Accuracy of Axisymmetric Drop Shape Analysis in Determining Surface and Interfacial Tensions

Surfactant additives in water-based metalworking fluids lead to strong biophysical inhibition of pulmonary surfactant film

As a class of very common and important industrial liquid materials, metalworking fluids (MWFs) are easy to be vaporized into the air and cause pulmonary toxic effects. The inhaled MWF aerosols should first interact with the pulmonary surfactant (PS) film that plays an essential role in maintaining the normal respiratory mechanics and pulmonary immunology in a human body. Here, to probe any potential adverse impacts of airborne MWFs on the biophysical and physiological functions of PS film that may help achieve a deep and comprehensive understanding of the pulmonary toxicology of MWFs, a systematic study on the interaction between an animal-derived natural PS (i.e., Calsurf) film and the aerosols of water-based MWFs and their different constituents is conducted using constrained drop surfactometry (CDS) capable of closely simulating normal tidal breathing and lung-related physiological conditions in vitro. It was found that the airborne MWFs can induce strong PS inhibitions once their accumulated amount in the environment attains 0.2 mg/cm3. And their inhibitory effects are demonstrated to mainly originate from the surfactant additives such as polyethylene glycol monooleate (PEGMO) that can desorb PS film from the air/water interface via competitive interfacial adsorption, although it has normally been regarded as an eco-friendly commercial reagent.

Highly Interfacial Active Gemini Surfactants as Simple and Versatile Emulsifiers for Stabilizing, Lubricating and Structuring Liquids

To date, locking the shape of liquids into non-equilibrium states usually relies on jamming nanoparticle surfactants at an oil/water interface. Here we show that a synthetic water-soluble zwitterionic Gemini surfactant can serve as an alternative to nanoparticle surfactants for stabilizing, structuring and additionally lubricating liquids. By having a high binding energy comparable to amphiphilic nanoparticles at the paraffin oil/water interface, the surfactant can attain near-zero interfacial tensions and ultrahigh surface coverages after spontaneous adsorption. Owing to the strong association between neighboring surfactant molecules, closely packed monolayers with high mechanical elasticity can be generated at the oil/water interface, thus allowing the surfactant to produce not only ultra-stable emulsions but also structured liquids with various geometries by using extrusion printing and 3D printing techniques. By undergoing tribochemical reactions at its sulfonic terminus, the surfactant can endow the resultant emulsions with favorable lubricity even under high load-bearing conditions. Our study may provide new insights into creating complex liquid devices and new-generation lubricants capable of combining the characteristics of both liquid and solid lubricants.

The shape of things to come: Axisymmetric drop shape analysis using deep learning

HYPOTHESIS

In the traditional approach to Axisymmetric Drop Shape Analysis (ADSA), the determination of surface tension or interfacial tension is constrained by computational speed and image quality. By implementing a machine learning-based approach, particularly using a convolutional neural network (CNN), it is posited that analysis of pendant drop images can be both faster and more accurate.

EXPERIMENTS

A CNN model was trained and used to predict the surface tension of drop images. The performance of our CNN model was compared to the traditional ADSA, i.e. direct numerical integration, in terms of precision, computational speed, and robustness in dealing with images of varying quality. Additionally, the ability of the CNN model to predict other drop properties such as Volume and Surface Area was evaluated.

FINDINGS

Our CNN demonstrated a significant enhancement in experimental fit precision, predicting surface tension with an accuracy of (+/-) 1.22×10-1 mN/m and at a speed of 1.50 ms-1, outpacing the traditional method by more than 5×103 times. The model maintained an average surface tension error of 2.42×10-1 mN/m even for experimental images with challenges such as misalignment and poor focus. The CNN model also demonstrated showcased a high degree of accuracy in determining other drop properties.

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Oil-in-Water Emulsions Stabilized by Hydrophilic Homopolymers

Most of the polymeric emulsifiers have diblock and triblock copolymer architecture containing hydrophilic and hydrophobic domains. In this work, we show that hydrophilic homopolymers can be effective stabilizers of oil-in-water emulsions. Using polyethelyne oxide and poly(vinylpyrrolidone) as model hydrophilic homopolymers and n-decane and n-hexane as model nonpolar phases, we show that high-molecular weight polymers can stabilize emulsions over 24 h beyond a threshold concentration. We highlight the role of the molecular weight and concentration of the polymer in the stability of emulsions through kinetic measurements of emulsion volume, microscopic analysis, interfacial tension, and dilational rheology. We explain the mechanism of stabilization to stem from buoyancy-driven creaming of emulsion drops and film drainage and dilational elasticity of the interface in relation to the molecular weights and concentrations of polymers. This study demonstrates that water-soluble homopolymers can stabilize oil-in-water emulsions and open avenues for the use of eco-friendly biopolymers, which are inherently hydrophilic, as an alternative to synthetic emulsifiers.

Amphiphiles Formed from Synthetic DNA-Nanomotifs Mimic the Stepwise Dispersal of Transcriptional Clusters in the Cell Nucleus

Stem cells exhibit prominent clusters controlling the transcription of genes into RNA. These clusters form by a phase-separation mechanism, and their size and shape are controlled via an amphiphilic effect of transcribed genes. Here, we construct amphiphile-nanomotifs purely from DNA, and we achieve similar size and shape control for phase-separated droplets formed from fully synthetic, self-interacting DNA-nanomotifs. Increasing amphiphile concentrations induce rounding of droplets, prevent droplet fusion, and, at high concentrations, cause full dispersal of droplets. Super-resolution microscopy data obtained from zebrafish embryo stem cells reveal a comparable transition for transcriptional clusters with increasing transcription levels. Brownian dynamics and lattice simulations further confirm that the addition of amphiphilic particles is sufficient to explain the observed changes in shape and size. Our work reproduces key aspects of transcriptional cluster formation in biological cells using relatively simple DNA sequence-programmable nanostructures, opening novel ways to control the mesoscopic organization of synthetic nanomaterials.

Phase transitions of the pulmonary surfactant film at the perfluorocarbon-water interface

Pulmonary surfactant is a lipid-protein complex that forms a thin film at the air-water surface of the lungs. This surfactant film defines the elastic recoil and respiratory mechanics of the lungs. One generally accepted rationale of using oxygenated perfluorocarbon (PFC) as a respiratory medium in liquid ventilation is to take advantage of its low surface tensions (14-18 mN/m), which was believed to make PFC an ideal replacement of the exogenous surfactant. Compared with the extensive studies of the phospholipid phase behavior of the pulmonary surfactant film at the air-water surface, its phase behavior at the PFC-water interface is essentially unknown. Here, we reported the first detailed biophysical study of phospholipid phase transitions in two animal-derived natural pulmonary surfactant films, Infasurf and Survanta, at the PFC-water interface using constrained drop surfactometry. Constrained drop surfactometry allows in situ Langmuir-Blodgett transfer from the PFC-water interface, thus permitting direct visualization of lipid polymorphism in pulmonary surfactant films using atomic force microscopy. Our data suggested that regardless of its low surface tension, the PFC cannot be used as a replacement of pulmonary surfactant in liquid ventilation where the air-water surface of the lungs is replaced with the PFC-water interface that features an intrinsically high interfacial tension. The pulmonary surfactant film at the PFC-water interface undergoes continuous phase transitions at surface pressures less than the equilibrium spreading pressure of 50 mN/m and a monolayer-to-multilayer transition above this critical pressure. These results provided not only novel biophysical insight into the phase behavior of natural pulmonary surfactant at the oil-water interface but also translational implications into the further development of liquid ventilation and liquid breathing techniques.

E-cigarette aerosol exposure of pulmonary surfactant impairs its surface tension reducing function

Introduction

E-cigarette (EC) and vaping use continue to remain popular amongst teenage and young adult populations, despite several reports of vaping associated lung injury. One of the first compounds that EC aerosols comes into contact within the lungs during a deep inhalation is pulmonary surfactant. Impairment of surfactant’s critical surface tension reducing activity can contribute to lung dysfunction. Currently, information on how EC aerosols impacts pulmonary surfactant remains limited. We hypothesized that exposure to EC aerosol impairs the surface tension reducing ability of surfactant.

Methods

Bovine Lipid Extract Surfactant (BLES) was used as a model surfactant in a direct exposure syringe system. BLES (2ml) was placed in a syringe (30ml) attached to an EC. The generated aerosol was drawn into the syringe and then expelled, repeated 30 times. Biophysical analysis after exposure was completed using a constrained drop surfactometer (CDS).

Results

Minimum surface tensions increased significantly after exposure to the EC aerosol across 20 compression/expansion cycles. Mixing of non-aerosolized e-liquid did not result in significant changes. Variation in device used, addition of nicotine, or temperature of the aerosol had no additional effect. Two e-liquid flavours, menthol and red wedding, had further detrimental effects, resulting in significantly higher surface tension than the vehicle exposed BLES. Menthol exposed BLES has the highest minimum surface tensions across all 20 compression/expansion cycles. Alteration of surfactant properties through interaction with the produced aerosol was observed with a basic e-liquid vehicle, however additional compounds produced by added flavourings appeared to be able to increase inhibition.

Conclusion

EC aerosols alter surfactant function through increases in minimum surface tension. This impairment may contribute to lung dysfunction and susceptibility to further injury.

Probing Surface and Interfacial Tension of Ionic Liquids in Vacuum with the Pendant Drop and Sessile Drop Method

We report on the surface and interface tension measurements of the two ionic liquids (ILs) [C8C1Im][PF6] and [m(PEGn)2Im]I (n = 2, 4, 6) in a surface science approach. The measurements were performed in a newly developed and unique experimental setup, which allows for surface tension (ST) measurements using the pendant drop method and for contact angle measurements using the sessile drop method under the well-defined conditions of a high vacuum (from 10−7 mbar). The setup also allows for in vacuum transfer to an ultrahigh vacuum system for surface preparation and analysis, such as in angle-resolved X-ray photoelectron spectroscopy. For [C8C1Im][PF6], we observe a linear decrease in the surface tension with increasing temperature. The ST measured under high vacuum is consistently found to be larger than under ambient conditions, which is attributed to the influence of water uptake in air by the IL. For [m(PEGn)2Im]I (n = 2, 4, 6), we observe a decrease in the ST with increasing polyethylene glycol chain length in a vacuum, similar to very recent observations under 1 bar Argon. This decrease is attributed to an increasing enrichment of the PEG chains at the surface. The ST data obtained under these ultraclean conditions are essential for a fundamental understanding of the relevant parameters determining ST on the microscopic level and can serve as a benchmark for theoretical calculations, such as molecular dynamic simulations. In addition to the ST measurements, proof-of-principle data are presented for sessile drop measurements in HV, and a detailed description and characterization of the new setup is provided.

Use of a Geometric Parameter for Characterizing Rigid Films at Oil-Water Interfaces

Interfacial tension and dilatational rheology are often used to characterize the mechanical response of a liquid interface using axisymmetric drop shape analysis (ADSA). It is important to note that for systems dominated by adsorption/desorption of surfactants, the contributions of extra mechanical stresses are negligible; thus, the Young-Laplace equation remains valid. However, for interfaces dominated by extra stresses, as in the case of particle monolayers or asphaltenes that clearly exhibit a skin (a rigid film), the nature of the elastic response is fundamentally different and the validity of the equation is questionable. Calculation of the interfacial tension and dilatational elasticity using drop shape analysis depends critically on the drop shape following the Young-Laplace equation. If the interface becomes more like a solid, the drop shape will deviate from being purely Laplacian. Indeed, the drop will exhibit a wrinkled surface as collapse continues. The geometric parameter R_V/A, defined as the ratio (dV/V)/(dA/A) with V is the volume of the drop and A is the area of the interface), allows one to measure the deviation of the drop shape from purely Laplacian. For a simple interface (pure liquids or surfactant solutions), R_V/A is quite close to the theoretical value of 1.5 of a perfect sphere. Nevertheless, if the molecules adsorbed at the interface begin to interact strongly, the ratio can vary. In the limit of long-time-scale experiments, R_V/A of some drops approaches 2. We studied the evolution of the parameter R_V/A for different systems, from simple to complex, as a function of oscillation frequencies and amplitudes of drop volume. The results obtained were compared to the values of the interfacial moduli and drop shape behavior to better characterize the regime change.

Current methods for studying intracellular liquid-liquid phase separation

Liquid-liquid phase separation (LLPS) is a ubiquitous process that drives the formation of membrane-less intracellular compartments. This compartmentalization contains vastly different protein/RNA/macromolecule concentrations compared to the surrounding cytosol despite the absence of a lipid boundary. Because of this, LLPS is important for many cellular signaling processes and may play a role in their dysregulation. This chapter highlights recent advances in the understanding of intracellular phase transitions along with current methods used to identify LLPS in vitro and model LLPS in situ.

Interfacial behavior of phospholipid monolayers revealed by mesoscopic simulation

A mesoscopic model with molecular resolution is presented for dipalmitoyl phosphatidylcholine (DPPC) and palmitoyl oleoyl phosphatidylcholine (POPC) monolayer simulations at the air-water interface using many-body dissipative particle dynamics (MDPD). The parameterization scheme is rigorously based on reproducing the physical properties of water and alkane and the interfacial property of the phospholipid monolayer by comparison with experimental results. Using much less computing cost, these MDPD simulations yield a similar surface pressure-area isotherm as well as similar pressure-related morphologies as all-atom simulations and experiments. Moreover, the compressibility modulus, order parameter of lipid tails, and thickness of the phospholipid monolayer are quantitatively in line with the all-atom simulations and experiments. This model also captures the sensitive changes in the pressure-area isotherms of mixed DPPC/POPC monolayers with altered mixing ratios, indicating that the model is promising for applications with complex natural phospholipid monolayers. These results demonstrate a significant improvement of quantitative phospholipid monolayer simulations over previous coarse-grained models.

Interaction of Two Amphipathic α-Helix Bundle Proteins, ApoLp-III and ApoE 3, with the Oil-Aqueous Interface

Protein-lipid interactions govern the structure and function of lipoprotein particles, which transport neutral lipids and other hydrophobic cargo through the blood stream. Apolipoproteins cover the surface of lipoprotein particles, including low-density (LDL) and high-density (HDL) lipoproteins, and determine their function. Previous work has focused on small peptides derived from these apolipoproteins or used such artificial lipid systems as Langmuir monolayers or the lipid disc assay to determine how apolipoproteins interact with the neutral lipid interface. Here, we focus on a recurring protein domain found in many neutral lipid-binding proteins, the amphipathic α-helix bundle. We use liquid droplet tensiometry to investigate protein-lipid interactions on an oil droplet, which mimics the real lipoprotein interface. The N-terminus of apoE 3 and full-length apoLp-III serve as model proteins. We find that each protein interacts with lipid monolayers at the oil-aqueous interface in unique ways. For the first time, we show that helix bundle unfolding is critical for proper protein insertion into the lipid monolayer at the oil-aqueous interface and that specific membrane lipids promote the rebinding of protein upon fluctuation in droplet size. These results shed new light on how amphipathic apolipoprotein α-helix bundles interact with neutral lipid particles.

The C-Terminus of Perilipin 3 Shows Distinct Lipid Binding at Phospholipid-Oil-Aqueous Interfaces

Lipid droplets (LDs) are ubiquitously expressed organelles; the only intracellular organelles that contain a lipid monolayer rather than a bilayer. Proteins localize and bind to this monolayer as they do to intracellular lipid bilayers. The mechanism by which cytosolic LD binding proteins recognize, and bind, to this lipid interface remains poorly understood. Amphipathic α-helix bundles form a common motif that is shared between cytosolic LD binding proteins (e.g., perilipins 2, 3, and 5) and apolipoproteins, such as apoE and apoLp-III, found on lipoprotein particles. Here, we use pendant drop tensiometry to expand our previous work on the C-terminal α-helix bundle of perilipin 3 and the full-length protein. We measure the recruitment and insertion of perilipin 3 at mixed lipid monolayers at an aqueous-phospholipid-oil interface. We find that, compared to its C-terminus alone, the full-length perilipin 3 has a higher affinity for both a neat oil/aqueous interface and a phosphatidylcholine (PC) coated oil/aqueous interface. Both the full-length protein and the C-terminus show significantly more insertion into a fully unsaturated PC monolayer, contrary to our previous results at the air-aqueous interface. Additionally, the C-terminus shows a preference for lipid monolayers containing phosphatidylethanolamine (PE), whereas the full-length protein does not. These results strongly support a model whereby both the N-terminal 11-mer repeat region and C-terminal amphipathic α-helix bundle domains of perilipin 3 have distinct lipid binding, and potentially biological roles.

Surface tensiometry of phase separated protein and polymer droplets by the sessile drop method

Phase separated macromolecules play essential roles in many biological and synthetic systems. Physical characterization of these systems can be challenging because of limited sample volumes, particularly for phase-separated proteins. Here, we demonstrate that a classic method for measuring the surface tension of liquid droplets, based on the analysis of the shape of a sessile droplet, can be effectively scaled down to measure the interfacial tension between a macromolecule-rich droplet phase and its co-existing macromolecule-poor continuous phase. The connection between droplet shape and surface tension relies on the density difference between the droplet and its surroundings. This can be determined with small sample volumes in the same setup by measuring the droplet sedimentation velocity. An interactive MATLAB script for extracting the capillary length from a droplet image is included in the ESI.

Compound Drop Shape Analysis with the Neumann Number

A compound droplet is composed of a traditional pendant drop (PD) or sessile drop (SD) sharing the interface with an immiscible phase of comparable sizes, which could be a solid particle, a gas bubble, or most often another droplet of an immiscible liquid. Over the past decade, the study of compound droplets has attracted increasing attention because of extensive applications in many research fields, such as complex fluids, microfluidics, foam and emulsion, and biomedical applications. Among all technical difficulties in characterizing compound droplets, a central problem in surface science is the prediction of its equilibrium shape, which requires knowledge of complicated boundary conditions. Existing dimensionless groups, such as the Bond number traditionally used to evaluate the shape of PDs and SDs, largely fail in predicting the shape of compound droplets. Here, we propose an alternative Bond number, termed the Neumann number, to characterize the shape of compound droplets. Using three dimensionless groups, that is, the Neumann number, the Bond number, and the Worthington number, we have quantitatively predicted and analyzed the shape of traditional PDs/SDs and various compound droplets, including a PD with a spherical particle suspending at the drop apex, a SD with its apex disturbed by a vertical cylinder, and a spherical SD (no gravity) with its apex disturbed by a fluid lens. It is found that the Neumann number can be readily adapted to quantitatively predict and analyze the shape of PDs/SDs and compound droplets.

Bond Number Revisited: Axisymmetric Macroscopic Pendant Drop

The numbers Rb_θ and Rb_r introduced recently (Berim, G.O.; Ruckenstein, E. J. Phys. Chem. 2019, 123, 10294-10300) to characterize the cylindrical pendant drops hanging from the solid surface are modified for characterization of axisymmetric pendant drops which are more common in experiment. Similar to the case of cylindrical drops, Rb_θ and Rb_r are defined on the basis of rigorous solution of Young-Laplace equation for the drop profile. As a consequence, they contain only the input parameters of the drop-solid system such as the volume of the drop, gravitational acceleration, fluid densities, surface tension, and contact angle. This constitutes the main difference between Rb numbers and traditional Bond and Bond-like numbers which involve the dimensions of the drop unknown prior to the experiment (e.g., height of the drop or radius of curvature at the drop apex). It is shown, that the new numbers can be used, for example, for predicting the breakup conditions of the drop, determining the surface tension, and estimating the deviation of drop shape from the circular one. A good agreement between theoretical predictions and experimental results known from the literature was demonstrated.

A method for measuring the interfacial tension for density-matched liquids

We propose a method to measure the interfacial tension characterizing the interface between two immiscible liquids of practically the same density. In this method, a cylindrical liquid bridge made of one the liquids is vibrated laterally inside a tank filled with the other. The first resonance frequency is determined and equated to the first eigenfrequency of the m=1 linear mode to infer the interfacial tension value. The method does not involve the density jump across the interface. Therefore, its accuracy is affected neither by the smallness of the Bond number nor by errors of the density difference. The experimental setup is relatively simple, and the procedure does not use image processing techniques. The results satisfactorily agree with those measured by TIFA-AI (Theoretical Fitting Image Analysis-Axisymmetric Interfaces) for the same liquid bridges when the density difference is sufficiently large for TIFA-AI to be valid. We conduct numerical simulations of the Navier-Stokes equations to determine the best parameter conditions for the proposed method. The transfer function characterizing the frequency response of the fluid configuration is measured in some experiments to quantify non-linear effects and to study the role played by the outer bath vibration.

Dynamic surface tension of xylem sap lipids

The surface tension of xylem sap has been traditionally assumed to be close to that of the pure water because decreasing surface tension is thought to increase vulnerability to air seeding and embolism. However, xylem sap contains insoluble lipid-based surfactants, which also coat vessel and pit membrane surfaces, where gas bubbles can enter xylem under negative pressure in the process known as air seeding. Because of the insolubility of amphiphilic lipids, the surface tension influencing air seeding in pit pores is not the equilibrium surface tension of extracted bulk sap but the local surface tension at gas-liquid interfaces, which depends dynamically on the local concentration of lipids per surface area. To estimate the dynamic surface tension in lipid layers that line surfaces in the xylem apoplast, we studied the time-dependent and surface area-regulated surface tensions of apoplastic lipids extracted from xylem sap of four woody angiosperm plants using constrained drop surfactometry. Xylem lipids were found to demonstrate potent surface activity, with surface tensions reaching an equilibrium at ~25 mN m-1 and varying between a minimum of 19 mN m-1 and a maximum of 68 mN m-1 when changing the surface area between 50 and 160% around the equilibrium surface area. It is concluded that xylem lipid films in natural conditions most likely range from nonequilibrium metastable conditions of a supersaturated compression state to an undersaturated expansion state, depending on the local surface areas of gas-liquid interfaces. Together with findings that maximum pore constrictions in angiosperm pit membranes are much smaller than previously assumed, low dynamic surface tension in xylem turns out to be entirely compatible with the cohesion-tension and air-seeding theories, as well as with the existence of lipid-coated nanobubbles in xylem sap, and with the range of vulnerabilities to embolism observed in plants.

Bond Number Revisited: Two-Dimensional Macroscopic Pendant Drop

The definition of the Bond number, which is widely used for the qualitative description of the shape and stability of a macroscopic pendant drop, is revisited. Two new numbers, named Rb_θ and Rb_λ, are introduced for two kinds of two-dimensional pendant drops, which have a constant contact angle and a constant width of drop-solid contact, respectively. These numbers contain only input parameters such as the volume of the drop, gravitational acceleration, fluid densities, surface tensions, and contact angles. This distinguishes them from the traditional Bond number, which involves, in addition to the input parameters, prior unknown experimentally derived characteristics of the drop, e.g., height of the drop or radius of the curvature at the drop apex. It is shown that, when properly defined, the new numbers can be used, in particular, for predicting the breakup conditions of the drop and estimating their shape.

Adsorption of Phospholipids at the Air-Water Surface

Phospholipids are ubiquitous components of biomembranes and common biomaterials used in many bioengineering applications. Understanding adsorption of phospholipids at the air-water surface plays an important role in the study of pulmonary surfactants and cell membranes. To date, however, the biophysical mechanisms of phospholipid adsorption are still unknown. It is challenging to reveal the molecular structure of adsorbed phospholipid films. Using combined experiments with constrained drop surfactometry and molecular dynamics simulations, here, we studied the biophysical mechanisms of dipalmitoylphosphatidylcholine (DPPC) adsorption at the air-water surface. It was found that the DPPC film adsorbed from vesicles showed distinct equilibrium surface tensions from the DPPC monolayer spread via organic solvents. Our simulations revealed that only the outer leaflet of the DPPC vesicle is capable of unzipping and spreading at the air-water surface, whereas the inner leaflet remains intact and forms an inverted micelle to the interfacial monolayer. This inverted micelle increases the local curvature of the monolayer, thus leading to a loosely packed monolayer at the air-water surface and hence a higher equilibrium surface tension. These findings provide novel insights, to our knowledge, into the mechanism of the phospholipid and pulmonary surfactant adsorption and may help understand the structure-function correlation in biomembranes.

Effect of Particle Size on the Rising Behavior of Particle-Laden Bubbles

The rising behavior of bubbles, initially half and fully coated with glass beads of various sizes, was investigated. The bubble velocity, aspect ratio, and oscillation periods were determined using high-speed photography and image analysis. In addition, the acting forces, drag modification factor, and modified drag coefficient were calculated and interpreted. Results show that the aspect ratio oscillation of the rising bubbles is similar, irrespective of the attached particle size. As the particle size is increased, the rising bubbles have a lower velocity and aspect ratio amplitude, with the time from release to each aspect ratio peak increasing. Higher particle coverage is shown to decrease the bubble velocity and dampen the oscillations, reducing the number of aspect ratio peaks observed. The highest rise velocities correspond to the lowest aspect ratios and vice versa, whereas a constant aspect ratio yields a constant rise velocity, independent of the particle size. Force analysis shows that the particle drag modification factor increases with the increased particle size and is greatest for fully laden bubbles. The modified drag coefficient of particle-laden bubbles increases with the increased particle size, although it decreases with the increased Reynolds number independent of the particle size. The drag force exerted by the particles plays a more dominant role in decreasing bubble velocities as the particle size increases. The results and interpretation produced a quantitative description of the behavior of rising particle-laden bubbles and the development of correlations will enhance the modeling of industrial applications.

Determining the surface dilational rheology of surfactant and protein films with a droplet waveform generator

Understanding rheological properties of surfactant and protein films plays a crucial role in a variety of industrial and research areas, such as food processing, cosmetics, and pharmacology. To determine the surface dilational modulus using drop shape analysis, one needs to measure the dynamic surface tension in response to a sinusoidal oscillation of the surface area of the droplet. Despite many applications of drop shape analysis in studying interfacial rheology, oscillation of the droplet surface area is usually controlled in an indirect manner. Existing methods are only capable of controlling volume oscillations of the droplet rather than its surface area. We have developed an arbitrary waveform generator (AWG) to directly oscillate the surface area of a millimeter-sized droplet in a predefined sinusoidal waveform. Here, we demonstrated the capacity of this AWG, in conjunction with constrained drop surfactometry (CDS), in studying the surface dilational rheology of adsorbed surfactant and protein films. It is found that the surface dilational modulus determined for a dilute surfactant (C₁₂DMPO) and two protein solutions (bovine serum albumin and β-casein) revealed their adsorption mechanisms. Our methods hold promise in studying the interfacial rheology of various thin-film materials, biomembranes, foams, and emulsions.

Reversible Phase Transitions in the Phospholipid Monolayer

The polymorphism of phospholipid monolayers has been extensively studied because of its importance in surface thermodynamics, soft matter physics, and biomembranes. To date, the phase behavior of phospholipid monolayers has been nearly exclusively studied with the classical Langmuir-type film balance. However, because of experimental artifacts caused by film leakage, the Langmuir balance fails to study the reversibility of two-dimensional surface phase transitions. We have developed a novel experimental methodology called the constrained drop surfactometry capable of providing a leakage-proof environment, thus allowing reversibility studies of two-dimensional surface phase transitions. Using dipalmitoylphosphatidylcholine (DPPC) as a model system, we have studied the reversibility of isothermal and isobaric phase transitions in the monolayer. It is found that not only the compression and expansion isotherms but also the heating and cooling isobars, completely superimpose with each other without hysteresis. Microscopic lateral structures of the DPPC monolayer also show reversibility not only during the isothermal compression and expansion processes but also during the isobaric heating and cooling processes. It is concluded that the two-dimensional surface phase transitions in phospholipid monolayers are reversible, which is consistent with the reversibility of phase transitions in bulk pure substances. Our results provide a better understanding of surface thermodynamics, phase change materials, and biophysical studies of membranes and pulmonary surfactants.

Droplet Oscillation as an Arbitrary Waveform Generator

Oscillating droplets and bubbles have been developed into a novel experimental platform for a wide range of analytical and biological applications, such as digital microfluidics, thin film, biophysical simulation, and interfacial rheology. A central effort of developing any droplet-based experimental platform is to increase the effectiveness and accuracy of droplet oscillations. Here, we developed a novel system of droplet-based arbitrary waveform generator (AWG) for feedback-controlling single-droplet oscillations. This AWG was developed through closed-loop axisymmetric drop shape analysis and based on the hardware of constrained drop surfactometry. We have demonstrated the capacity of this AWG in oscillating the volume and surface area of a millimeter-sized droplet to follow four representative waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating the surface area of a droplet across the frequency spectrum makes the AWG an ideal tool for studying interfacial rheology. The AWG was used to determine the surface dilational modulus of a commonly studied nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based AWG developed in this study is expected to achieve accuracy, versatility, and applicability in a wide range of research areas, such as thin film and interfacial rheology.

Melting of the Dipalmitoylphosphatidylcholine Monolayer

Langmuir monolayer self-assembled at the air-water interface represents an excellent model for studying phase transition and lipid polymorphism in two dimensions. Compared with numerous studies of phospholipid phase transitions induced by isothermal compression, there are very scarce reports on two-dimensional phase transitions induced by isobaric heating. This is mainly due to technical difficulties of continuously regulating temperature variations while maintaining a constant surface pressure in a classical Langmuir-type film balance. Here, with technological advances in constrained drop surfactometry and closed-loop axisymmetric drop shape analysis, we studied the isobaric heating process of the dipalmitoylphosphatidylcholine (DPPC) monolayer. It is found that temperature and surface pressure are two equally important intensive properties that jointly determine the phase behavior of the phospholipid monolayer. We have determined a critical point of the DPPC monolayer at a temperature of 44 °C and a surface pressure of 57 mN/m. Beyond this critical point, no phase transition can exist in the DPPC monolayer, either by isothermal compression or by isobaric heating. The melting process of the DPPC monolayer studied here provides novel insights into the understanding of a wide range of physicochemical and biophysical phenomena, such as surface thermodynamics, critical phenomena, and biophysical study of pulmonary surfactants.