Micro-Surface and -Interfacial Tensions Measured Using the Micropipette Technique: Applications in Ultrasound-Microbubbles, Oil-Recovery, Lung-Surfactants, Nanoprecipitation, and Microfluidics

Abstract

This review presents a series of measurements of the surface and interfacial tensions we have been able to make using the micropipette technique. These include: equilibrium tensions at the air-water surface and oil-water interface, as well as equilibrium and dynamic adsorption of water-soluble surfactants and water-insoluble and lipids. At its essence, the micropipette technique is one of capillary-action, glass-wetting, and applied pressure. A micropipette, as a parallel or tapered shaft, is mounted horizontally in a microchamber and viewed in an inverted microscope. When filled with air or oil, and inserted into an aqueous-filled chamber, the position of the surface or interface meniscus is controlled by applied micropipette pressure. The position and hence radius of curvature of the meniscus can be moved in a controlled fashion from dimensions associated with the capillary tip (~5–10 μm), to back down the micropipette that can taper out to 450 μm. All measurements are therefore actually made at the microscale. Following the Young–Laplace equation and geometry of the capillary, the surface or interfacial tension value is simply obtained from the radius of the meniscus in the tapered pipette and the applied pressure to keep it there. Motivated by Franklin’s early experiments that demonstrated molecularity and monolayer formation, we also give a brief potted-historical perspective that includes fundamental surfactancy driven by margarine, the first use of a micropipette to circuitously measure bilayer membrane tensions and free energies of formation, and its basis for revolutionising the study and applications of membrane ion-channels in Droplet Interface Bilayers. Finally, we give five examples of where our measurements have had an impact on applications in micro-surfaces and microfluidics, including gas microbubbles for ultrasound contrast; interfacial tensions for micro-oil droplets in oil recovery; surface tensions and tensions-in-the surface for natural and synthetic lung surfactants; interfacial tension in nanoprecipitation; and micro-surface tensions in microfluidics.

Acid Catalysis with Alkane/Water Microdroplets in Ionic Liquids

“… 47 , 48 The mixture of the highly polar but soft IL with the apolar, extremely soft C 14–18 n -alkanes, generates a calculated interfacial tension γ AB > 15 mN·m –1 , which is within the accepted values to spontaneously generate stable microdroplets in liquid–liquid mixtures. 49 − 52 The slight decrease of interfacial tension produced by the increasing temperature is apparently compensated by the higher mobility of the organic molecules in the IL, to give the final microstructured IL/alkane material. 53 …”

Antibacterial Action of Nanoparticles by Lethal Stretching of Bacterial Cell Membranes

“…From the values of the surface tension and the bilayer contact angle θ, which were obtained from pendant drop measurements or optical microscopy images (see Figure 6), respectively, the bilayer tension can be calculated using Young's equation. [ 74 ] …”

Formation of nanocrystalline graphite in polymer-derived SiCN by polymer infiltration and pyrolysis at a low temperature

“…These pore spaces were completely filled by precursor solution after the infiltration process which stayed within them in the following process due to surface tension. For a cylindrical capillary tube, the pressure (P) on the liquid originated from surface tension can be expressed by [35]:…”