Instrumentation

Surfactants are ubiquitous interfacial materials that govern the behavior of countless biological, chemical, and industrial systems. Among them, pulmonary surfactant represents a particularly important biological example, forming a complex phospholipid–protein film that maintains alveolar stability by lowering surface tension at the lung air–liquid interface. Understanding the biophysical behavior of such surfactant systems requires specialized instrumentation capable of probing interfacial properties with high sensitivity, temporal resolution, and physiological relevance. Consequently, the development of advanced surfactometry techniques has become a critical area of research. These techniques provide indispensable tools for quantifying surface tension, adsorption kinetics, film compressibility, phase behavior, and dynamic interfacial responses, thereby enabling mechanistic studies of surfactant function, dysfunction, and therapeutic intervention. 

We have developed a next-generation droplet-based surface tensiometry and surfactometry platform termed constrained drop surfactometer (CDS). CDS is capable of studying both Langmuir monolayers formed by spreading insoluble surfactants and Gibbs films formed by adsorption of soluble surfactants.

Powered by closed-loop axisymmetric drop shape analysis (CL-ADSA), the CDS platform enables a broad range of surface, interfacial, and biophysical characterizations that are difficult or impossible to implement using conventional techniques. CDS transforms traditional drop shape analysis from an offline surface-tension measurement method into a real-time, intelligent, fully controlled characterization platform.

Fundamental technical advantages of CDS include:

• Ultrafast real-time surface tension measurements (sub-millisecond per image)

• Dynamic surface tension and contact angle measurements

• Compatibility with both the air-liquid surfaces and liquid-liquid interfaces

• Real-time feedback control of surface tension, droplet volume, and surface area

• Minimal sample consumption (nanoliter range for spreading; microliter range for adsorption)

• Miniaturized Langmuir film balance

• Capability for measurements at ultralow, near-zero surface and interfacial tensions

• Fully controlled environmental conditions, including physiologically relevant environments

• Evaporimetry under controlled environmental conditions

• Continuous environmental temperature variation

• Subphase replacement

• In situ Langmuir–Blodgett (LB) transfer

• Small-amplitude harmonic oscillation

• Quantitative characterization of interfacial dilatational rheology

Signature applications of CDS include:

• High-fidelity biophysical simulation of natural pulmonary surfactant function

• Evaporation resistance of the tear film lipid layer

• Inhalation toxicology of airborne particles and pathogens

• Interfacial rheology of surfactant systems

• Surface thermodynamics of phospholipid monolayers

• Molecular and colloidal self-assembly at the oil–water interface

• Xylem surfactants and plant biointerfaces