Interfacial Shear Rheology Research Articles

Peptide‐Guided Self‐Assembly: Fabrication of Tailored Spiral‐Like Nanostructures for Precise Inorganic Templating

AbstractSelf‐assembling peptides (SAPs) are versatile building blocks in the creation of hierarchical nanostructures. Although SAPs promise precise control over assembled morphology and dimensions, experimental validation is still crucial. In this sense, recent efforts have focused on nanospiral structures, which mimic natural architectures and hold potential for biomedical and nanotechnological applications. Here, it is demonstrated that SARS‐CoV‐2 fusion peptides are able to form specific spiral‐like structures using interfacial assembly as a fabrication methodology, along with fine‐tuning of the spiral ensembles through an imposed interfacial flow due to the uniaxial compression in a Langmuir–Blodgett trough. Molecular characterization by atomic force microscopy, neutron reflectometry, interfacial shear rheology, and infrared spectroscopy, highlighted how variations in fusion peptide sequences influence assembly, highlighting the importance of molecular design. These spiral structures are subsequently modified to serve as templates for metallic replicas, expanding the potential of peptide‐guided self‐assembly in fabricating tailored nanostructured surfaces with sub‐10 nm dimensions over cm2 areas.

PH-induced conformational changes of lupin protein-pectin mixtures and its effect on air-water interfacial properties and foaming functionality

Lupin protein isolate (LPI) has high nutritional value and good foaming properties around neutral pH; however, its functionality becomes poor at acidic pH, due to reduced protein solubility. The addition of pectin to LPI can increase its solubility at acidic pH and hence improve protein functionality. Here, we investigated the air-water interfacial and foaming properties of LPI-pectin (1:1) mixtures at pH 3.5–7.0. We used interfacial shear and dilatational rheology, characterized the air-water interfacial microstructure with AFM of Langmuir-Blodgett films, and linked the results to the foaming properties of the LPI-pectin mixtures. Based on the phase diagram, LPI and pectin formed co-soluble mixtures at pH 6.0 and 7.0, while LPI-pectin electrostatic complexes were formed at pH 3.5 and 4.0. In the co-soluble mixtures, proteins diffused faster towards the air-water interface than the electrostatic complexes, due to smaller particle sizes of the proteins. Their air-water interfaces showed distinct differences with respect to microstructure and mechanical properties. The interfaces stabilized by co-soluble mixtures were dominated by protein aggregates, leading to weaker interfaces in response to shear and dilatational deformation, while the complexes formed thicker and denser polymeric air-water interfaces that were stiffer and more solid-like. As a result, the complex-stabilized foams were more stable than those stabilized with co-soluble mixtures. Findings from this study indicate that soluble LPI-pectin complexes formed at pH 3.5 and 4.0 were more efficient in improving interfacial and foaming properties of LPI than the co-soluble mixtures at pH 6.0 and 7.0, which can be used to tailor the properties of acid aerated products stabilized by LPI.

(Non)linear Interfacial Rheology of Tween, Brij and Span Stabilized Oil-Water Interfaces: Impact of the Molecular Structure of the Surfactant on the Interfacial Layer Stability.

During emulsification and further processing (e.g., pasteurizing), the oil-water interface is mechanically and thermally stressed, which can lead to oil droplet aggregation and coalescence, depending on the interfacial properties. Currently, there is a lack of insights into the impact of the molecular structure (headgroup and FA chain) of low molecular weight emulsifiers (LME) on the resulting interfacial properties. Additionally, the crystallization/melting of the oil/the emulsifier is often neglected within interfacial rheological experiments. Within this study, the stability of interfaces formed by Tween, Span or Brij was determined as a function of their molecular structure, taking crystallization effects of the LME into account. The headgroup was kept constant while varying the FA, or vice versa. The interfacial film properties (viscoelasticity) were investigated at different temperatures using dilatational and interfacial shear rheology. Both the headgroup and the FA chain impacted the interfacial properties. For the same FA composition, a rather small hydrophobic headgroup resulted in a higher packed interface. The interfacial elasticity increased with increased FA chain length (C12 to C18). This seemed to be particularly the case when the emulsifier crystallized on the interface among cooling. In the case of a densely packed interface, network formation due to chain crystallization of the LME's FA chains occurs during the cooling step. The resulting interface shows predominantly elastic behavior.

Application of high-pressure homogenization-assisted pH-shift to enhance techno-functional and interfacial properties of lentil protein isolate

High-pressure homogenization (HPH) is a promising physical non-thermal approach to improve protein techno-functionality. This study aims to examine the effects of HPH on the lentil proteins through the perspective of the interfacial adsorption mechanism. The impact of HPH treatment on lentil protein isolate (LPI) at varying pressure levels (0–150 MPa) was determined using several analytical techniques, including SDS-PAGE, FTIR, solubility, and techno-functional properties (foaming and emulsifying properties), alongside analyses of interfacial tension and interfacial shear rheology at the o/w and a/w interfaces for two pH values (2.0 and 4.5). Results reveal that HPH treatment up to 100 MPa effectively unfolds lentil proteins by disrupting disulfide-bonded subunits into lower molecular weight fractions and unfolding highly-ordered secondary structures into random coils. LPI's capacity to produce emulsions and foams was found to be enhanced concurrently with these physicochemical changes, particularly at pressures up to 50 MPa. The findings aligned with the interfacial tension and shear rheology analyses, which show that proteins can form interfacial viscoelastic films on both o/w and a/w interfaces. Furthermore, the interfacial behavior of LPI and the effect of HPH on the interfacial behavior were found to be pH-dependent. The lower interfacial tension and the higher interfacial viscoelastic moduli (G′ and G″) were recorded at 50 MPa and 0 MPa at pH 2.0 and 4.5, respectively. These results stated that the effects of the HPH on the technofunctionality of LPI can be further enlightened by investigating the interfacial adsorption kinetics.

Goose liver protein emulsion with enhanced interfacial stabilization by facile core-shell curcumin complexation

The role of facile curcumin dispersion and its hydrophobic complexation onto GLP, in the form of shell (GLPC-E), core (GLPE-C) and with synergy (GLP-ECE), on the protein interfacial and emulsion stabilization was investigated. Turbiscan instability index, microrheological elasticity, viscosity and solid-liquid balance values showed that the O/W emulsion stability was in the order of GLP-E < GLPC-E < GLPE-C < GLP-ECE. GLP-ECE also gave the most reduced D [4, 3] (8.11 ± 0.14 μm) with lowest indexes of flocculation (2.80 ± 0.05 %) and coalescence (2.83 ± 0.10 %) at day 5. Interfacial shear rheology suggested the GLP-curcumin complexation fortified the GLP interfacial gelling and then the efficiency as steric stabilizer, especially of core-shell complexation (14.2 mN/m) that showed the most sufficient in-plane protein interaction against strain. Dilatational elasticity and desorption observation revealed the synergistic curcumin complexation facilitated GLP unfolding and macromolecular association at O/W interface, as was also verified from SEM image and surface hydrophobicity (from 36.23 to 76.04). Overall, this study firstly reported the facile curcumin bi-physic dispersion and GLP complexation in improving the emulsion stabilizing efficiency of the protein by advancing its interfacial stabilization.

Interfacial mechanics of β-casein and albumin mixed protein assemblies at liquid-liquid interfaces

Protein emulsifiers play an important role in formulation science, from food product development to emerging applications in biotechnologies. The impact of mixed protein assemblies on surface composition and interfacial shear mechanics remains broadly unexplored, in comparison to the impact that formulation has on dilatational mechanics and surface tension or pressure. In this report, we use interfacial shear rheology to quantify the evolution of interfacial shear moduli as a function of composition in bovine serum albumin (BSA)/β-casein mixed assemblies. We present the pronounced difference in mechanics of these two protein, at oil interfaces, and observe the dominance of β-casein in regulating interfacial shear mechanics. This observation correlates well with the strong asymmetry of adsorption of these two proteins, characterised by fluorescence microscopy. Using neutron reflectometry and fluorescence recovery after photobleaching, we examine the architecture of corresponding protein assemblies and their surface diffusion, providing evidence for distinct morphologies, but surprisingly comparable diffusion profiles. Finally, we explore the impact of crosslinking and sequential protein adsorption on the interfacial shear mechanics of corresponding assemblies. Overall, this work indicates that, despite comparable surface densities, BSA and β-casein assemblies at liquid–liquid interfaces display almost 2 orders of magnitude difference in interfacial shear storage modulus and markedly different viscoelastic profiles. In addition, co-adsorption and sequential adsorption processes are found to further modulate interfacial shear mechanics. Beyond formulation science, the understanding of complex mixed protein assemblies and mechanics may have implications for the stability of emulsions and may underpin changes in the mechanical strength of corresponding interfaces, for example in tissue culture or in physiological conditions.

Interfacial Shear Rheology of the Waxy Oil-Water Interfacial Layer Construction by Span80/60/65: Influence of Hydrophobic Chain Structure

Interfacial Shear Rheology of the Waxy Oil-Water Interfacial Layer Construction by Span80/60/65: Influence of Hydrophobic Chain Structure

Type of pectin affects the functionality of potato protein-pectin conjugates in emulsions

This study investigated the effect of the molecular structure of pectin on conjugate formation and its functionality in plant protein-based emulsions. For this purpose, high-methoxylated, low-methoxylated, amidated citrus pectin and their mixtures with potato protein were vacuum-dried at defined process conditions. The change in free amino groups, colour and molecular weight distribution assessed conjugation. The side reaction, such as pectin degradation, was monitored by the degree of methoxylation, galacturonic acid content and intrinsic viscosity. In addition, FT-IR and hydrophobicity measurements were performed to determine the changes in chemical structure due to the occurring reactions. The ζ-potential, solubility and oil droplet size reflected the functionality of the conjugated samples.The results confirm that, apart from the conjugation of protein and pectin, where the most significant decrease in free amino groups was obtained with an increasing degree of methoxylation or presence of amide groups, undesirable degradation of pectin occurring, resulting in a broad molecular weight distribution of the conjugate samples. The conjugate formation was the highest with amidated pectin. The FT-IR measurements confirmed the formation of Maillard conjugates. The FT-IR spectra showed a decrease in the intensity signals of amide I and amide II and the formation of new C–N bond maxima. In addition, the conjugation caused the shift of the protein's isoelectric point to the acidic environment, resulting in higher solubility and improved emulsion stability at the isoelectric point of the protein.Regarding emulsion stability, the conjugates with amidated pectin performed best and achieved the narrowest oil droplet size distribution over a wider pH range. Emulsion-stabilising properties of conjugates with high- and low-methoxylated pectin differed only slightly. To better understand the underlying mechanisms for emulsion stabilisation, future research will focus on the interfacial properties of the conjugates as determined by interfacial shear rheology and pendant drop analysis.

Interfacial rheology of polyelectrolyte microgel monolayers: Correlation between mechanical properties and phase behavior at oil-water interfaces

Microgels are confined to fluid interfaces in numerous applications, yet many aspects of the microgel-covered interface remain unclear. In this work, we use interfacial shear and dilatational rheology to study the effects of electrostatics on the mechanical characteristics of polyelectrolyte microgel monolayers at oil-water interfaces as a function of the microgel interfacial concentration. We find a clear correlation between the influence of charges on the mechanical properties of the monolayers and the influence of charges on their two-dimensional phase behavior. At lower microgel interfacial concentrations, the moduli of uncharged monolayers are larger than those of charged monolayers. Consistent with our previous findings on the phase behavior, here, the mechanical response of the interfacial layer is controlled by in-plane interactions of the microgels within the interface. At higher microgel interfacial concentrations, the moduli of charged monolayers are larger than those of uncharged monolayers. The mechanical response becomes dominated by out-of-plane interactions between the fractions of the adsorbed microgels further from the interface. Evidently, electrostatic interactions do not contribute directly to the mechanical response of the interfacial layer, that is, through charge repulsion, but indirectly through the difference in the swelling properties of uncharged and charged microgels. These results advance our understanding of how the charge-dependent microstructure of the interfacial layer affects its mechanical properties, which is not only important from a fundamental point of view but is also relevant to applications where polyelectrolyte microgels are used as responsive emulsion stabilizers.

Interfacial rheological properties of pepsin-hydrolyzed lentil protein isolate at oil-water interfaces

Lentil protein isolate (LPI) was investigated for its potential as a plant protein-based emulsifier. LPI consists mainly of globulins with high molecular weights which are relatively slow to adsorb and stabilize oil-water interfaces. Smaller proteins, such as whey protein, can stabilize the interface much faster, quickly forming a viscoelastic film that slows down the rate of droplet recoalescence, leading to smaller droplets. To increase the rate of adsorption, LPI was enzymatically hydrolyzed by pepsin at 1.5% and 4.5% degree of hydrolysis (DH), to obtain small peptides. The behavior of these peptides was studied at the oil-water interface in comparison to whey protein isolate (WPI) and native lentil protein isolate (LPI). Interfacial properties were investigated using dilatational and interfacial shear rheology in the linear viscoelastic regime (LVE) and nonlinear viscoelastic regime (NLVE) and these were related to protein characteristics and emulsion formation. The 1.5% and 4.5%DH LPI showed high surface hydrophobicity, and consisted mainly of low molecular weight peptides, which contributed to faster adsorption kinetics and consequently covered the interface faster than LPI. In dilatation, the native and hydrolyzed LPI-stabilized interfaces had comparable stiffness and were clearly weaker than WPI-stabilized interfaces, which contrasts with the results in shear deformations, where the former both had higher stiffness than WPI. Both hydrolyzed LPI and WPI formed more brittle and less stretchable interfaces compared to native LPI in both dilatational and shear rheology. In emulsification tests, native LPI produces the largest droplets, and the 4.5% DH LPI and WPI have the smallest droplet size. But, bridging flocculation was observed when emulsions were prepared with both hydrolyzed LPI, which may be attributed to inter-droplet hydrophobic interactions between small polypeptides. This can be linked to an increase in the surface hydrophobicity of hydrolyzed LPI. Thus, the use of native LPI as a stabilizer in emulsion preparation is preferable over the hydrolyzed samples, despite its larger droplet size.

Interfacial shear and elongational rheology of immiscible multi-micro-nanolayered polymers: contribution for probing the effect of highly mismatched viscoelastic properties and modeling interfacial tension properties

Interfacial shear and elongational rheology of immiscible multi-micro-nanolayered polymers: contribution for probing the effect of highly mismatched viscoelastic properties and modeling interfacial tension properties

Effect of surfactants on the interfacial viscoelasticity and stability of silk fibroin at different oil-water interfaces.

The biocompatible amphiphilic silk fibroin, extracted from domesticated silkworms, can adsorb at the oil-water interface and form elastic interfacial layers. In this study, three surfactants (cationic cetyltrimethylammonium bromide, the nonionic polyoxyethylene sorbitan monolaurate, and the anionic sodium dodecyl sulfate) were selected to investigate, through interfacial shear rheology, the influences of surfactants on the interfacial viscoelasticity and stability of silk fibroin at the interfaces between water and two different oils. The presence of surfactant prolongs the equilibration time and enhances the interfacial elastic modulus and toughness of the interfacial silk fibroin layers, especially at the nonpolar dodecane-water interface. However, when the surfactant exceeds a critical concentration, the shear modulus and stability of interfacial silk fibroin layers begin to decrease due to the competitive adsorption of surfactant molecules and the weakening of the protein network. Owing to electrostatic interactions, the ionic surfactants cetyltrimethylammonium bromide and sodium dodecyl sulfate can form more hydrophobic complexes with silk fibroin, which results in higher shear moduli than for silk fibroin and silk fibroin-polyoxyethylene sorbitan monolaurate mixture. Both the surfactant type and oil polarity play important roles in the adsorption, shear viscoelasticity, and stability of silk fibroin at the oil-water interface. Enhanced interactions between a silk fibroin-surfactant mixture and the oil phase delay the equilibration of the adsorption layers but strengthen the stability of interfacial layers. © 2023 Society of Chemical Industry.

Incorporating soy protein hydrolysate and temperature-induced gelling polysaccharide as partial egg replacements for enhanced texture in sponge cake

This study investigated the influence of hydroxypropyl methylcelluloses (HPMCs), temperature-induced gelling polysaccharides, on the air-water interfacial properties and foaming properties of four soy proteins. Two HPMCs with different relative molecular masses (F50, lower; K4M, higher) were combined with soy protein isolate (SPI), 7S fraction (7S), selective hydrolysate by pepsin (SPSH), and limited hydrolysate by papain (SPLH). Interfacial shear rheology results showed that SPSH exhibited the highest G′ value, indicating a multi-layer adsorption structure formed due to both 7S and low molecule peptides in SPSH. Adding F50 to SPI, 7S, and SPSH improved foam expansion and stability while adding K4M negatively impacted the foam expansion of SPI, 7S, and SPSH but enhanced the foam expansion of SPLH. These findings suggest that the interfacial interaction patterns between HPMCs and soy proteins are significantly influenced by the molecular weight of HPMCs and the structure of the protein. Incorporating F50 into SPSH decreased the gelation temperature, shortened the formation time of a solid-like layer at the air-water interface, and expanded the linear viscoelastic region during structural fraction testing. Consequently, the combination of F50 and SPSH was chosen as the viable alternative to egg protein in sponge cake, as it facilitated rapid foam stability and prevented collapse during baking. The resulting sponge cake, with half of the egg replaced by the SPSH/F50 mix, exhibited a similar appearance and texture to that with only the egg added. Thus, combining temperature-induced gelling polysaccharides with SPSH offers a promising alternative to egg protein in aerated bakery food production.

Interfacial rheology of linearly growing polyelectrolyte multilayers at the water-air interface: from liquid to solid viscoelasticity.

Despite the long history of investigations of polyelectrolyte multilayer formation on solid or liquid surfaces, important questions remain open concerning the construction of the first set of layers. These are generally deposited on a first anchoring layer of different chemistry, influencing their construction and properties. We propose here an in-depth investigation of the formation of NaPSS/PAH multilayers at the air/water interface in the absence of a chemically different anchoring layer, profiting from the surface activity of NaPSS. To analyse the mechanical properties of the different layers, we combine recently established analysis techniques of an inflating/deflating bubble exploiting simultaneous shape and pressure measurement: bubble shape elastometry, general stress decomposition and capillary meniscus dynanometry. We complement these measurements by interfacial shear rheology. The obtained results allow us to confirm, first of all, the strength of the aforementioned techniques to characterize complex interfaces with non-linear viscoelastic properties. Furthermore, their sensitivity allows us to show that the multilayer properties are highly sensitive to the temporal and mechanical conditions under which they are constructed and manipulated. We nevertheless identify a robust trend showing a clear transition from a liquid-like viscoelastic membrane to a solid-like viscoelastic membrane after the deposition of 5 layers. We interpret this as the number of layers required to create a fully connected multilayer, which is consistent with previous results obtained on solid or liquid interfaces.

Enhanced foamability and stability of aqueous foams through novel pickering fat globules formulated with solid lipid particles

Pickering fat globules (PFGs), known for their remarkable stability via the Pickering effect, eco-friendliness, and ability to carry hydrophobic flavors and nutrients, play a pivotal role in the advancement of food foam systems. This study investigates highly unsaturated and exceptionally stable aqueous foams stabilized by PFGs with interfacial activity. These PFGs are derived from two types of solid lipid particles (SLPs): fully hydrogenated soybean oil with sodium caseinate (FHSO-SC-SLPs-PFGs) and glyceryl stearate citrate (GSC-SLPs-PFGs), which were formulated with varying SLPs concentrations (0.5 wt%, 2.5 wt% and 5 wt%) while coating 10 wt% and 20 wt% oil droplets. Remarkably, 0.5 wt% SLPs could accommodate up to 20 wt% of low-melting-point liquid oil, enhancing the unsaturation of PFGs. GSC-SLPs-PFGs generated foams with overrun values reaching up to 235%, exhibiting stability for at least 40 days. In contrast, foams derived from FHSO-SC-SLPs-PFGs, with the highest overrun value of 145%, remained stable for only 2 h. Compared with FHSO-SC-SLPs-PFGs, GSC-SLPs-PFGs exhibited superior foaming performance and stability attributed to their properties such as smaller particle size, enhanced hydrophilicity, higher apparent viscosity, increased surface potential, and superior surface tension reduction. Furthermore, increasing the concentration of SLPs and/or reducing the oil phase can also improve the properties of SLPs-PFGs, resulting in greater foamability and foam stability. Surface dynamic adsorption, interfacial shear rheology, and microstructural observations revealed robust adsorption of all SLPs-PFGs at the air-liquid interface, indicative of pronounced Pickering stabilization, with GSC-SLPs-PFGs also exhibited three-dimensional network structure. These SLPs-PFGs stabilized foams presented heightened stability and reduced saturated fat content, paving the way for novel and healthier foam-based food products.

Role of the pea protein aggregation state on their interfacial properties

HypothesisPlant protein ingredients from similar sources can vary in functionality not only because of compositional differences, but also because of differences in their structure depending on their processing history. It is essential to understand these distinctions to develop novel food emulsion using plant proteins. It is hypothesized that differing interfacial properties can be attributed to their structures, aggregation, and colloidal states. ExperimentsThe adsorption behavior of a commercial protein isolate, homogenized or non-homogenized, was compared to a mildly extracted isolate to evaluate the effect of aggregation state and structural differences. After characterization of the particle size and protein composition, the interfacial properties were compared. FindingsAtomic force microscopy provided evidence of interfaces packed with protein oligomers regardless of the treatment. Differences in adsorption kinetics and interfacial shear rheology depending on oil polarity suggested different interfacial structures. A polydisperse mixture of protein oligomers resulted in increased rearrangements and protein-protein interactions at the interface. Homogenization of commercial proteins resulted in a lower interfacial tension and less elastic interfaces compared to those of native proteins due to the presence of larger aggregates. This study highlights how the interfacial properties can be related to the protein aggregation state resulting from differences in processing history.

Air-water interface properties and foam stabilization by mildly extracted lentil protein

Lentil proteins have shown potential as substitutes for animal proteins in foam-based food products, although the mechanisms by which the proteins stabilize the air-water interface are largely unexplored. Here, we systematically studied the air-water interfacial behavior (including adsorption kinetics, and interfacial dilatational and shear rheology), interfacial structures and foaming properties of mildly extracted lentil protein isolate (LPI). We compared these properties of LPI with two animal-based proteins commonly used in food industry, i.e., dairy-derived whey protein isolate (WPI) and egg-derived egg white protein isolate (EPI). The mildly extracted LPI had high protein content (85.2%), high protein solubility (91%) and high nativity, with a globulin-to-albumin ratio of 81:19. LPI adsorbed fast at the air-water interface, showing a foam overrun of 285% that is comparable with WPI and EPI. Although LPI formed interfaces with lower stiffness than EPI, it showed comparable foam half-life time (115 min) with EPI, due to the higher thickness of the interface formed by LPI, which is dominated by lentil globular protein particles with a size around 11 nm. The lentil globular proteins appear to have weaker in-plane interactions than smaller WPI molecules at the air-water interface, resulting in a pronouncedly lower interfacial stiffness and foam half-life time of LPI compared to WPI (295 min). However, the LPI-formed interface is more stretchable than the WPI-formed interface and shows less disruption in both large dilatational and shear deformations. Thus, LPI might have comparable or better performance than WPI in stabilizing foam under processing conditions. Overall, lentil proteins have good foaming properties and could be applied as an animal-based protein replacer in producing foam-based food products.

Apparent Failures in Interpretation of Interfacial Characterization When Formulating Emulsions Stabilized by Cellulose Nanocrystals.

Cellulose nanocrystals (CNCs) are sustainable particles that are effective at stabilizing emulsions by adsorbing at droplet interfaces and providing a steric barrier to coalescence. However, CNCs have surface charges that reduce the coverage of the emulsion droplets due to the electrostatic repulsion between CNCs. In such cases, adding salt is a typical (and straightforward) way to adjust the formulation so that the charges are screened, allowing increased coverage of the droplets. At the outset of this work, we hypothesized that characterization of the interfacial tension and interfacial shear rheology of the oil-water interface would be correlated to interfacial coverage and therefore predictive of the optimal salt concentration for emulsion stability. Included in the methods section as a useful reference to others is the presentation of a detailed derivation for the equations needed to compute interfacial shear moduli in a custom, double-gap geometry. In contrast to our hypothesis, we found that interfacial tension did not correlate well with emulsion stability and that the native surface-active compounds in corn oil overwhelmed any influence of the CNCs on the interfacial tension. Additionally, we found that interfacial shear rheology (which can be painstakingly difficult to measure) was not a useful tool for formulating these emulsions. This is because at commonly used concentrations of CNCs, the bulk rheology is increased to a much greater degree than that of the interface, making the details of the interfacial rheology unimportant. Finally, we found that at concentrations of CNCs that are typical in industrial processes, characterizing the bulk viscoelastic properties of the aqueous suspending phase without added oil (a relatively simple measurement) is sufficient to predict the influence of NaCl concentration on charge screening between the CNCs and, by extension, increased surface coverage of droplets for greater emulsion stability.

Interfacial shear rheology of glassy polymers at liquid interfaces

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.

Antibodies Adsorbed to the Air-Water Interface Form Soft Glasses.

When monoclonal antibodies are exposed to an air-water interface, they form aggregates, which negatively impacts their performance. Until now, the detection and characterization of interfacial aggregation have been difficult. Here, we exploit the mechanical response imparted by interfacial adsorption by measuring the interfacial shear rheology of a model antibody, anti-streptavidin immunoglobulin-1 (AS-IgG1), at the air-water interface. Strong viscoelastic layers of AS-IgG1 form when the protein is adsorbed from the bulk solution. Creep experiments correlate the compliance of the interfacial protein layer with the subphase solution pH and bulk concentration. These, along with oscillatory strain amplitude and frequency sweeps, show that the viscoelastic behavior of the adsorbed layers is that of a soft glass with interfacial shear moduli on the order of 10-3 Pa m. Shifting the creep compliance curves under different applied stresses forms master curves consistent with stress-time superposition of soft interfacial glasses. The interfacial rheology results are discussed in the context of the interface-mediated aggregation of AS-IgG1.