Air-water Surface Research Articles

Influence of Solvent Polarity on Crocin Content and Surface Properties of Saffron (Crocus sativus L.) Extracts

The saffron composition is being widely studied for authenticity and traceability, but very few works have been carried out to investigate the relationship between the chemical and physico-chemical properties of saffron solutes and their technological functionality in colloidal systems. This study aims at evaluating the surface properties of saffron extracts obtained using solvents of different polarities to achieve extracts with different compositions in terms of the pattern and content of polar and medium polarity crocins. The air–water surface was evaluated alone and in the presence of Tween 20 at different surfactant–extract ratios. Saffron extracts were able to decrease the surface tension of the aqueous phase, indicating the presence of surface-active compounds. In the mixed saffron extract–Tween 20 systems, competitive adsorption at the air–water interface occurred when the surfactant was present at a low concentration, while at concentrations higher than the CMC, Tween 20 hindered the adsorption of the extract surface-active compounds. The results highlight the interesting technological functionality of saffron extracts for applications in colloidal systems. To better exploit their use in the design and development of formulated foods, nutraceutics and pharma products, further studies are needed to unravel the relationship between the composition of saffron extracts and corresponding surface activity.

Coupled effects of temperature and suction on the anisotropic elastic shear moduli of unsaturated soil

Elastic shear moduli of soil at various temperatures and suctions are important for analysing the serviceability limit state of energy piles and many other structures. Up to now, however, the coupled effects of temperature and suction on elastic shear modulus and the stiffness anisotropy, have not been well understood. This experimental study investigated the anisotropic elastic shear modulus of a compacted lateritic clay. A temperature and suction-controlled triaxial apparatus equipped with bender element probes and local strain measurements was used. Soil suctions from 0 to 300 kPa, and a temperature range of 5–40 °C were applied. The results at saturated and unsaturated conditions consistently reveal that the shear modulus is smaller after heating at a given stress and suction. Several mechanisms may contribute to this thermal-induced reduction in shear modulus, such as the heating-induced reduction of interparticle force and air–water surface tension. Moreover, the reduction in shear modulus upon heating depends on the shear plane and the degree of anisotropy changes.

A Rare Mono-Rhamnolipid Congener Efficiently Produced by Recombinant Pseudomonas aeruginosa YM4 via the Expression of Global Transcriptional Regulator irrE.

Rhamnolipids (RLs) are widely used biosurfactants produced mainly by Pseudomonas aeruginosa and Burkholderia spp. in the form of mixtures of diverse congeners. The global transcriptional regulator gene irrE from radiation-tolerant extremophiles has been widely used as a stress-resistant element to construct robust producer strains and improve their production performance. A PrhlA-irrE cassette was constructed to express irrE genes in the Pseudomonas aeruginosa YM4 of the rhamnolipids producer strain. We found that the expression of irrE of Deinococcus radiodurans in the YM4 strain not only enhanced rhamnolipid production and the strain's tolerance to environmental stresses, but also changed the composition of the rhamnolipid products. The synthesized rhamnolipids reached a maximum titer of 26 g/L, about 17.9% higher than the original, at 48 h. The rhamnolipid production of the recombinant strain was determined to be mono-rhamnolipids congener Rha-C10-C12, accounting for 94.1% of total products. The critical micelle concentration (CMC) value of the Rha-C10-C12 products was 62.5 mg/L and the air-water surface tension decreased to 25.5 mN/m. The Rha-C10-C12 products showed better emulsifying activity on diesel oil than the original products. This is the first report on the efficient production of the rare mono-rhamnolipids congener Rha-C10-C12 and the first report that the global regulator irrE can change the components of rhamnolipid products in Pseudomonas aeruginosa.

Surface Chemistry Study of Normal and Diseased Human Meibum Films Prior to and after Supplementation with Tear Mimetic Eyedrop Formulation

Ophthalmic nanoemulsions that can treat the deficiencies of meibum (MGS) in Meibomian gland disease and restore its functionality in the tear film are greatly sought. The Rohto Dry Aid (RDA) formulation employs TEARSHIELD TECHNOLOGYTM, which uses a multicomponent oil phase of polar and non-polar lipid-like molecules selected to mimic the profiles of healthy meibum. Thus, the interactions of RDA with “diseased” Meibomian (dMGS) films merit deeper analysis, as these interactions might offer important clues for both the development of new ocular formulations and the processes behind the therapeutic action of the nanoemulsions. Pseudobinary dMGS/RDA films were spread at the air–water surface of the Langmuir trough. Surface pressure-area isocycles and stress relaxations were used to access the layer’s response to blink-like cycling and dilatational viscoelasticity, respectively, while film morphology was recorded via Brewster angle microscopy. It was found that RDA is able to reverse the brittleness and to restore the stability of “diseased” MGS films and thus to revert the layer’s properties to the functionality of healthy Meibomian lipids. Therefore, in order to effectively treat dry eyes with MGS-oriented therapy, ophthalmic nanoemulsions warrant more research.

Interfacial Activity and Surface pKa of Perfluoroalkyl Carboxylic Acids (PFCAs).

Perfluoroalkyl carboxylic acids (PFCAs) are widely used synthetic chemicals that are known for their exceptional stability and interfacial activity. Despite their industrial and environmental significance, discrepancies exist in the reported pKa values for PFCAs, often spanning three to four units. These disparities stem from an incomplete understanding of how pH influences the ionized state of PFCA molecules in the bulk solution and at the air-water interface. Using pH titration and surface tension measurements, we show that the pKa values of the PFCAs adsorbed at the air-water interface differ from the bulk. Below the equivalence point, the undissociated and dissociated forms of the PFCAs exist in equilibrium, driving to the spontaneous adsorption and reduced air-water surface tension. Conversely, above the equivalence point, the complete ionization of the headgroup into the carboxylate form renders PFCAs highly hydrophilic, resulting in reduced interfacial activity of the molecules. The distinction in the chemical environments at the interface and bulk results in differences in the pKa of PFCA molecules in the bulk phase and at the air-water interface. We explore the effects of the fluoroalkyl tail length of PFCAs on their surface pKa and interfacial activity across a broad pH range. We further demonstrate the influence of pH-dependent ionized state of PFCAs on their foamability and the rate of microdroplet evaporation, understanding of which is crucial for optimizing their industrial applications and developing effective strategies for their environmental remediation. This study underscores the potential significance of pH in directing the interfacial activity of PFCAs and prompts the inclusion of pH as a key determinant in the predictions of their fate and potential risks in the environment.

Experimental study and modeling of water film thickness in aero-engine under air–water mist flow

The water film not only plays an important role in the mass, momentum, and energy transfer between the air–water-surface but also determines the on-wing washing effect of the aero-engine. In view of this, air–water mist flow visualization experiments have been conducted at different gas velocities and water-to-air ratios in a compressor cascade, and the microscopic water film images have been analyzed to extract the transient water film thickness data by the Matlab code. It was found that the transient water film thickness fluctuation has no obvious association with the gas velocity, and the water film fluctuation is more affected by the water-to-air ratio. As the water-to-air ratio increases, the fluctuation magnitude of the water film thickness increases. The average water film thickness has been studied in relation to gas velocity and water flow rate, i.e., the average thickness of water film decreases with increasing gas velocity and increase with the increasing water flow rate. On the basis of the water film flow equation and taking the droplet collection efficiency into account, i.e., from the perspective of the physical mechanism of water film formed, a new model for predicting the water film thickness of a compressor blade surface under the air–water mist flow condition was proposed and validated. This model predicted, with a root mean square error and the mean absolute percentage error of 11.6% and 9.15%, respectively, under the present experimental flow conditions.

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.

Hydromechanical tensile strength modelling at particle size level for non cohesive granular materials

The aim of this paper is to propose a robust yet simple model to predict the variation of the tensile strength of granular materials with the degree of saturation. Because capillary phenomenon and air–water surface tension govern interparticle bonding in granular materials, they are explicitly taken into consideration. These vary with the degree of saturation as with all geotechnical structures that are subjected to wetting–drying processes. To predict the influence of capillary and air–water surface tension effects, mathematical equations are derived to simulate a porous model. The model is used to predict the drainage process in the porous network of the material and thus determine the relationship between interparticle bonding and the degree of saturation of the granular material. These hydraulic processes are expressed in mechanical terms using the effective stress concept thus providing the key for hydromechanical coupling. The parameters for the resulting equations can be obtained from conventional well-known laboratory tests: the water retention curve, the grain-size distribution of the material and its void ratio. The numerical results are confronted to laboratory tests showing that this approach predicts accurately the tensile strength of granular materials.

Movements of Magnetite-Encapsulated Graphene Particles at Air-Water Interface and Their Cell Growths under Dynamic Magnetic Field.

The motion of magnetic particles under magnetic fields is an object to be solved in association with basic and practical phenomena. Movement phenomena of magnetite-encapsulated graphene particles at air-water interfaces were evaluated by manufacturing a feedback control system of the magnetic field to cause the motion of particles due to magnetic torque. A homogeneous magnetic field was generated using two pairs of electromagnets located perpendicular to each other, which were connected to an electronic switch. The system influenced the translational movement and the self-rotational speed of magnetic particles located at a center on the surface of fluid media in a continuous duty cycle. Operating the particle at a remote control in the same duty cycle at the air-water surface, the short and elongated magnetic particles successfully rotated. In addition, the rotational speed of the curved particle was slower than that of the elongated particle. The results indicate that the translational and self-rotational movements of magnetite-encapsulated graphene particles at the air-water interface under the external magnetic field are size- and shape-dependent for the speed and the direction. A short magnetic particle was used as a target particle to rotate on cancer cell lines, aiming to study the advantage of this method to induce the growth of HeLa cells. It was monitored for up to 4 days with and without magnetic particles by checking the viability and morphology of cells before and after the electromagnetic treatment. As an outcome, the movement of magnetic particles reduced the number of biological cells, at least on HeLa cells, but it was inactive on the viability of HeLa cells.

Distinctive Heterogeneous Reaction Mechanism of ClNO2 on the Air-Water Surface Containing Cl.

The heterogeneous reaction of nitryl chloride (ClNO2) on the air-water surface plays a significant role in the chloride lifecycle. The air-water surface is ubiquitous on ice surfaces under supercooled conditions, affecting the uptake and heterogeneous reaction processes of trace gases. Previous studies suggest that ClNO2 is formed on Cl-doped ice surfaces following the N2O5 uptake. Herein, a distinctive heterogeneous reaction mechanism of ClNO2 is suggested on an air-water surface containing Cl under supercooled conditions using combined classic molecular dynamics (MD) and Born-Oppenheimer MD simulations. It is found that N2O5 dissociates into a NO2+ and NO3- ionic pair on the top air-water surface. In the top layer of the surface containing barely any Cl-, NO2+ proceeds through hydrolysis and produces H3O+ and HNO3. Thus, surface acidification appears because of H3O+ yields. With NO2+ diffusion to the deep layer of the surface, NO2+ reacts with Cl- and forms ClNO2. Note that ClNO2 formation competes with NO2+ hydrolysis, and the rate of ClNO2 formation is 27.7[Cl-] larger than that of NO2+ hydrolysis. Afterward, the reaction of ClNO2 with Cl- becomes barrierless with the catalysis by H3O+, which is not feasible on a neutral surface. Cl2 is thus generated and escapes into the atmosphere (low solubility of Cl2), contributing to the Cl radical. The proposed mechanism bolsters the current understanding of ClNO2's fate and its role in Cl chemistry in extremely cold environments like the Arctic and other high-latitude regions in wintertime.

One-dimensional drift-flux model for two-phase flow in NaCl solutions: The role of surface mobility

The presence of dissolved salts in water inhibits bubble coalescence, facilitating the generation of microbubbles, while also maintaining a tangentially mobile air-water surface. In this study, we investigated the variation of bubble size and gas holdup with gas velocity in NaCl solutions using a rectangular bubble column and interpreted the experimental data using the one-dimensional drift-flux model. For the generated bubbles at the size between 0.4 mm and 1 mm, we found that the Moore model assumes mobile air-water surfaces, rather than the Schiller and Naumann model, should be used to describe the bubble terminal velocity. The latter model, which assumes an immobile air-water surface, has been used successfully to predict two-phase flow in surfactant solutions but would overestimate the gas holdup by almost 100 % for NaCl solutions. This finding emphasizes the crucial role of surface mobility in determining gas holdup. Thus, we propose the use of a one-dimensional drift-flux model, incorporating the Moore model to describe bubble terminal velocity, for microbubble systems with a mobile air-water interface.

Disturbance Observer-Based Robust Take-Off Control for a Semi-Submersible Permeable Slender Hybrid Unmanned Aerial Underwater Quadrotor

The development of hybrid unmanned aerial underwater vehicles (HAUVs) compatible with the advantages of the aerial vehicles and the underwater vehicles is of great significance. This paper presents the first study on a new HAUV layout using four rotors to realize the medium crossing motion of a transverse slender body similar to the fuselage of a missile or a submarine, that is, the hybrid aerial underwater quadrotor (HAUQ). Then, a robust control strategy is proposed for the take-off HAUQ on the water in the presence of unknown disturbances and complex model dynamic uncertainties. As a semi-submersible HAUQ rises straight from the water, the inside of the slender fuselage placed horizontally is filled with water. The center of the mass, the moment of inertia, and the arm of the force of the HAUQ will change rapidly in the take-off phase from the water because of the rapid nonuniform change in mass caused by the passive fast drainage. It is difficult to establish an accurate mathematical model of the complex dynamic changes caused by the multi-media dynamics, the fast changing buoyancy, and the added mass crossing the air–water surface. Therefore, an uncertain kinematic and dynamic model is established through the passive, fast, nonuniform change and the complex dynamics are considered as the unknown terms, and the external disturbances of gust and other factors are assumed as the bounded disturbance input. A robust design approach is introduced to deal with the fast time-varying mass disturbance based on the input-to-state stability (ISS) theorem. The complex dynamics are estimated using the basis function and the unknown weight parameters, and the adaptive laws are adopted for the on-line estimation of the unknown weight parameters. Considering the residual disturbance of the uncertain nonlinear system as a total disturbance term, a disturbance observer is introduced for disturbance observation. The numerical simulation shows the feasibility and robustness of the proposed algorithm.

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.

Analysis of fluid dynamic behavior and impact load on oblique water entry of a two-dimensional seaplane based on VOF method

Seaplane landing is a typical water entry impact phenomenon. However, our understanding on the development of jet flow and air cavity, and corresponding load characteristic is still limited. In this paper, the impact problem of oblique water entry of a two-dimensional seaplane is studied to clarify the relationship between theses complex flows and loads. A gas-liquid two-phase model is established to model these complex fluid flows, where the air-water interface is captured based on a VOF algorithm. The accuracy is validated by the published experimental data. Some hydrodynamic impact characteristics related to seaplane model under different oblique water entry postures, including air-water free surface and local impact pressure are discussed. Complicated flows (air cavity and jet flow impact) are the internal cause to some unusual load characteristics such as multiple pressure peaks, negative pressure, simultaneity of impact load on the airfoil, and great difference of load at different positions on the cabin. The evolution of jet flow and air cavity is further discussed through combing free surface and pressure distribution. Results clarified the mechanism of complex flows under the influence of bottom cabin and inclined postures, which will be helpful to further understand the impact phenomenon of seaplane landing.

The Performance of Carbonate-Modified Nonionic Surfactants in Microplastic Flotation

For a group of nonionic surfactants with polar head groups synthesized by the copolymerization of ethylene oxide and carbon dioxide, the potential for their application in the foam flotation of microplastic particles was investigated. Their performance was compared to conventional that of nonionic surfactants and correlated with their adsorption behavior at the water–air surface through the maximum-bubble-pressure technique. The carbonate content of the surfactant head groups increased the biodegradability of the surfactants and affected their foaming properties and their performance in the flotation process. Since the inserted carbonate units increased the hydrophilicity of the whole surfactant but decreased the polarity of their headgroups, the number of carbonate units can be considered as an independent tuning parameter for adjusting these surfactants to a particular application. With this feature, the countercurrent trends of the two most important performance indicators, namely the removal rate and enrichment factor, can be resolved.

An Integrated Interferometric Fiber Optic Sensor Using a 638 nm Semiconductor Laser for Air-Water Surface Velocity Measurements.

An integrated interferometric fiber optic velocimetry sensor has been proposed and demonstrated at the central wavelength of 638 nm. The sensor is based on the principle of two laser-beams' interference. The light signal scattered from the particles or vapor is demodulated to measure the water surface velocity and water vapor velocity. Three velocity measurement experiments are carried out to measure the velocity, and the experimental data shows that the velocity increases linearly in the range of 4 mm·s-1 to 100 mm·s-1, with a slope of linear fitting curve of 0.99777 and the R-Square of 1.00000. The velocity calculated from frequency shift fits well with the reference velocity. The maximum average relative error in the three velocity measurements is less than 2.5%. In addition, the maximum speed of 4.398 m·s-1 is confirmed in the rotating disk calibration experiment, which expands the sensor's velocity measurement range. To solve the problem that it is difficult to directly measure the velocity of small-scale water surface flow velocity, especially from the aspect of the low velocity of air-water surface, the interferometric fiber optic sensor can be applied to the measurement of water surface velocity and wind velocity on the water surface.

How effective is alternating pressure (active) air surfaces for preventing pressure ulcers? A Cochrane Review summary with commentary.

Pressure ulcers may develop in people with impaired mobility, sensation, or cognition. Alternating pressure (active) air beds, overlays and mattresses are commonly used to prevent pressure ulcers. This Cochrane Review aimed to determine the effects of alternating pressure (active) air beds, overlays or mattresses compared with any support surface in preventing pressure ulcers. The population addressed was people at risk of and with existing pressure ulcers. Studies comparing alternating pressure (active) air surfaces with any beds, overlays or mattresses were included. The outcomes studied were pressure ulcer incidence, patient support-surface-associated comfort, adverse events, health-related quality of life and cost-effectiveness. There were 32 studies with a total of 9058 participants. There is low certainty evidence that alternating pressure (active) air surfaces compared with foam surfaces may reduce the incidence of pressure ulcers. It is uncertain whether there is a difference in the proportion of people developing new pressure ulcers between alternating pressure (active) air surfaces and reactive water-filled, fibre, air, gel or standard hospital surfaces. The use of alternating pressure (active) air surfaces may reduce the incidence of pressure ulcers compared to foam surfaces. However, it is uncertain if it is superior to reactive air surfaces, water surfaces and fiber surfaces in preventing pressure ulcers.

Effect of Model Tear Film Lipid Layer on Water Evaporation.

A majority of in vitro models were incapable of reproducing the evaporation resistance of tear film lipid layer (TFLL) in vivo. The purpose of this research is to develop a novel in vitro model to study the effect of TFLL on water evaporation. A ventilated, closed-chamber, droplet evaporimeter with a constant surface area has been invented to study the evaporation resistance of TFLL. This evaporimeter ensures a rigorous control of environmental conditions, including the temperature, relative humidity, airflow rate, surface area, and surface pressure, thus allowing for reproducible water evaporation measurements over a time period of only 5 minutes. The volumetric evaporation rate of this droplet evaporimeter is less than 2.7 µL/min, comparable to the basal tear production of healthy adults. Together with direct film imaging using atomic force microscopy (AFM), we have studied the effect of a model TFLL on water evaporation, as a function of the lipid composition and surface pressure. A model TFLL composed of 40% wax esters, 40% cholesteryl esters, and 20% polar lipids was capable of reducing the water evaporation rate by 11% at surface pressure 47 mN/m. AFM revealed that the model TFLL at high surface pressures consists of discrete droplets/aggregates of the nonpolar lipids residing atop a polar lipid monolayer with phase separation. The TFLL may resist water evaporation with a combined mechanism by increasing film compactness of the polar lipid film at the air-water surface, and, to a lesser extent, by increasing film thickness of the nonpolar lipid film.

A Molecular Dynamic Study of the Effects of Surface Partitioning on the OH Radical Interactions with Solutes in Multicomponent Aqueous Aerosols

The surface-bulk partitioning of small saccharide and amide molecules in aqueous droplets was investigated using molecular dynamics. The air-particle interface was modeled using a 80 Å cubic water box containing a series of organic molecules and surrounded by gaseous OH radicals. The properties of the organic solutes within the interface and the water bulk were examined at a molecular level using density profiles and radial pair distribution functions. Molecules containing only polar functional groups such as urea and glucose are found predominantly in the water bulk, forming an exclusion layer near the water surface. Substitution of a single polar group by an alkyl group in sugars and amides leads to the migration of the molecule toward the interface. Within the first 2 nm from the water surface, surface-active solutes lose their rotational freedom and adopt a preferred orientation with the alkyl group pointing toward the surface. The different packing within the interface leads to different solvation shell structures and enhanced interaction between the organic molecules and absorbed OH radicals. The simulations provide quantitative information about the dimension, composition, and organization of the air-water interface as well as about the nonreactive interaction of the OH radicals with the organic solutes. It suggests that increased concentrations, preferred orientations, and decreased solvation near the air-water surface may lead to differences in reactivities between surface-active and surface-inactive molecules. The results are important to explain how heterogeneous oxidation mechanisms and kinetics within interfaces may differ from those of the bulk.

Critical point for membrane bilayer formation

Unilamellar liposomes often are employed in investigations of lipid-protein interactions and the delivery of drugs in therapies for disease. Also, related lipid-containing nanoparticles have been developed as elements of a new class of mRNA vaccines. We show that only unilamellar films form in equilibrium lipid dispersions, at temperature values {T*} that depend on the identities of the lipids (e.g., T* ≈ 29 °C for DMPC). Thermodynamic analysis confirms that films at air-water surfaces can be used to monitor the properties of the lipid vesicles that form in the dispersion. When T > T*, critical exponents describing film properties as T approaches T* are μ ≈ 1.4 and ν ≈ 0.7, which are close to values for the interfacial tension and the correlation length of density fluctuations at fluid interfaces. These results, and observations that within the bilayer the lateral diffusion of fluorescent lipid probes demonstrates increases at T*, suggest that unilamellar vesicles at T* are a transition state between two different multilamellar structures. We generalize the thermodynamic arguments to explain the linkage between lipid structures in the surface and bulk dispersion within more complex samples, showing that dispersions containing total lipid extracts of cell membranes have properties similar to those in dispersions containing single lipids. Information from various independent studies indicates that T* noted for bilayer membranes of a population of cells is identical to the temperature at which the growth or gestation of the cells occurs in vivo. Examples include whole-cell lipid extracts obtained from bacteria, and poikilothermic and homeothermic animals.