Abdelhamid Maali

Hydrodynamics of oscillating atomic force microscopy cantilevers in viscous fluids

We present a study of thermal noise of commercially available atomic force microscopy (AFM) cantilevers in air and in water. The purpose of this work is to investigate the oscillation behavior of a clamped AFM microlever in liquids. Up to eight vibration modes are recorded. The experimental results are compared to theoretical predictions from the hydrodynamic functions corresponding to rigid transverse oscillations of an infinitely long rectangular beam. Except for the low-frequency modes, the known hydrodynamic functions cannot describe the amount of dissipated energy due to the liquid motion induced by the cantilever oscillation. The observed variation of the damping coefficient is smaller than the one predicted. The difference at higher modes between the mentioned theoretical description and experimental results is discussed with the help of numerical solutions of the three-dimensional Navier–Stokes equation.

Boundary slip study on hydrophilic, hydrophobic, and superhydrophobic surfaces with dynamic atomic force microscopy.

Slip length has been measured using the dynamic atomic force microscopy (AFM) method. Unlike the contact AFM method, the sample surface approaches an oscillating sphere with a very low velocity in the dynamic AFM method. During this process, the amplitude and phase shift data are recorded to calculate the hydrodynamic damping coefficient, which is then used to obtain slip length. In this study, a glass sphere with a large radius was glued to the end of an AFM cantilever to measure the slip length on rough surfaces. Experimental results for hydrophilic, hydrophobic, and superhydrophobic surfaces show that the hydrodynamic damping coefficient decreases from the hydrophilic surface to the hydrophobic surface and from the hydrophobic one to the superhydrophobic one. The slip lengths obtained on the hydrophobic and superhydrophobic surfaces are 43 and 236 nm, respectively, which indicates increasing boundary slip from the hydrophobic surface to the superhydrophobic one.

Measurement of slip length on superhydrophobic surfaces

In this paper, a review of different techniques used to measure the slip length on superhydrophobic surfaces with large slip length is presented. First, we present the theoretical models used to calculate the effective slip length on superhydrophobic surfaces in different configurations of liquid flow. Then, we present the different techniques used to measure the slip past these superhydrophobic surfaces: rheometry, particle image velocimetry, pressure drop, surface force apparatus and atomic force microscopy.

Atomic force microscopy measurement of boundary slip on hydrophilic, hydrophobic, and superhydrophobic surfaces

Reduction in drag is important in fluid flow applications. So called boundary slip, a measure of relative fluid velocity at the solid-fluid interface, affects the drag. The slip is a function of the degree of hydrophobicity. In this study, boundary slip was studied through slip length measurements on hydrophilic, hydrophobic, and superhydrophobic surfaces in de-ionized water with atomic force microscopy. On the hydrophilic surface, the experimental data are consistent with no-slip boundary conditions. However, boundary slip is observed on hydrophobic and superhydrophobic surfaces. Experimental results obtained with different squeezing velocities show that the slip length is independent of squeezing velocity. Moreover, the degree of boundary slip is observed to increase when the surface was changed from the hydrophobic surface to the superhydrophobic one. The increasing degree of boundary slip from a hydrophobic surface to a superhydrophobic surface is believed to be because the increasing hydrophobicity f...

Measurement of the slip length of water flow on graphite surface

We present measurements of the hydrodynamic damping of an atomic force microscopy cantilever-tip immersed in water and approaching a mica surface or a graphite surface. Water completely wets the mica surface while it partially wets the graphite surface with a contact angle of 74°. The measurements show that the damping is higher on mica than on graphite giving a slip length of about 8nm on this latter surface.

Intrinsic fluorescence from individual silver nanoparticles

Optical microscopy and spectroscopy on individual silver nanoparticles reveals strong emission for blue laser excitation. The silver nanoparticles (15 nm in size) are spin-coated on glass. The emitted light shows blinking and spectral fluctuations under continuous excitation. Identical spectral behaviour was observed on oxidised silver powder indicating that the emissive sites are small silver clusters (few atoms) photo-activated by light illumination of the oxide.

The Microcantilever: A Versatile Tool for Measuring the Rheological Properties of Complex Fluids

Silicon microcantilevers can be used to measure the rheological properties of complex fluids. In this paper two different methods will be presented. In the first method the microcantilever is used to measure the hydrodynamic force exerted by a confined fluid on a sphere that is attached to the microcantilever. In the second method the measurement of the microcantilever’s dynamic spectrum is used to extract the hydrodynamic force exerted by the surrounding fluid on the microcantilever. The originality of the proposed methods lies in the fact that not only may the viscosity of the fluid be measured but also the fluid’s viscoelasticity, i.e., both viscous and elastic properties, which are key parameters in the case of complex fluids. In both methods the use of analytical equations permits the fluid’s complex shear modulus to be extracted and expressed as a function of shear stress and/or frequency.

Nanorheology and boundary slip in confined liquids using atomic force microscopy

The physical properties of materials at the nanometer scale can be completely different from those of the bulk. Here we review some of the properties of confined liquids using an atomic force microscope (AFM). We present different experimental schemes used to study layering of the liquid confined between an AFM tip and solid substrate. Then we consider the liquid flow close to the solid wall and report some experimental measurements of the liquid slip on different surfaces using colloidal particles glued to an AFM cantilever. Nanobubble formation on hydrophobic surfaces is also described at the end of the paper.

Improved acoustic excitation of atomic force microscope cantilevers in liquids

A simple modification of the existing setup used in the commercial atomic force microscopes (AFM) is presented with the aim of improving the piezoacoustic excitation in liquid used by the AFM community. The improvement removes the spurious peaks not corresponding to the resonance frequencies of the cantilever oscillation. To illustrate the benefits of such a clean excitation, very fine effects like the structuring of mesitylene confined between the oscillating AFM tip and a highly oriented pyrolitic graphite surface could be measured with subangstrom oscillation amplitudes and with very high accuracy.

Coalescence and movement of nanobubbles studied with tapping mode AFM and tip?bubble interaction analysis

Imaging of a polystyrene (PS) coated silicon wafer immersed in deionized (DI) water was conducted using atomic force microscopy (AFM) in the tapping mode (TMAFM). As reported earlier, spherical cap-like domains, referred to as nanobubbles, were observed to be distributed on the PS surface. Experiments reveal that, in addition to the well-known parameter of scan load, scan speed is also an important parameter which affects nanobubble coalescence. The process of nanobubble coalescence was studied. It was found that during coalescence, small nanobubbles were easily moved and merged into bigger ones. Based on the interaction between the AFM cantilever tip and a bubble in the so-called force modulation mode of TMAFM, bubble height and adhesive force information for a given bubble was extracted. A viscoelastic model is used to obtain the interaction stiffness and damping coefficient, which provides a method to obtain the mechanical properties of nanobubbles. The model was further used to study the effect of surface tension force on attractive interaction force and contact angle hysteresis on the changes of the interaction damping coefficient during tip–bubble interaction.