http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

For several centuries fluid dynamics studies have relied upon the assumption that when a liquid flows over a solid surface, the liquid molecules adjacent to the solid are stationary relative to the solid. This no-slip boundary condition (BC) has been applied successfully to model many macroscopic experiments, but has no microscopic justification. In recent years there has been an increased interest in determining the appropriate BCs for the flow of Newtonian liquids in confined geometries, partly due to exciting developments in the fields of microfluidic and microelectromechanical devices and partly because new and more sophisticated measurement techniques are now available. An increasing number of research groups now dedicate great attention to the study of the flow of liquids at solid interfaces, and as a result a large number of experimental, computational and theoretical studies have appeared in the literature. We provide here a review of experimental studies regarding the phenomenon of slip of Newtonian liquids at solid interfaces. We dedicate particular attention to the effects that factors such as surface roughness, wettability and the presence of gaseous layers might have on the measured interfacial slip. We also discuss how future studies might improve our understanding of hydrodynamic BCs and enable us to actively control liquid slip.

https://iopscience.iop.org/article/10.1088/0034-4885/68/12/R05/pdf

Boundary slip in Newtonian liquids: a review of experimental studies

We present here a brief review of direct force measurements between hydrophobic surfaces in aqueous solutions. For almost 70 years, researchers have attempted to understand the hydrophobic effect (the low solubility of hydrophobic solutes in water) and the hydrophobic interaction or force (the unusually strong attraction of hydrophobic surfaces and groups in water). After many years of research into how hydrophobic interactions affect the thermodynamic properties of processes such as micelle formation (self-assembly) and protein folding, the results of direct force measurements between macroscopic surfaces began to appear in the 1980s. Reported ranges of the attraction between variously prepared hydrophobic surfaces in water grew from the initially reported value of 80–100 A to values as large as 3,000 A. Recent improved surface preparation techniques and the combination of surface force apparatus measurements with atomic force microscopy imaging have made it possible to explain the long-range part of this interaction (at separations >200 A) that is observed between certain surfaces. We tentatively conclude that only the short-range part of the attraction (<100 A) represents the true hydrophobic interaction, although a quantitative explanation for this interaction will require additional research. Although our force-measuring technique did not allow collection of reliable data at separations <10 A, it is clear that some stronger force must act in this regime if the measured interaction energy curve is to extrapolate to the measured adhesion energy as the surface separation approaches zero (i.e., as the surfaces come into molecular contact).

https://www.pnas.org/content/pnas/103/43/15739.full.pdf

Recent progress in understanding hydrophobic interactions

Form and functionality of starch

Surface nanobubbles are nanoscopic gaseous domains on immersed substrates which can survive for days. They were first speculated to exist about 20 years ago, based on stepwise features in force curves between two hydrophobic surfaces, eventually leading to the first atomic force microscopy (AFM) image in 2000. While in the early years it was suspected that they may be an artifact caused by AFM, meanwhile their existence has been confirmed with various other methods, including through direct optical observation. Their existence seems to be paradoxical, as a simple classical estimate suggests that they should dissolve in microseconds, due to the large Laplace pressure inside these nanoscopic spherical-cap-shaped objects. Moreover, their contact angle (on the gas side) is much smaller than one would expect from macroscopic counterparts. This review will not only give an overview on surface nanobubbles, but also on surface nanodroplets, which are nanoscopic droplets (e.g., of oil) on (hydrophobic) substrates immersed in water, as they show similar properties and can easily be confused with surface nanobubbles and as they are produced in a similar way, namely, by a solvent exchange process, leading to local oversaturation of the water with gas or oil, respectively, and thus to nucleation. The review starts with how surface nanobubbles and nanodroplets can be made, how they can be observed (both individually and collectively), and what their properties are. Molecular dynamic simulations and theories to account for the long lifetime of the surface nanobubbles are then reported on. The crucial element contributing to the long lifetime of surface nanobubbles and nanodroplets is pinning of the three-phase contact line at chemical or geometric surface heterogeneities. The dynamical evolution of the surface nanobubbles then follows from the diffusion equation, Laplace’s equation, and Henry’s law. In particular, one obtains stable surface nanobubbles when the gas influx from the gas-oversaturated water and the outflux due to Laplace pressure balance. This is only possible for small enough surface bubbles. It is therefore the gas or oil oversaturation ζ that determines the contact angle of the surface nanobubble or nanodroplet and not the Young equation. The review also covers the potential technological relevance of surface nanobubbles and nanodroplets, namely, in flotation, in (photo)catalysis and electrolysis, in nanomaterial engineering, for transport in and out of nanofluidic devices, and for plasmonic bubbles, vapor nanobubbles, and energy conversion. Also given is a discussion on surface nanobubbles and nanodroplets in a nutshell, including theoretical predictions resulting from it and future directions. Studying the nucleation, growth, and dissolution dynamics of surface nanobubbles and nanodroplets will shed new light on the problems of contact line pinning and contact angle hysteresis on the submicron scale.

/pdf/surface-nanobubbles-and-nanodroplets-17od1qfe93.pdf

Surface nanobubbles and nanodroplets

Gas bubbles of nanometer size were produced on atomically flat solid surfaces and imaged by atomic force microscopy (AFM) in tapping mode in water. In AFM images, nanobubbles appeared like blight spheres. Some of the bubbles remained stable for hours during the experiments. The bubbles were disturbed under high load during AFM imaging. A related mechanism is discussed. (C) 2000 American Vacuum Society. [S0734-211X(00)02505-1].

Nanobubbles on solid surface imaged by atomic force microscopy

Manipulating DNA molecules to form molecular patterns on a nanometer scale is a subject with wide prospects By applying a modified “molecular combing” technique and imaging in air with atomic force microscope, we aligned DNA molecules on a mica surface which was chemically modified with a small organic molecule, (3-aminopropyl)-triethanoxysilane Two-dimensional patterns of DNA molecules were also constructed

Molecular patterns by manipulating DNA molecules

Starch molecules were stretched and aligned by a special method on a mica surface and imaged with atomic force microscopy (AFM). The AFM images have shown the “chain” structure of the starch molecules. The results also show that there exist complicated structures in the molecules of natural starch and that the glucose units may form thicker “chains.” The length of the chains may reach 1–2 μm.

Study on the chain structure of starch molecules by atomic force microscopy

DNA molecules were stretched on silanized mica surface with the molecular combing technique, and detected with fluorescence microscopy and atomic force microscopy. Meantime, DNA molecules were stretched with a modified dynamic molecular combing technique and studied with atomic force microscopy. The results indicate that, compared with the dynamic molecular combing technique, the modified dynamic molecular combing technique has advantages of less-sample demand and less contamination to sample; as compared with the molecular combing technique, it has better aligning effect and reproducibility. Combination of this kind of DNA molecular manipulating technique with the single DNA molecule detecting technique by atomic force microscopy and fluorescence microscopy will play an important role in the basic research of molecular dynamics and the application of gene research.

Stretching and imaging studies of single DNA molecules

Manipulation of macromolecules in nanometer scale is becoming an interesting research field. An approach to manipulate supramolecular assemblies is reported in this article. Linear phage viruses were aligned in one direction on atomically flat surfaces by a special method called “molecular combing.” Atomic force microscopy was used to check the results. Most of the phage strands were found to be stretch straight from one end to another. A related mechanism is also discussed.

Zhenqian Ouyang

Papers

Nanobubbles on solid surface imaged by atomic force microscopy

Molecular patterns by manipulating DNA molecules

Study on the chain structure of starch molecules by atomic force microscopy

Stretching and imaging studies of single DNA molecules

Stretch and align virus in nanometer scale on an atomically flat surface