Surface Rheology Research Articles

Surfactant monolayers at oil–water interfaces. Behavior upon compression and relation to emulsion stability

AbstractSurfactant monolayers modify the surface tension of fluid surfaces, and they confer to the surfaces a resistance to mechanical perturbances, such as compression or dilatation. The resistance can be characterized by elastic and viscous moduli, that affect the phenomena involving the motion of liquid surfaces such as wetting, two‐phase flow, foaming and emulsification, rheology and stability of foams and emulsions. Some of these phenomena, for instance coalescence of bubbles or drops, are extremely rapid, and although coalescence seems correlated with the elastic compression modulus, no quantitative comparison has been made. Indeed, this modulus is frequency dependent, and cannot be measured by most conventional devices at the short timescales during which coalescence events occur. In this paper, we first review the current understanding on foam and emulsion stability, highlighting the role of surface rheology in the stability of these systems, important for their formulation. We present tension measurements for various surfactant systems, nonionic and cationic, at oil–water interfaces. We show calculations of the high frequency elastic modulus using the Gibbs adsorption equation and compare the results to previous measurements at air–water interfaces. The moduli measured imposing a rapid expansion of the monolayers are consistently smaller, in particular at oil–water interfaces. We discuss the possible origin of the differences between the two types of interfaces.

Exploring the influence of acetylenic acetogenin unsaturation patterns on lipid interface interactions: Insights from surface chemistry, rheology, and spectroscopy

This study explores how the unsaturation patterns of three acetogenins from Porcelia macrocarpa (2S,3R,4R)-3-hydroxy-4-methyl-2-(eicos-11′-yn-19′-enyl)-butanolide (1), (2S,3R,4R)-3-hydroxy-4-methyl-2-(eicos-11′-ynyl)-butanolide (2), and (2S,3R,4R)-3-hydroxy-4-methyl-2-eicosyl-butanolide (3)—affect their interactions with lipid interfaces, using Langmuir monolayers of dipalmitoyl-phosphoethanolamine (DPPE) as a model for protozoal membranes. The hypothesis posits that unsaturated acetogenins (1 and 2) will exhibit distinct behaviors compared to the saturated acetogenin (3) due to structural differences. Experiments incorporated each acetogenin into DPPE monolayers, employing techniques like tensiometry, dilatational rheology, infrared spectroscopy, and Brewster angle microscopy (BAM) to assess interactions and changes in film properties. Results showed that unsaturated acetogenin 1, active against Leishmania infantum and Trypanosoma cruzi, destabilized the film and induced rapid molecular reorganization, while acetogenin 2, inactive against these parasites, uniquely increased the viscous contribution to viscoelasticity. Saturated acetogenin 3 did not integrate into the film. All acetogenins enhanced fluidity, reducing elasticity and viscosity, with BAM revealing phase separation only in acetogenin 2. Infrared spectroscopy underscored the impact of drug-lipid interactions, leading to the conclusion that unsaturated acetogenins 1 and 2 have a more significant influence on thermodynamic and structural properties than acetogenin 3, offering insights into their potential therapeutic mechanisms.

Cardamom Pickering emulsions stabilized by cellulose nanostructures as disinfection agents against bacteria and SARS-CoV-2

This study introduces a new disinfectant agent utilizing cardamom essential oil stabilized by cellulose nanomaterials (CN). The Pickering emulsion technique prevented oil degradation and extended its storage duration. Cellulose nanocrystals (CNC) or cellulose nanofibers (CNF) were utilized as stabilizers, and the emulsions were evaluated for surface tension, rheology, morphology, antimicrobial and antiviral activity against S. enterica, S. aureus, P. aeruginosa, E. coli, and SARS-CoV-2. Surface tension measurements indicated an electrostatic stabilization mechanism in CNC emulsions, exhibiting fluid behavior confirmed by rheological tests. On the other hand, CNF emulsions displayed drop packaging, characteristic of shear-thinning behavior, as confirmed by the Ostwald-de-Waele model. The emulsions exhibited antibacterial activity against S. aureus and P. aeruginosa, particularly the CNC samples, suggesting that nanocellulose alters the regulation of oil migration into the medium. Testing against SARS-CoV-2 revealed that cardamom essential oil required 30 min of contact for viral protein denaturation, while CNC and CNF emulsions took 60 and 50 min, respectively. This indicates that CN acts as an encapsulating agent, prolonging the oil’s release time and necessitating longer contact periods with the virus. These findings demonstrate the potential of these emulsions in developing new antimicrobial and antiviral products.

(Invited) Zwitter Ion-Promoted Hybrid Sepiolite Membranes for Energy Applications

Functionalized natural clays exhibit significant potential as electrode materials, solid-state electrolytes and separators in energy storage and conversion devices. Features that evoke interest in clay minerals are their (1) porous structures and adsorption properties, (2) tunable surface areas, (3) remarkable thermal and mechanical stabilities, (4) abundant natural reserves, and (5) cost. By tweaking such features along with functionalization with desired molecules, they can be used as an ideal candidate for solid-state electrolyte and battery separators. Naturally occurring clays possess layered silicate structures with interchangeable, intercalated ions and tunable chemical properties. Among them, sepiolite, which is a hydrated magnesium silicate, is one of the most appealing clays having a high-aspect ratio. The blocks and channels in sepiolite with discontinuity in the external silica sheet expose a significant number of surface-silanol groups directly accessible to functionalize with organic and inorganic species. Moreover, its exceptionally high surface area and high cation exchange capacity (CEC) compared to other clays enables easy functionalization, thereby imparting exceptional mechanical durability and material properties that can be used for myriads of applications.In this work a sustainable, cost-effective, environmentally benign, carbon-neutral, and bio-friendly sepiolite clay composites have been developed using betaine and ionic liquid without the use of polymers, metals, or carbon-based precursors.Betaine-modified sepiolite clay composites have been synthesized to fabricate flexible and mechanically robust membranes. These membranes are characterized using powder X-ray diffraction, ATR-FTIR, XPS, surface area analyses, rheology, and imaging techniques (e.g., SEM). Morphological analyses reveal sepiolite possesses needle-shaped morphology in a highly aggregated state. The addition of betaine disaggregates such bundles, forming a 3D network with unique “clay anemone” structures. The XRD patterns are consistent with the literature reflecting no change in the d-spacing and corresponding 2θ value (d100 2θ = 7.4 ͦ) before and after betaine treatment. This provides strong evidence of modification limited to the external surface. ATR-FTIR spectra of the sepiolite-betaine composite membranes show the presence of 935, 1330,1391, and 1616 cm-1 frequencies that correspond to C-N-C, N-C-H, C-N and C=Ostretching respectively. This suggests the successful functionalization of sepiolite clay by betaine through an intercalation mechanism. Further, the electrochemical properties of betaine-modified sepiolite membranes are investigated by electrochemical impedance spectroscopy (EIS) measurements to study the ion dynamics of the hybrid membranes. The room temperature ionic conductivities of these membranes are measured using lithium perchlorate (LiClO4) in ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed solvent (1:1). The concentration of LiClO4 is varied to monitor the change in ionic conductivity and ease of ion transfer through the membranes. The highest ionic conductivity is observed in 0.1M LiClO4 concentration which is superior to the ionic conductivity of sepiolite or hybrid materials with carbon-based precursors, metal ions, quantum dots, or polymers. Viscoelastic properties of the sepiolite slurries with varying concentrations of betaine are also studied to investigate the interaction mechanism between the sepiolite-betaine system and correlate the impact of these interactions on the structure and ionic conductivity of the hybrid sepiolite membranes. Contrary, the IL-sepiolite membranes exhibit superior ionic conductivity compared to betaine-sepiolite membranes, which gives a distinct advantage in the physicochemical and electrochemical properties. The composites are synthesized with varying concentrations of ionic liquid-to-clay ratios and pressed into pellets for conductivity measurements. The highest ionic conductivity of 1 x 10-2 S/cm is achieved by modifying the clay with 10 wt. % IL specimens. This helps to improve the ionic conductivity of the IL-clays. The XRD spectra of sepiolite-IL composites show almost no change in the 2 theta values and peak intensities compared to the pristine sepiolite, confirming the crystalline structure of the sepiolite clay is intact after functionalization with IL. The ATR-FTIR confirms the IL-sepiolite modification with the appearance of a new band at 1345 cm-1 that is attributed to -CH stretching frequencyfrom the alkyl chains of IL present in the clay structures.We have developed a new approach using earth-abundant silicate-based materials to synthesize hybrid zwitterion-modified sepiolite clay membranes. Such materials are not only flexible and mechanically durable, but they also exhibit enhanced ionic conductivity and show exceptional thermal and chemical durability at temperatures above 300 °C and with most aggressive organic solvents, making them a superior candidate for an ergonomic and eco-friendly membrane-based separator for energy devices and gas absorption. Current work is in progress to understand the fundamentals underlying the unique structure-property relationship of hybrid sepiolite, zwitterion-silicate interactions, and conductivity variance with sepiolite architectures, thereby exploiting newer and better properties for application in catalysis, flexible electronics, and biomedical fields. Figure 1

Langmuir monolayers provide an effective strategy for studying molecular recognition of nucleobases using alkylated nucleotides

Molecular Recognition in nucleotides is crucial for medicine, underpinning precise interactions in genetic replication and therapy. Alkylated nucleotides, in particular, play a key role in modifying DNA to inhibit cancer cell growth. In this study, we focused on an alkylated nucleotide, PNM2 (3′,4′,6’-O-tristearoyl uridine or uridine tri-stearate), to investigate the interaction between adenine molecules in the aqueous subphase and PNM2 Langmuir monolayers. Utilizing techniques such as tensiometry, Brewster angle microscopy, infrared spectroscopy, surface potential measurements, and dilatational surface rheology, we found compelling evidence of molecular Recognition between the polar head of the insoluble amphiphile (uridine) in the monolayer and adenine in the aqueous subphase, attributed to hydrogen bonding. These interactions significantly influenced the physicochemical properties of the air-water interface, including monolayer expansion upon molecular recognition, decreased dilatational modulus, increased tensiometric stability of the monolayer when compressed to relevant surface pressures, and decreased surface potential. These findings are noteworthy for drug development, providing crucial insights into the mechanisms of nucleotide interactions.

Exploring proteins at soft interfaces and in thin liquid films – From classical methods to advanced applications of reflectometry

The history of the topic of proteins at soft interfaces dates back to the 19th century, and until the present day, it has continuously attracted great scientific interest. A multitude of experimental methods and theoretical approaches have been developed to serve the research progress in this large domain of colloid and interface science, including the area of soft colloids such as foams and emulsions. From classical methods like surface tension adsorption isotherms, surface pressure-area measurements for spread layers, and surface rheology probing the dynamics of adsorption, nowadays, advanced surface-sensitive techniques based on spectroscopy, microscopy, and the reflection of light, X-rays and neutrons at liquid/fluid interfaces offers important complementary sources of information. Apart from the fundamental characteristics of protein adsorption layers, i.e., surface tension and surface excess, the nanoscale structure of such layers and the interfacial protein conformations and morphologies are of pivotal importance for extending the depth of understanding on the topic. In this review article, we provide an extensive overview of the application of three methods, namely, ellipsometry, X-ray reflectometry and neutron reflectometry, for adsorption and structural studies on proteins at water/air and water/oil interfaces. The main attention is placed on the development of experimental approaches and on a discussion of the relevant achievements in terms of notable experimental results. We have attempted to cover the whole history of protein studies with these techniques, and thus, we believe the review should serve as a valuable reference to fuel ideas for a wide spectrum of researchers in different scientific fields where proteins at soft interface may be of relevance.

Using Passive Microrheology to Measure the Evolution of the Rheological Properties of NIST mAb Formulations during Adsorption to the Air-Water Interface.

The development of novel protein-based therapeutics, such as monoclonal antibodies (mAbs), is often limited due to challenges associated with maintaining the stability of these formulations during manufacturing, storage, and clinical administration. An undesirable consequence of the instability of protein therapeutics is the formation of protein particles. MAbs can adsorb to interfaces and have the potential to undergo partial unfolding as well as to form viscoelastic gels. Further, the viscoelastic properties may be correlated with their aggregation potential. In this work, a passive microrheology technique was used to correlate the evolution of surface adsorption with the evolution of surface rheology of the National Institute of Standards and Technology (NIST) mAb reference material (NIST mAb) and interface-induced subvisible protein particle formation. The evolution of the surface adsorption and interfacial shear rheological properties of the NIST mAb was recorded in four formulation conditions: two different buffers (histidine vs phosphate-buffered saline) and two different pHs (6.0 and 7.6). Our results together demonstrate the existence of multiple stages for both surface adsorption and surface rheology, characterized by an induction period that appears to be purely viscous, followed by a sharp increase in protein molecules at the interface when the film rheology is viscoelastic and ultimately a slowdown in the surface adsorption that corresponds to the formation of solid-like or glassy films at the interface. When the transitions between the different stages occurred, they were dependent on the buffer/pH of the formulations. The onset of these transitions can also be correlated to the number of protein particles formed at the interface. Finally, the addition of polysorbate 80, an FDA-approved surfactant used to mitigate protein particle formation, led to the interface being surfactant-dominated, and the resulting interface remained purely viscous.

Oil-water interface and emulsion stabilising properties of rapeseed proteins napin and cruciferin studied by nonlinear surface rheology

HypothesisTwo major protein families are present in rapeseed, namely cruciferins and napins. The structural differences between the two protein families indicate that they might behave differently when their mixture stabilises oil–water interfaces. Therefore, this work focuses on elucidating the role of both proteins in interface and emulsion stabilisation. ExperimentsProtein molecular properties were evaluated, using SEC, DSC, CD, and hydrophobicity analysis. The oil–water interface mechanical properties were studied using LAOS and LAOD. General stress decomposition (GSD) was used as a novel method to characterise the nonlinear response. Additionally, to evaluate the emulsifying properties of the rapeseed proteins, emulsions were prepared using pure napins or cruciferin and also their mixtures at 1:3, 1:1 and 3:1 (w:w) ratios. FindingsCruciferins formed stiff viscoelastic solid-like interfacial layers (Gs′ = 0.046 mN/m; Ed′ = 30.1 mN/m), while napin formed weaker and more stretchable layers at the oil–water interface (Gs′ = 0.010 mN/m; Ed′ = 26.4 mN/m). As a result, cruciferin-formed oil droplets with much higher stability against coalescence (coalescence index, CI up to 10%) than napin-stabilised ones (CI up to 146%) during two months of storage. Both proteins have a different role in emulsions produced with napin-cruciferin mixtures, where cruciferin provides high coalescence stability, while napin induces flocculation. Our work showed the role of each rapeseed protein in liquid–liquid multiphase systems.

Stability and thinning of liquid jets in the presence of soluble surfactants.

The dynamics of many multiphase fluid systems involve the thinning and eventual break up of a slender fluid filament or a liquid jet. The interfacial instability that controls the rate of jet thinning depends on the relative magnitudes of capillary, viscous, and inertial stresses. Surfactants add an additional layer of physicochemical dynamics by reducing the surface tension of the interface and introducing reverse Marangoni flows in response to surface concentration gradients. Surfactants may also introduce an intrinsic surface rheology that affects jet thinning. Quantifying these effects has been a significant problem in chemical physics and a topic of key research interest. Recent studies have shown that insoluble surfactants delay thread thinning and suppress instabilities in Newtonian jets. However, the role of surfactant solubility in liquid jet stability is still unknown. In this work, we use linear stability analysis to quantitatively show the stabilizing effects of Marangoni stresses, surfactant adsorption and desorption time, and intermolecular forces upon adsorption. We highlight the seemingly indistinguishable way in which various surfactant properties result in the same outcome. We also identify a surface dissipative contribution that arises from the interplay of Marangoni flows with finite adsorption and desorption, which acts as an "apparent" surface viscosity. We verify predictions of our linear stability results against numerical simulations and conclude by noting that tuning surface activity and kinetics of adsorbed surfactants or particles can potentially suppress droplet formation, which is of significant impact in the printing industry and in the control of the spread of aerosols.

Dynamic properties of the layers of cupin-1.1 aggregates at the air/water interface

Spread layers of amorphous aggregates of the structural domain of plant protein vicilin, cupin-1.1, at the water – air interface were studied by the surface tensiometry, dilational surface rheology, Brewster angle and atomic force microscopy. The layer properties differed strongly from the results for the layers of previously studied proteins. The dependency of the dynamic elasticity of the layer on surface pressure had two local maxima with the second peak being four times higher than the first one. In the region of the first maximum the obtained results are similar to those for dispersions of polymer microgels with a hairy corona. At the beginning of surface compression separate threads of the corona are stretched along the surface and the surface elasticity increases. The further compression results in the formation of loops and tails leading to a decrease of the elasticity. The second local maximum of the dynamic surface elasticity is presumably caused by the interactions of the rigid cores of the aggregates leading finally to the formation of multilayer structures at high surface pressures. In this case, the surface elasticity starts to decrease as a result of the segment exchange between different layers at the interface.

Rheological investigations on frequency selective surface carbon composite microwave absorber.

A high-performance stealth platform is one of the crucial requirements in defence technology that could practically be realized by building effective microwave frequency selective surface (FSS) absorbers. Herein, we report the design and manufacturing of an absorber by tuning the rheology of cell architecture. Initially, a fan-shaped cell (10.4 mm2) was designed for its surface and bulk rheology. The FSS overlayer composition was investigated using SEM, EDX, and XRD and tuned for 0.25% carbon: 1.5% silver to achieve the ink resistivity ∼255 Ω □-1. The bulk rheology was optimized for air (Roha) spacer (thickness ∼2.8 mm), interlayer dielectrics (0.2 mm each), carbon composition (5%), and cell dimension (10.2 mm). Analyses are presented for absorption loss (RC, dB), bandwidth (GHz), resonance dispersion, and constitutive (ε, μ) parameters, compounded with an equivalent circuit model with the settings R = 273.55 Ω, L = 2.25 nH, C = 0.057 pF and the Fabry-Perot reactance mode@10 GHz. The bi-modal response was investigated for induced polarization, electromagnetic fields, volume power distribution, and angular (Θ = 0°-50°) and rotational stability (Φ = 0°-90°) against TE/TM incidences. The FSS pattern was implemented using a screen printing technique to fabricate a prototype absorber and subjected to the free space measurements in an anechoic chamber. The prototype behaviour was found to be commensurate with the simulated performance, thereby achieving a figure of merit of RC ∼-25 dB@10 GHz, accessible bandwidth 4 GHz (in X band) by using the thickness of 0.057 λ0. Details are presented in this study.

Effect of surface viscoelasticity on top jet drops produced by bursting bubbles.

Jet drops resulting from bubble bursting at a liquid surface play a key role in various mass transfer processes across the interface, including sea spray aerosol generation and pathogen transmission. However, the impact of structurally compound interfaces, characterized by complex surface rheology introduced by surface-active contaminants, on the jet drop ejection still remains unclear. Here, we experimentally investigate the influence of surface viscoelasticity on the size and velocity of the top jet drops from surface bubble bursting, examining both pure protein and mixed protein-surfactant solutions. We document that for bubble bursting at a pure-protein-laden surface where surface elasticity dominates, the increase in Ec, i.e. the interfacial elastocapillary number as the ratio between the effects of interfacial elasticity and capillarity, efficiently increases the radius and decreases the velocity of the top jet drop, ultimately inhibiting the jet drop ejection. On the other hand, considering the mixed protein-surfactant solution, we show that the top jet drop radius and velocity exhibit a different variation trend with Ec, which is attributed to the additional dissipation on the capillary waves as well as the retardation and resistance on the converging flow for jet formation from surface viscoelasticity. Our work may advance the understanding of bubble bursting dynamics at contaminated liquid surfaces and shed light on the potential influence of surface viscoelasticity on the generation of bubble bursting aerosols.

Structural changes in layers of lipid mixtures at low surface tensions

Layers of pulmonary lipids on an aqueous substrate at non-equilibrium conditions can decrease the surface tension of water to quite low values. This is connected with different relaxation processes occurring at the interface and the associated changes in the surface layer structure. Results of measurements by the combination of methods like surface rheology, ellipsometry, Brewster angle microscopy, and IRRAS for spread layers of lipid mixtures open a possibility to specify the dynamics of structural changes at conditions close to the physiological state. At sufficiently low surface tension values (below 5 mN/m) significant changes in the ellipsometric signal were observed for pure DPPC layers, which can be related to a transition from 2D to 3D structures caused by the layer folding. The addition of other lipids can accelerate the relaxation processes connected with squeezing-out of molecules or multilayer stacks formation hampering thereby a decrease of surface tension down to low values corresponding to the folding of the monolayer.

Noninvasive biosensing 3D scaffold to monitor degradation: The potential of fluorescent PCL and PLGA for tissue engineering

AbstractThe nondestructive localization and traceability of polymers by fluorescent tagging has become a valuable tool for biomedical applications. Integration of fluorescent molecule to the pristine polymers could modify polymers' degradation rate which is still unpredictable from a scaffold application standpoint. The current study focused to understand the material perspective of fluorescently tagged biodegradable polymers such as polycaprolactone (PCL) and poly (d,l‐lactide‐co‐glycolide) (PLGA) with fluorescein amine isomer I (FITC). PCL‐FITC and PLGA‐FITC were characterized using FTIR for surface chemistry analysis and rheology for their mechanical properties. The grafted materials were utilized to form 3‐dimentional scaffolds, and their degradation was monitored under accelerated degradation conditions triggered by pH. It was found that PCL and PCL‐FITC had a very slow degradation rate, when compared to PLGA and PLGA‐FITC. Both the FITC tagged materials displayed a faster degradation rate compared to their respective pristine material. Biocompatibility of the FITC conjugated polymers was tested using human‐adipose derived stem cells (hADSCs) revealing that the sub products from the degradation of the polymers over 7 days did not negatively affect the cellular metabolic activity. This work highlights the significance of initial characterization of fluorescent modified polymers for future biomedical application.

Evaluation of Cassia fistula seed galactomannan as tablet-binder in formulation of diclofenac sodium-loaded monolithic matrix tablet

The present study aimed to evaluate Cassia fistula seed galactomannan (CFSG) as a tablet-binder in the formulation of a monolithic matrix tablet using diclofenac sodium as a model drug. Initially, CFSG was extracted and purified from the seeds of the Cassia fistula tree and then screened for phytochemicals. Native CFSG was characterized with polysaccharide content determination, monosaccharide composition analysis, elemental analysis, FTIR, solid-state 13C NMR, molecular weight, zeta potential, DSC, TGA-DTA, XRD, viscosity, pH and surface tension, rheology, SEM and acute oral toxicity study. Prior to formulation, the drug-CFSG compatibility was checked by FTIR, DSC, and XRD. Diclofenac sodium-loaded granules were prepared by the wet granulation method and evaluated for various granule properties. Finally, granules were compressed into tablets and evaluated for binding and other tablet properties. The granules showed to have optimum micromeritic properties. Tablet hardness and friability were found to be approximately 7 kg/m2 and 0.3 %, respectively, which substantiate the excellent binding capacity of CFSG. Other tablet properties were also found to be within the Pharmacopoeial compliance limit. The tablets with a minimum concentration of CFSG (2.5%w/w) as binder showed appreciable mechanical strength and faster drug release, which ratifies CFSG as an alternative tablet binder.

Evaporation-Induced Temperature Gradient in a Foam Column.

Various parameters affect foam stability: surface and bulk rheology of the solution, gravitational drainage, mechanical vibrations, bubble gas composition, and also evaporation. Evaporation is often considered through the prism of liquid loss but also induces a cooling effect due to the enthalpy of vaporization. In this study, we combine a theoretical and experimental approach to explore the temperature field in a foam column evaporating from the top. We show that a measurable temperature profile exists in this geometry, with temperatures at the interface lower than the environmental temperature by a few degrees. We demonstrate that the temperature profile is the result of a balance between the enthalpy of vaporization and heat fluxes originating from the thermal conduction of foam and air and thermal radiation. For small foam thicknesses compared to the radius, we found that the temperature gradient is established over the foam thickness, while for large aspect ratios, the gradient spans over a length scale comparable to the tube radius.

Secondary Bubble Entrainment via Primary Bubble Bursting at a Viscoelastic Surface.

Bubble bursting at liquid surfaces is ubiquitous and plays a key role for the mass transfer across interfaces, impacting global climate and human health. Here, we document an unexpected phenomenon that when a bubble bursts at a viscoelastic surface of a bovine serum albumin solution, a secondary (daughter) bubble is entrapped with no subsequent jet drop ejection, contrary to the counterpart experimentally observed at a Newtonian surface. We show that the strong surface dilatational elastic stress from the viscoelastic surface retards the cavity collapse and efficiently damps out the precursor waves, thus facilitating the dominant wave focusing above the cavity nadir. The onset of daughter bubble entrainment is well predicted by an interfacial elastocapillary number comparing the effects of surface dilatational elasticity and surface tension. Our Letter highlights the important role of surface rheology on free surface flows and may find important implications in bubble dynamics with a contaminated interface exhibiting complex surface rheology.

Dynamic Properties of Pulmonary Lipid Monolayers on the Surfaces of Sodium Polystyrene Sulfonate and Polydiallyldimethylammonium Chloride Solutions

Pulmonary surfactant, which is a complex mixture of lipids and proteins, plays a key role in the functional properties of the respiratory system. Lipids form complexes with proteins to maintain low values of surface tension at continuous compression/expansion deformations. However, interactions that lead to the complexation still remain to be unknown thus significantly hampering the development of synthetic analogs of the natural pulmonary surfactant. In this work, the methods of surface rheology and ellipsometry have been employed to study the dynamic properties of model phospholipid monolayers applied onto the surfaces of synthetic polyelectrolyte solutions. It has been shown, that electrostatic or hydrophobic interactions between lipids and macromolecules are insufficient for the efficient complexation and maintenance of low surface tension values.

Impact of surface rheology on droplet coalescence in uniaxial compressional flow

Impact of surface rheology on droplet coalescence in uniaxial compressional flow

Self-aggregation, dilational surface rheology and foaming properties of 1-O-dodecyl diglyceryl ether compared to n-dodecyl-β-D-maltoside and pentaethyleneglycol monododecyl ether

Diglyceryl alkyl esters are good foaming agents but the ester linker can be sensitive to pH, chemicals and temperature unlike the ether function. In this work, the foaming properties of a diglyceryl ether, namely 1-O-dodecyl diglyceryl ether (C12Gly2), are compared to those of n-dodecyl-β-D-maltoside (C12Glc2) and pentaethyleneglycol monododecyl ether (C12E5). The self-aggregation behaviour of C12Gly2 and adsorption at the air–water interface has been first investigated. For the three surfactants, the dynamic response of the interface, measured in oscillatory bubble interfacial rheology experiments below CMC, is compared and put in relation with their foaming properties. In particular, the foamability by air sparging at a concentration of 10 times the CMC, the foam stability over 1 h and the foam density are quantified.It is shown that C12Gly2 forms liquid crystals at low concentration (∼10CMC). C12E5, with a lower elasticity high frequency limit ε0, forms unstable foam with quick drainage and breakdown, whereas higher ε0 surfactants C12Gly2 and C12Glc 2 form much more stable foams, resulting from hydrogen bonds between the polar heads of C12Glc 2 and C12Gly2. Differences in C12Gly2 and C12Glc2 foams lay in initial bubble size (smaller for the C12Glc 2). In C12Gly2 foam, the main destabilization phenomenon is coalescence over drainage, and the foam volume only decreases by 30% in 1 h.