Alexandre Chau

Three-dimensional model for capillary nanobridges and capillary forces

This paper presents a model for the computation of capillary forces (applied to water capillary condensation). For simple geometries (planes, spheres, cones, etc), this model complies well with the literature results. But the literature only provides results for simple shapes and meniscii geometries. Our model allows the computation of capillary force for non-axisymmetrical shapes, with a meniscus fulfilling the Kelvin equation (i.e. we do not assume the profile of the meniscus).Currently the model takes into account the contact angles, the relative humidity, temperature and the geometrical description of the problem.The complexity of the problem can result from object shape (modelling for example an AFM tip) and/or from geometrical configuration. Using the model, this article shows that the tilt angle of a tip cannot be neglected when computing capillary forces. It is also shown that the difference between a cone and a pyramid has a significant effect on the computation of the force. The authors propose a simplified formula to determine capillary forces for a range of tips from existing results for similar tips.Eventually, the paper also shows that this geometrical effect can be used to control the force between a tip and an object, allowing to pick it up and release it.

Theoretical and Experimental Study of the Influence of AFM Tip Geometry and Orientation on Capillary Force

Adhesion issues are present in many disciplines such as, for example, surface science, microrobotics or MEMS design. Within this framework, this paper presents a study on capillary forces due to capillary condensation. A simulation tool had already been presented using Surface Evolver and Matlab to compute the shape of a meniscus in accordance with the Kelvin equation and contact angles. The numerical results of this simulation complied well with literature results. One very important result is the ability to compute the evolution of the capillary force depending on the tilt angle of the gripper with respect to the object. The main contribution of this new paper is a test bench and the related experimental results which validate these numerical results. We present here new experimental results illustrating the role of humidity and tilt angle in capillary forces at the nanoscale.

Influence of geometrical parameters on capillary forces

As miniaturization of objects and systems is further carried on, adhesion appears to be one major problem during the assembly and/or fabrication of micro-components. This paper presents a model for the computation of capillary forces. For simple geometries, this model complies with literature results. In addition, it allows the computation of capillary force for non-axisymmetrical shapes. The complexity can arise from object shape (modelling for example an AFM tip) and/or from geometrical configuration. One very important result is the ability to compute the evolution of the capillary force depending on the tilt angle of the gripper with respect to the object. Using this results, it could be possible to manipulate small (a few tens of mum of characteristic dimension) objects with capillary condensation grippers. Currently the model takes into account the contact angles, the relative humidity, temperature and the geometrical description of the problem. It is shown that it is possible to reach forces up to a few hundreds of nanonewton in magnitude. This paper also presents a test bed developed in order to validate the models.

Effects of relative humidity on capillary force and applicability of these effects in micromanipulation

This contribution presents the experimental validation of a computational model of capillary forces based on the Laplace equation for the meniscus geometry and on the Kelvin equation for capillary condensation. We apply this to a tip of AFM cantilever ended by a 100 nm curvature tip, and describe the effect of both humidity rate and relative tilt between the cantilever and the substrate. We propose to pick and place components at the considered scale (100 nm) by varying the capillary force.

Behaviour of Flexure Hinges for Use as Articulations in High Precision Mechanisms

For micromechanical engineering purpose, the classical articulations have reached their limits in terms of precision: their movement is corrupted by backlash and imprecisions in assembly. It also becomes increasingly difficult to realise the assembly within a reasonable cost.