Understanding and determination of surface free energies of both liquid and solid surfaces play an important role in a wide range of scientific and industrial areas, such as material sciences, polymers and particles, sprays and coatings, textiles and adhesives, soaps and detergents, oil recovery, printing industry, semiconductor industry, paper industry, food industry, cosmetics, pharmaceuticals, and biocompatibility of medical implants. The surface free energy of a liquid is measured by its surface tension and the surface free energy of a solid surface can be revealed by contact angle measurements.

Research in our lab is centered on developing experimental methodologies, both hardware and software, of measuring surface tension and contact angle using drop shape analysis. Principle of drop shape analysis is: in equilibrium, the shape of a drop/bubble is determined by the balance between gravity, which tends to deform the drop (elongate a pendant drop or flatten a sessile drop), and surface tension forces, which tend to hold the drop spherical. This force balance is governed by the Laplace equation of capillarity. If the shape of a drop/bubble is known (e.g., by photographing), it is possible to determine surface tension by inversely solving the Laplace equation.

Schematic of drop shape analysis and typical applications centered around the study of surface energetics. Pulmonary surfactant should decrease surface tension down to near-zero, thus maintaining the normal respiratory mechanics. For soaps and detergents, it is important to measure dynamic surface tension during adsorption and to determine the critical micelle concentration. Medical implants, such as artificial joints, should be wet with body fluids for lubrication; while periscope windows need to be nonwetting for a clear view

Based on drop shape analysis, we have developed advanced surface tensiometry techniques, such as the captive bubble surfactometer (CBS) and the constrained sessile drop (CSD), that are capable of measuring both static and dynamic surface tensions. These methods are ideal for studying lung surfactant and other biological fluids in terms of the ability of closely mimicking the physiological conditions and the capacity of fully automatic measurements.

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Summary of four experimental methods for studying surface tension of lung surfactant. They are the Langmuir-Wilhelmy balance (LWB), the pulsating bubble surfactometer (PBS), the captive bubble surfactometer (CBS), and the newly developed constrained sessile drop (CSD). All these methods are capable of studying the three most important biophysical properties of lung surfactant, i.e., rapid adsorption, near-zero surface tension upon compression, and limited surface tension increase upon expansion. The relative merits and flaws of these tensiometry techniques are discussed. The figure is adapted from a poster by Zuo et al. for the first International Conference on Lung Innate Immunity and Pulmonology, in Toronto 2008.

The key of our experimental method is Axisymmetric Drop Shape Analysis (ADSA). ADSA is an advanced software package capable of determining surface tension and contact angle from all three commonly used drop/bubble configurations: pendant drop, sessile drop, and captive bubble. ADSA is superior to other commercial software of drop shape analysis in a number of key points. For example, ADSA utilizes a unique mathematical algorithm that ensures convergence and rapid calculation. The average analyzing time is less than 1 sec per image, thus permitting real-time measurements. In addition, ADSA features an advanced image analysis scheme that allows accurate and fully automatic measurement. With a user-friendly PC interface, ADSA allows rapid surface tension and contact angle measurements on one click without the need of previous training and knowledge of surface science.

The ADSA software package is commercially available. Please contact Dr. Zuo or Dr. Neumann, for details.

This demo shows how ADSA analyzes a simple captive bubble image. Input parameters can be readily set up and saved for analyzing multiple images later. The accuracy of the measurement can be examined by checking the superimposed experimental and theoretical profiles. If demo does not play, just refresh this web page by clicking F5.

Research in our lab focuses on the biophysical study of lung surfactant from both macroscopic and microscopic points of view. On the one hand, we study surface tension of lung surfactant using advanced surface tensiometry. On the other hand, we probe direct molecular interactions between phospholipids and proteins at the interface using atomic force microscopy (AFM). This multidisciplinary approach allows us to explore the biophysical properties of lung surfactant in detail. The ultimate goal of this research is to translate the fundamental study to biomedical and clinical practice of lung surfactant in treating infant and adult respiratory diseases. It should be noted that the approach we developed in studying lung surfactant films is also applicable to the study of other biomembranes and self-assembled monolayers, bilayers, and multilayers.

This demo shows how ADSA analyzes multiple images with only one set of user-specific parameters. Once the input parameters are set, multiple images in the same experimental run can be analyzed automatically without further human intervention. ADSA also has a function of analyzing multiple runs of images, with which a large amount of data generated in studying dynamic surface tension or contact angle can be processed with ease. If demo does not play, just refresh this web page by clicking F5.