, the polymer–solvent potential requires the presence of attractions between solvent particles and monomers, in accordance with previous experimental findings. In addition, we observe that the thermophoretic mobility is independent of polymer length in the observed regime, again in agreement with experiments. Finally, we investigate the thermophoretic behavior of block copolymers, demonstrating that the thermophoretic mobility can be obtained by linear interpolation, weighted by the relative lengths of the two blocks.

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Tuning the mechanical properties of organophilic clay dispersions: particle composition and preshear history effects

Nikolaos A. Burger, Benoit Loppinet, Andrew Clarke and George Petekidis

 https://doi.org/10.1122/8.0000854

Clay minerals are abundant natural materials used widely in coatings, construction materials, ceramics, as well as being a component of drilling fluids. Here, we present the effect of steady and oscillatory preshear on organophilic modified clay gels in synthetic oil. Both platelet and needle-like particles are used as viscosifiers in drilling fluid formulations. For both particles the plateau modulus exhibits a similar concentration dependence, , whereas the yield strain is for the platelets and  for the needles. Mixtures of the two follow an intermediate behavior: at low concentrations their elasticity and yield strain follows that of needle particles while at higher concentrations it exhibits a weaker power law dependence. Furthermore, upon varying the preshear history, the gel viscoelastic properties can be significantly tuned. At lower (higher) clay concentrations, preshear at specific oscillatory strain amplitudes or steady shear rates, may induce a hardening (softening) of the dispersions and, at all concentrations, a lowering of the shear strain. Hence, in needle dispersions preshear resulted in changes in the volume fraction dependence of the elastic modulus from to and of the yield strain from to. However, small angle X-ray scattering showed not much structural changes, within the q-range covered. Our findings indicate ways to design colloidal organoclay dispersions with a mechanical response that can be tuned at will.

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High-frequency Optimally Windowed Chirp rheometry for rapidly evolving viscoelastic materials: application to a crosslinking thermoset

Thanasis Athanasiou, Michela Geri, Patrice Roose, Gareth H. McKinley and George Petekidis

https://doi.org/10.1122/8.0000793

Knowledge of the evolution of mechanical properties of the curing matrix is of great importance in composite parts or structure fabrication. Conventional rheometry, based on small amplitude oscillatory shear is limited by long interrogation times. In rapidly evolving materials, time sweeps can provide a meaningful measurement albeit at a single frequency. To overcome this constraint we utilize a combined frequency and amplitude-modulated chirped strain waveform in conjunction with a home-made sliding plate piezo-operated (PZR) and a dual-head commercial rotational rheometer (Anton Paar MCR 702) to probe the linear viscoelasticity of these time-evolving materials. The direct controllability of the PZR resulting from the absence of any kind of firmware and the microsecond actuator-sensor response renders this device ideal for exploring the advantages of this technique. The high frequency capability allows us to extend the upper limits of the accessible linear viscoelastic spectrum and most importantly, to shorten the length of the interrogating strain signal (OWCh-PZR) to sub-second scales, while retaining a high time-bandwidth product. This short duration ensures that the mutation number (NMu) is kept sufficiently low, even in fast curing resins. The method is validated via calibration tests in both instruments and the corresponding limitations are discussed. As a proof of concept the technique is applied to a curing vinylester resin. The linear viscoelastic (LVE) spectrum is assessed every 20 seconds to monitor the rapid evolution of the time- and frequency-dependence of the complex modulus. Finally, FTIR spectroscopy is utilized to gain insights on the evolution of the chemical network while the gap-dependence of the evolving material properties in these heterogeneous systems is also investigated.

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