Scaling Pairwise Force Modelling of Surface Tension Toward Capillary Microbot Simulator

  • Barbot, Antoine (Femto-st)

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Microrobotics aims at finding relevant compact actuation and sensing methods at the microscale. Surface tension forces at fluid interfaces become dominant at this scale and could be used in many micro/nano mechanisms from structure self-assembly to active control of micro-optics [1]. However the design of such capillary micro-mechanisms is really challenging due to the lack of human intuition and important design challenge at this scale. This renders trial and error by real experimentation too time consuming. We are therefore interested in developping a robotic simulator capable of modelling surface tension forces and their interaction with free moving solids.Smooth Particles Hydrodynamics SPH seems an interesting choice for its inherent ability to model topology changes, changes in wetting solid conditions as well as multiphysics (fluid and solid mechanics) problems. Our approach is to model surface tension as a pairwise force, inspired by the definition from Li et al. [2]. We first propose a correction to their formulation which keeps constant the surface tension when the model resolution is increased (by increasing particle numbers). We also detail how to tun this pairwise force to get the desired value of the surface tension in the liquid and contact angle with the solid. Both of this features are evaluated by simulating oscillating drop problems from which the results are well known. We then extend the pairwise capillary force to interactions with free rigid solid partially wetted. This allows us to simulate the movement of an object floating on a drop thanks to capillary forces in order to deduce its equilibria position and study its resonant frequency. This simulator is the first step toward a simulations tool for the microrobotics community dealing with surface tension forces. By adding structure deformation to the model, we also plan to extend it to elasto-capillary problems (such as thin film floating on a drop or beetle walk on glass plates [3].