DNA properties investigated by dynamic force microscopy

In this work, we show that by varying the experimental conditions, the driving amplitude, a dynamic force microscope allows DNA properties to be selectively imaged. The substrate on which the DNA is fixed is a silica surface grafted with silanes molecules ended with amine groups. Use of small oscillation amplitudes favors the attractive interaction between the tip and the sample, while use of large amplitudes renders the contribution of the attractive interaction negligible. Particularly, at small amplitudes, the images show that the attractive interaction is strongly enhanced along the DNA. This enhancement is found to be amenable with a model considering a narrow strip of randomly oriented dipoles on each side of the molecule. This work should provide new insights on the DNA interaction and conformational changes with localized charges

Stability of an oscillating tip in Non-Contact Atomic Force Microscopy: theoretical and numerical investigations.

This paper is a theoretical and a numerical investigation of the stability of a tip-cantilever system used in Non-Contact Atomic Force Microscopy (NC-AFM) when it oscillates close to a surface. No additional dissipative force is considered. The theoretical approach is based on a variationnal method exploiting a coarse grained operation that gives the temporal dependence of the nonlinear coupled equations of motion in amplitude and phase of the oscillator. Stability criterions for the resonance peak are deduced and predict a stable behavior of the oscillator in the vicinity of the resonance. The numerical approach is based on results obtained with a virtual NC-AFM developped in our group. The effect of the size of the stable domain in phase is investigated. These results are in particularly good agreement with the theoretical predictions. Also they show the influence of the phase shifter in the feedback loop and the way it can affect the damping signal

Precise damping and stiffness extraction in acoustic driven cantilever in liquid

In this paper, we first explain how to extract accurately the driving force acting on the acoustic driven atomic force microscope cantilever in liquid from the measured resonance curve. We present a model that includes the driving force to extract precisely the damping and the stiffness of the tip sample interaction. The model is validated by an experimental test based on two independent methods to measure the hydrodynamic drag coefficient of a sphere moving perpendicular to flat surface.Détermination des propriétés rhéologiques d'un fluide à l'aide de vibration de microlevie

DNA properties investigated by dynamic force microscopy

In this work, we show that by varying the experimental conditions, the driving amplitude, a dynamic force microscope allows DNA properties to be selectively imaged. The substrate on which the DNA is fixed is a silica surface grafted with silanes molecules ended with amine groups. Use of small oscillation amplitudes favors the attractive interaction between the tip and the sample, while use of large amplitudes renders the contribution of the attractive interaction negligible. Particularly, at small amplitudes, the images show that the attractive interaction is strongly enhanced along the DNA. This enhancement is found to be amenable with a model considering a narrow strip of randomly oriented dipoles on each side of the molecule. This work should provide new insights on the DNA interaction and conformational changes with localized charges

Nonlinear dynamical properties of an oscillating tip-cantilever system in the tapping mode

The dynamical properties of an oscillating tip-cantilever system are now widely used in the field of scanning force microscopy. The aim of the present work is to get analytical expressions describing the nonlinear dynamical properties of the oscillator in noncontact and intermittent contact situations in the tapping mode. Three situations are investigated: the pure attractive interaction, the pure repulsive interaction, and a mixing of the two. The analytical solutions obtained allow general trends to be extracted: the noncontact and the intermittent contact show a very discriminate variation of the phase. Therefore the measurement of the phase becomes a simple way to identify whether or not the tip touches the surface during the oscillating period. It is also found that the key parameter governing the structure of the dynamical properties is the product of the quality factor by a reduced stiffness. In the attractive regime, the reduced stiffness is the ratio of an attractive effective stiffness and the cantilever one. In the repulsive regime, the reduced stiffness is the ratio between the contact stiffness and the cantilever one. The quality factor plays an important role. For large values of the quality factor; it is predicted that a pure topography can be obtained whatever the value of the contact stiffness. For a smaller quality factor, the oscillator becomes more sensitive to change of the local mechanical properties. As a direct consequence, varying the quality factor, for example with a vacuum chamber, would be a very interesting way to investigate soft materials either to access topographic information or nanomechanical properties

Stability criterions of an oscillating tip-cantilever system in dynamic force microscopy

This work is a theoretical investigation of the stability of the non-linear behavior of an oscillating tip-cantilever system used in dynamic force microscopy. Stability criterions are derived that may help to a better understanding of the instabilities that may appear in the dynamic modes, Tapping and NC-AFM, when the tip is close to a surface. A variational principle allows to get the temporal dependance of the equations of motion of the oscillator as a function of the non-linear coupling term. These equations are the basis for the analysis of the stability. One find that the branch associated to frequencies larger than the resonance is always stable whereas the branch associated to frequencies smaller than the resonance exhibits two stable domains and one unstable. This feature allows to re-interpret the instabilities appearing in Tapping mode and may help to understand the reason why the NC-AFM mode is stable

Nanomécanique aux interfaces (applications à l'étude de couches de phospholipides et à l'interface air-liquide)

La microscopie à force atomique est utilisée dans de nombreuses applications. Ce travail a ici pour but d'étudier, à l'échelle du nanomètre, les propriétés mécaniques de monocouches et de multicouches de phospholipides avec un microscope à force atomique en mode dynamique. Le comportement non linéaire du système pointe-levier, proche des couches de phospholipides, est étudié avec un modèle de granulation. Le MFA est aussi utilisé pour étudier l'influence d'un peptide membranaire, la syntaxine, et la structure des couches de phospholipides autoassemblées sur du mica. Dans un second temps, nous étudions l'oscillation d'un microlevier de MFA clampé en milieu liquide. Les résultats sont comparés aux prédictions théoriques des simulations numériques de la résolution des équations de Navier-Stokes à trois dimensions, pour mesurer l'influence du mouvement du fluide sur le comportement d'un levier oscillant de MFA. Nous montrons aussi comment la force hydrodynamique peut être réduite d'au moins un ordre de grandeur en usinant, à l'aide d'un faisceau d'ions focalisés (FIB), la largeur du levier. Enfin, nous présentons une description analytique qui détermine le mouvement d'un levier excité acoustiquement en milieu liquide. La troisième partie est une étude du comportement dynamique d'un nanoménisque avec une nanopointe oscillante. La nanoaiguille, découpée avec un faisceau d'ions focalisés, est approchée d'une interface air-liquide et seule son extrémité oscille dans le liquide (Eau-Glycérol-PDMS). Il est montré qu'il est possible d'imager une interface de liquide avec une résolution à l'échelle du nanomètre.Atomic force microscopy (AFM) is used for various applications. This work aims here at studying mechanical properties, at the nanometer scale, of phospholipids monolayers and multilayers with an atomic force microscope in the dynamic mode. The non-linear dynamic behaviour of an oscillating tip-cantilever system, near phopholipids layers, is analyzed based on a coarse graining model. AFM is also applied to study the influence of a membrane peptid, Syntaxyn, and the features of self-assembled phospholipids layers on mica. In a next step, we investigate the oscillation behaviour of a clamped AFM microlever in liquids. The experimental results are compared to theoretical predictions from the numerical solutions of the three-dimensional Navier-Stokes equation, to measure the influence of the fluid motion on the oscillating behaviour of an AFM cantilever. We also show that the drag hydrodynamic force can be reduced by almost an order of magnitude when reducing the cantilever width.using focused ion beam (F.I.B.) milling. Then, we present an analytical description that enables determining the motion of an acoustic-driven atomic force microscope cantilever in liquid. The third part is a study of the dynamical behavior of a nanomeniscus with an oscillating nanoneedle. The nanoneedle, carved with a focus ion beam, is approached to the air-liquid interface and the very end of the tip oscillates in the liquid (Water-Gycerol-PDMS). Shown is the capability to record height images of the liquid interface with resolutions at nanometer scale.BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF