Abstract
The surface forces apparatus (SFA) was developed in the late 1960s as a powerful tool for investigating molecular interactions across apposing surfaces including the first measurement of van der Waals forces and interactions in biologic and liquid media. However, the SFA has two major disadvantages. First, it traditionally uses white light interference between back-silvered muscovite mica surfaces to measure distances and to infer forces from distance shifts during interaction of two surfaces. Hence, distance shifts and force measurement are not decoupled. Second, productive SFA interferometers are so far limited to measuring across mica versus mica or mica versus metal. Direct gold-gold configurations were suggested in the late 1990s but not experimentally achieved as proof-of-principle until recently using a templating technique. In this work, we show how we solve these two disadvantages. First, we present a new SFA design that decouples force and distance measurements with similar resolution. The presented SFA design is inexpensive and can be home-built with mostly commercially available parts. Second, we present an alternative physical vapor deposition approach to construct a stable gold-gold interferometer and demonstrate its performance showing hydrophobic interactions, bubble formation, hemifusion of bilayers, and friction experiments. The presented system is easy to use. The obtained results show excellent reproducibility, indicating that the designed SFA and the three-mirror gold-gold interferometer functions as well as or even better than the traditional interferometer configurations used in SFA. This opens SFA to a wide range of options for various possible applications. Specifically, the gold-gold configuration allows a broad range of surface modifications for studying biophysical interactions as demonstrated in this work.
Highlights
All active systems that undergo change, motion, or flow of matter are subject to fundamental forces that drive and steer the way in which atoms, molecules, and macroscopic structures develop, evolve, adapt, and age
We demonstrate the performance of the sensing SFA (sSFA) and the three-mirror interferometer with a set of three different experiments, increasing in complexity of the surface architecture and simultaneously measured parameters
The measured interaction is not related to a true short range hydrophobic effect between hydrophobic surfaces immersed in an electrolyte
Summary
All active systems that undergo change, motion, or flow of matter (i.e., all biological systems and all mechanical systems) are subject to fundamental forces that drive and steer the way in which atoms, molecules, and macroscopic structures develop, evolve, adapt, and age. The study of interactive forces is a shared and fundamental interest in seemingly unrelated fields such as biophysics and adhesion, or surface science and stem cell research. Techniques such as atomic force microscopy or using optical tweezers can measure forces at molecular-scale and nanoscale, while they do not actively track distances. The core of the SFA is multiple beam interferometry (MBI), an optical high precision distance measuring technique based on the interference of white light passing through an optical cavity. The spacer is necessary to generate a cavity that is in the range of the wavelength of white light. Muscovite mica was used, as it can be cleaved into uniformly thick spacer layers with an atomically smooth surface
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