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.

The biophysical function of pulmonary surfactant.

The type II pneumocytes of the lungs secrete a mixture of lipids and proteins that together acts as a surfactant. The material forms a thin film on the surface of the liquid layer that lines the alveolar air sacks. When compressed by the decreasing alveolar surface area during exhalation, the films reduce surface tension to exceptionally low levels. Pulmonary surfactant is essential for preserving the integrity of the barrier between alveolar air and capillary blood during normal breathing. This review focuses on the major biophysical processes by which endogenous pulmonary surfactant achieves its function and the mechanisms involved in those processes. Vesicles of pulmonary surfactant adsorb rapidly from the alveolar liquid to form the interfacial film. Interfacial insertion, which requires the hydrophobic surfactant protein SP-B, proceeds by a process analogous to the fusion of two vesicles. When compressed, the adsorbed film desorbs slowly. Constituents remain at the surface at high interfacial concentrations that reduce surface tensions well below equilibrium levels. We review the models proposed to explain how pulmonary surfactant achieves both the rapid adsorption and slow desorption characteristic of a functional film.

pH-Mediated Mucus Penetration of Zwitterionic Polydopamine-Modified Silica Nanoparticles.

Zwitterionic polymers have emerged as promising trans-mucus nanocarriers due to their superior antifouling properties. However, for pH-sensitive zwitterionic polymers, the effect of the pH microenvironment on their trans-mucus fate remains unclear. In this work, we prepared a library of zwitterionic polydopamine-modified silica nanoparticles (SiNPs-PDA) with an isoelectric point of 5.6. Multiple-particle tracking showed that diffusion of SiNPs-PDA in mucus with a pH value of 5.6 was 3 times faster than that in mucus with pH value 3.0 or 7.0. Biophysical analysis found that the trans-mucus behavior of SiNPs-PDA was mediated by hydrophobic and electrostatic interactions and hydrogen bonding between mucin and the particles. Furthermore, the particle distribution in the stomach, intestine, and lung demonstrated the pH-mediated mucus penetration behavior of the SiNPs-PDA. This study reveals the pH-mediated mucus penetration behavior of zwitterionic nanomaterials, which provides rational design strategies for zwitterionic polymers as nanocarriers in various mucus microenvironments.

Biophysical function of pulmonary surfactant in liquid ventilation.

Liquid ventilation is a mechanical ventilation technique in which the entire or part of the lung is filled with oxygenated perfluorocarbon (PFC) liquids rather than air in conventional mechanical ventilation. Despite its many ideal biophysicochemical properties for assisting liquid breathing, a general misconception about PFC is to use it as a replacement for pulmonary surfactant. Because of the high PFC-water interfacial tension (59 mN/m), pulmonary surfactant is indispensable in liquid ventilation to increase lung compliance. However, the biophysical function of pulmonary surfactant in liquid ventilation is still unknown. Here, we have studied the adsorption and dynamic surface activity of a natural surfactant preparation, Infasurf, at the PFC-water interface using constrained drop surfactometry. The constrained drop surfactometry is capable of simulating the intra-alveolar microenvironment of liquid ventilation under physiologically relevant conditions. It was found that Infasurf adsorbed to the PFC-water interface reduces the PFC-water interfacial tension from 59 mN/m to an equilibrium value of 9 mN/m within seconds. Atomic force microscopy revealed that after de novo adsorption, Infasurf forms multilayered structures at the PFC-water interface with an average thickness of 10-20 nm, depending on the adsorbing surfactant concentration. It was found that the adsorbed Infasurf film is capable of regulating the interfacial tension of the PFC-water interface within a narrow range, between ∼12 and ∼1 mN/m, during dynamic compression-expansion cycles that mimic liquid ventilation. These findings have novel implications in understanding the physiological and biophysical functions of the pulmonary surfactant film at the PFC-water interface, and may offer new translational insights into the development of liquid ventilation and liquid breathing techniques.

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.

Constrained drop surfactometry for studying adsorbed pulmonary surfactant at physiologically relevant high concentrations.

Exogenous surfactant therapy has been used as a standard clinical intervention for treating premature newborns with respiratory distress syndrome. The phospholipid concentrations of exogenous surfactants used in clinical practice are consistently higher than 25 mg/mL; while it was estimated that the phospholipid concentration of endogenous surfactant is approximately in the range between 15 and 50 mg/mL. However, most in vitro biophysical simulations of pulmonary surfactants were only capable of studying surfactant concentrations up to 3 mg/mL, one order of magnitude lower than the physiologically relevant concentration. Using a new in vitro biophysical model, called constrained drop surfactometry, in conjunction with atomic force microscopy and other technological advances, we have investigated the biophysical properties, ultrastructure, and topography of the pulmonary surfactant film adsorbed from the subphase at physiologically relevant high surfactant concentrations of 10-35 mg/mL. It was found that the effect of surfactant concentration on the dynamic surface activity of the surfactant film was only important when the surface area of the surfactant film varied no more than 15%, mimicking normal tidal breathing. The adsorbed surfactant film depicts a multilayer conformation consisting of a layer-by-layer assembly of stacked bilayers with the height of the multilayers proportional to the surfactant concentration. Our experimental data suggest that the biophysical function of these multilayer structures formed after de novo adsorption is to act as a buffer zone to store surface-active materials ejected from the interfacial monolayer under extreme conditions such as deep breathing.NEW & NOTEWORTHY An in vitro biophysical model, called constrained drop surfactometry, was developed to study the biophysical properties, ultrastructure, and topography of the pulmonary surfactant film adsorbed from the subphase at physiologically relevant high surfactant concentrations of 10-35 mg/mL. These results suggest that the biophysical function of multilayers formed after de novo adsorption is to act as a buffer zone to store surface-active materials ejected from the interfacial monolayer under extreme conditions such as deep breathing.

Adverse Biophysical Impact of e-Cigarette Flavors on Pulmonary Surfactant.

The attractiveness and abundance of flavors are primary factors eliciting youth to use e-cigarettes. Emerging studies in recent years revealed the adverse health impact of e-cigarette flavoring chemicals, including disruption of the biophysical function of pulmonary surfactants in the lung. Nevertheless, a comprehensive understanding of the biophysical impact of various flavoring chemicals is still lacking. We used constrained drop surfactometry as a new alternative method to study the biophysical impact of flavored e-cigarette aerosols on an animal-derived natural pulmonary surfactant. The dose of exposure to e-cigarette aerosols was quantified with a quartz crystal microbalance, and alterations to the ultrastructure of the surfactant film were visualized using atomic force microscopy. We have systematically studied eight representative flavoring chemicals (benzyl alcohol, menthol, maltol, ethyl maltol, vanillin, ethyl vanillin, ethyl acetate, and ethyl butyrate) and six popular recombinant flavors (coffee, vanilla, tobacco, cotton candy, menthol/mint, and chocolate). Our results suggested a flavor-dependent inhibitory effect of e-cigarette aerosols on the biophysical properties of the pulmonary surfactant. A qualitative phase diagram was proposed to predict the hazardous potential of various flavoring chemicals. These results provide novel implications in understanding the environmental, health, and safety impacts of e-cigarette aerosols and may contribute to better regulation of e-cigarette products.

Microplastics and Nanoplastics Impair the Biophysical Function of Pulmonary Surfactant by Forming Heteroaggregates at the Alveolar-Capillary Interface.

Microplastics (MPs) are ubiquitous environmental pollutants produced through the degradation of plastic products. Nanoplastics (NPs), commonly coexisting with MPs in the environment, are submicrometer debris incidentally produced from fragmentation of MPs. We studied the biophysical impacts of MPs/NPs derived from commonly used commercial plastic products on a natural pulmonary surfactant extracted from calf lung lavage. It was found that in comparison to MPs/NPs derived from lunch boxes made of polypropylene or from drinking water bottles made of poly(ethylene terephthalate), the MP/NP derived from foam packaging boxes made of polystyrene showed the highest adverse impact on the biophysical function of the pulmonary surfactant. Accordingly, intranasal exposure of MP/NP derived from the foam boxes also induced the most serious proinflammatory responses and lung injury in mice. Atomic force microscopy revealed that NP particles were adsorbed on the air-water surface and heteroaggregated with the pulmonary surfactant film. These results indicate that although the incidentally formed NPs only make up a small mass fraction, they likely play a predominant role in determining the nano-bio interactions and the lung toxicity of MPs/NPs by forming heteroaggregates at the alveolar-capillary interface. These findings may provide novel insights into understanding the health impact of MPs and NPs on the respiratory system.

Biophysical properties of tear film lipid layer I. Surface tension and surface rheology.

Tear film lipid layer (TFLL) is the outmost layer of the tear film. It plays a crucial role in stabilizing the tear film by reducing surface tension and retarding evaporation of the aqueous layer. Dysfunction of the TFLL leads to dysfunctional tear syndrome, with dry eye disease (DED) being the most prevalent eye disease, affecting 10%-30% of the world population. To date, except for treatments alleviating dry eye symptoms, effective therapeutic interventions in treating DED are still lacking. Therefore, there is an urgent need to understand the biophysical properties of the TFLL with the long-term goal to develop translational solutions in effectively managing DED. Here, we studied the composition-function correlations of an artificial TFLL, under physiologically relevant conditions, using a novel experimental methodology called constrained drop surfactometry. This artificial TFLL was composed of 40% behenyl oleate and 40% cholesteryl oleate, representing the most abundant wax ester and cholesteryl ester in the natural TFLL, respectively, and 15% phosphatidylcholine and 5% palmitic-acid-9-hydroxy-stearic-acid (PAHSA), which represent the two predominant polar lipid classes in the natural TFLL. Our study suggests that the major biophysical function of phospholipids in the TFLL is to reduce the surface tension, whereas the primary function of PAHSA is to optimize the rheological properties of the TFLL. These findings have novel implications in better understanding the physiological and biophysical functions of the TFLL and may offer new translational insight to the treatment of DED.

Biophysical properties of tear film lipid layer II. Polymorphism of FAHFA.

Fatty acid esters of hydroxy fatty acids (FAHFAs) are a newly discovered class of endogenous lipids that consist of two acyl chains connected through a single ester bond. Being a unique species of FAHFAs, (O-acyl)-ω-hydroxy fatty acids (OAHFAs) differ from other FAHFAs in that their hydroxy fatty acid backbones are ultralong and their hydroxy esterification is believed to be solely at the terminal (ω-) position. Only in recent years with technological advances in lipidomics have OAHFAs been identified as an important component of the tear film lipid layer (TFLL). It was found that OAHFAs account for approximately 4 mol% of the total lipids and 20 mol% of the polar lipids in the TFLL. However, their biophysical function and contribution to the TFLL is still poorly understood. Here we studied the molecular biophysical mechanisms of OAHFAs using palmitic-acid-9-hydroxy-stearic-acid (PAHSA) as a model. PAHSA and OAHFAs share key structural similarities that could result in comparable biophysical properties and molecular mechanisms. With combined biophysical experiments, atomic force microscopy observations, and all-atom molecular dynamics simulations, we found that the biophysical properties of a dynamic PAHSA monolayer under physiologically relevant conditions depend on a balance between kinetics and thermal relaxation. PAHSA molecules at the air-water surface demonstrate unique polymorphic behaviors, which can be explained by configurational transitions of the molecules under various lateral pressures. These findings could have novel implications in understanding biophysical functions that FAHFAs, in general, or OAHFAs, specifically, play in the TFLL.

Menthol in electronic cigarettes causes biophysical inhibition of pulmonary surfactant.

With an increasing prevalence of electronic cigarette (e-cigarette) use, especially among youth, there is an urgent need to better understand the biological risks and pathophysiology of health conditions related to e-cigarettes. A majority of e-cigarette aerosols are in the submicron size and would deposit in the alveolar region of the lung, where they must first interact with the endogenous pulmonary surfactant. To date, little is known whether e-cigarette aerosols have an adverse impact on the pulmonary surfactant. We have systematically studied the effect of individual e-cigarette ingredients on an animal-derived clinical surfactant preparation, bovine lipid extract surfactant, using a combination of biophysical and analytical techniques, including in vitro biophysical simulations using constrained drop surfactometry, molecular imaging with atomic force microscopy, chemical assays using carbon nuclear magnetic resonance and circular dichroism, and in silico molecular dynamics simulations. All data collectively suggest that flavorings used in e-cigarettes, especially menthol, play a predominant role in inhibiting the biophysical function of the surfactant. The mechanism of biophysical inhibition appears to involve menthol interactions with both phospholipids and hydrophobic proteins of the natural surfactant. These results provide novel insights into the understanding of the health impact of e-cigarettes and may contribute to better regulation of e-cigarette products.

Quantitative Determination of the Hydrophobicity of Nanoparticles.

The hydrophobicity of nanoparticles (NPs) is one of the most important physicochemical properties that determines their agglomeration state under various environmental conditions. When studying nano-bio interactions, it is found that the hydrophobicity of NPs plays a predominant role in mediating the biological response and toxicity of the NPs. Although many methods have been developed to qualitatively or quantitatively determine hydrophobicity, there is not yet a scientific consensus on the standard of characterizing the hydrophobicity of NPs. We have developed a novel optical method, called the maximum particle dispersion (MPD), for quantitatively characterizing the hydrophobicity of NPs. The principle of measurement of the MPD method lies in the control of the aggregation state of the NPs via manipulating the van der Waals interactions between NPs across a dispersion liquid. We have scrutinized the mechanism of the MPD method using a combination of dynamic light scattering and atomic force microscopy and further verified the MPD method using a completely independent dye adsorption method. The MPD method demonstrated great promise to be developed into an easy-to-use and cost-effective method for quantitatively characterizing the hydrophobicity of NPs.

Atomic Force Microscopy Imaging of Adsorbed Pulmonary Surfactant Films.

Pulmonary surfactant (PS) is a lipid-protein complex that adsorbs to the air-water surface of the lung as a thin film. Previous studies have suggested that the adsorbed PS film is composed of an interfacial monolayer, plus a functionally attached vesicular complex, called the surface-associated surfactant reservoir. However, direct visualization of the lateral structure and morphology of adsorbed PS films using atomic force microscopy (AFM) has been proven to be technically challenging. To date, all AFM studies of the PS film have relied on the model of Langmuir monolayers. Here, we showed the first, to our knowledge, AFM imaging of adsorbed PS films under physiologically relevant conditions using a novel, to our knowledge, experimental methodology called constrained drop surfactometry. In conjunction with a series of methodological innovations, including subphase replacement, in situ Langmuir-Blodgett transfer, and real-time surface tension control using closed-loop axisymmetric drop shape analysis, constrained drop surfactometry allowed the study of lateral structure and topography of animal-derived natural PS films at physiologically relevant low surface tensions. Our data suggested that a nucleation-growth model is responsible for the adsorption-induced squeeze-out of the PS film, which likely results in an interfacial monolayer enriched in dipalmitoylphosphatidylcholine with the attached multilayered surface-associated surfactant reservoir. These findings were further supported by frequency-dependent measurements of surface dilational rheology. Our study provides novel, to our knowledge, biophysical insights into the understanding of the mechanisms by which the PS film attains low surface tensions and stabilizes the alveolar surface.

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.

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.

An Optical Method for Quantitatively Determining the Surface Free Energy of Micro- and Nanoparticles.

Surface free energy (SFE) of micro- and nanoparticles plays a crucial role in determining the hydrophobicity and wettability of the particles. To date, however, there are no easy-to-use methods for determining the SFE of particles. Here, with the application of several inexpensive, easy-to-use, and commonly available lab procedures and facilities, including particle dispersion, settling/centrifugation, pipetting, and visible-light spectroscopy, we developed a novel technique called the maximum particle dispersion (MPD) method for quantitatively determining the SFE of micro- and nanoparticles. We demonstrated the versatility and robustness of the MPD method by studying nine representative particles of various chemistries, sizes, dimensions, and morphologies. These are triethoxycaprylylsilane-coated zinc oxide nanoparticles, multiwalled carbon nanotubes, graphene nanoplatelets, molybdenum(IV) sulfide flakes, neodymium(III) oxide nanoparticles, two sizes of zeolites, poly(vinylpolypyrrolidone), and polystyrene microparticles. The SFE of these micro- and nanoparticles was found to cover a range from 21 to 36 mJ/m2. These SFE values may find applications in a broad spectrum of scientific disciplines including the synthesis of these nanomaterials, such as in liquid-phase exfoliation. The MPD method has the potential to be developed into a standard, low-cost, and easy-to-use method for quantitatively characterizing the SFE and hydrophobicity of particles at the micro- and nanoscale.

Poly(amidoamine) Dendrimer as a Respiratory Nanocarrier: Insights from Experiments and Molecular Dynamics Simulations.

Pulmonary drug delivery is superior to the systemic administration in treating lung diseases. An optimal respiratory nanocarrier should be able to efficiently and safely cross the pulmonary surfactant film, which serves as the first biological barrier for respiratory delivery and plays paramount roles in maintaining the proper mechanics of breathing. In this work, we focused on the interactions between poly(amidoamine) (PAMAM) dendrimers and a model pulmonary surfactant. With combined Langmuir monolayer experiments and coarse-grained molecular dynamics simulations, we studied the effect of environmental temperature, size, and surface property of PAMAM dendrimers (G3-OH, G3-NH2, G5-OH, and G5-NH2) on the dipalmitoylphosphatidylcholine (DPPC) monolayer. Our simulations indicated that the environmental temperature could significantly affect the influence of PAMAM dendrimers on the DPPC monolayer. Therefore, results obtained at room temperature cannot be directly applied to elucidate interactions at body temperature. Simulations at body temperature found that all tested PAMAM dendrimers can easily penetrate the lipid monolayer during the monolayer expansion process (mimicking "inhalation"), and the cationic PAMAM dendrimers (-NH2) show promising penetration ability during the monolayer compression process (mimicking "expiration"). Larger PAMAM dendrimers (G5) adsorbed onto the lipid monolayer tend to induce structural collapse and inhibit normal phase transitions of the lipid monolayer. These adverse effects could be mitigated in the subsequent expansion-compression cycle. These findings suggest that the PAMAM dendrimer may be used as a potential respiratory drug nanocarrier.

Xylem Surfactants Introduce a New Element to the Cohesion-Tension Theory.

Vascular plants transport water under negative pressure without constantly creating gas bubbles that would disable their hydraulic systems. Attempts to replicate this feat in artificial systems almost invariably result in bubble formation, except under highly controlled conditions with pure water and only hydrophilic surfaces present. In theory, conditions in the xylem should favor bubble nucleation even more: there are millions of conduits with at least some hydrophobic surfaces, and xylem sap is saturated or sometimes supersaturated with atmospheric gas and may contain surface-active molecules that can lower surface tension. So how do plants transport water under negative pressure? Here, we show that angiosperm xylem contains abundant hydrophobic surfaces as well as insoluble lipid surfactants, including phospholipids, and proteins, a composition similar to pulmonary surfactants. Lipid surfactants were found in xylem sap and as nanoparticles under transmission electron microscopy in pores of intervessel pit membranes and deposited on vessel wall surfaces. Nanoparticles observed in xylem sap via nanoparticle-tracking analysis included surfactant-coated nanobubbles when examined by freeze-fracture electron microscopy. Based on their fracture behavior, this technique is able to distinguish between dense-core particles, liquid-filled, bilayer-coated vesicles/liposomes, and gas-filled bubbles. Xylem surfactants showed strong surface activity that reduces surface tension to low values when concentrated as they are in pit membrane pores. We hypothesize that xylem surfactants support water transport under negative pressure as explained by the cohesion-tension theory by coating hydrophobic surfaces and nanobubbles, thereby keeping the latter below the critical size at which bubbles would expand to form embolisms.

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.