Multimodal Miniature Surface Forces Apparatus (μSFA) for Interfacial Science Measurements

Abstract

Advances in the research of intermolecular and surface interactions result from the development of new and improved measurement techniques and combinations of existing techniques. Here, we present a new miniature version of the Surface Force Apparatus -the µSFA -that has been designed for ease of use and multi-modal capabilities with retention of the capabilities of other SFA models including accurate measurement of surface separation distance and physical characterization of dynamic and static physical forces (i.e., normal, shear, and friction) andinteractions (e.g., van der Waals, electrostatic, hydrophobic, steric, bio-specific). The small physical size of the µSFA, compared to previous SFA models, makes it portable and suitable for integration into commercially available optical and fluorescence light microscopes, as demonstrated here. The large optical path entry and exit ports make it ideal for concurrent force measurements and spectroscopy studies. Examples of the use of the µSFA in combination with surface plasmon resonance (SPR) and Raman spectroscopy measurements are presented. Due to the short working distance constraints associated with Raman spectroscopy, an interferometric technique was developed and applied for calculating the inter-surface separation distance based on Newton's rings. The introduction of the µSFA will mark a transition in SFA usage from primarily physical characterization to concurrent physical characterization with in situ chemical and biological characterization to study interfacial phenomena, including (but not limited to) molecular adsorption, fluid flow dynamics, determination of surface species and morphology, and (bio-)molecular binding kinetics.

A Method to Quantify Molecular Diffusion within Thin Solvated Polymer Films: A Case Study on Films of Natively Unfolded Nucleoporins

“…It should be noted here that the generation of confined spaces using a plane–sphere geometry is a well-known procedure. The surface force apparatus, for example, combines this geometry with exquisite sensitivity in force and separation distance, to measure interaction forces between functionalized surfaces; more recently, this approach has also been combined with optical analysis for concurrent studies of the molecular organization and diffusion within the confined fluids by microscopy and spectroscopy techniques. − Plane-sphere geometries have also already been combined with optical microscopy for improved single-molecule imaging − or to visualize dynamic processes under confinement as diverse as blood clot formation, lubricant transfer during interfacial shear, and capillary condensation . Distinct aspects of the here-described method are the application to solvated polymer films and its ease of integration with existing microscopes and imaging modalities, where a substantially static and constant contact force is beneficial to control the compression of the polymer film.…”

Interface-Sensitive Raman Microspectroscopy of Water via Confinement with a Multimodal Miniature Surface Forces Apparatus

“…The curved surface approached the flat one while images of NR were recorded with the CMOS camera. Following the experiment, distances were measured using the NR analysis described recently 29 and summarized in Figure 3B. Briefly, we acquire images of the NR during the force measurement, which are radially-averaged around their symmetry point to obtain the radial intensity profile I(r).…”

“…As such, we combine one measurement from each technique that has high confidence: the relative distance measurement from the NR (which is accurate to within 0.5 nm 29 ) and the relative OH stretch integrated intensity from the Raman, allowing us to estimate the absolute separation distance at contact Do. We performed a regression analysis using Equation 4 on the measured values of ∆x and Ro/i over the full distance regime between glass-glass immersed in 10 mM KCl, as shown in Figure 6.…”

“…We and others showed that the μSFA is easily coupled with various techniques such as fluorescence microscopy, surface plasmon resonance, and Raman spectroscopy. 29 For coupling with Raman spectroscopy, we implement a sphere-flat configuration (instead of the standard SFA crossedcylinder geometry), and image analysis of Newton's Rings (NR) is used to measure the separation distance with sub-nanometer accuracy 29 rather than the multiple beam interferometry between back-silvered mica surfaces that is used in the standard SFA technique.…”

Boc‐Protection on L‐DOPA: an Easy Way to Promote Underwater Adhesion

“…[8,[11][12][13][14] The role of hydrophobic groups in the removal of water has not been exploited likewise in designing underwater adhesives. It is worth noticing that in water the adhesive forces required to separate two hydrophobic surfaces are very high, even higher than the adhesive forces necessary to separate mfps from model mica surfaces, [15] and it is also much easier to remove water from two hydrophobic than hydrophilic surfaces. [16,17] The design of adhesives which include hydrophobic groups is likely to improve adhesion in wet environments, as recently reported.…”