Thermomechanical coupling at the fingertip: investigating the thermal dynamics during contact

Details

Supervisors

Delhaye, Benoit

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil biomédical, à finalité spécialisée

Abstract

enThe dynamics of heat transfer that occur at the finger-object interface during touch are difficult to describe with precision. A deep understanding of this interaction, however, is necessary, for example, to provide realistic thermal feedback through haptic devices. For this reason, many models have been developed with the aim of describing the transient dynamics that characterize this heat transfer. Unfortunately, however, no current model is able to accurately capture this dynamic interaction, as the microscopic deformation of the fingertip resulting from the application of loads is never taken into account. This thesis aims to study the thermomechanical coupling at the finger-object interface, with the goal of providing insights into the physics governing it. To this end, an analytical model based on physics equations was first built to predict this dynamic interaction. Then, to validate these predictions, a dedicated experimental setup was developed: using this setup, it was possible to acquire data on the applied normal forces, the resulting heat flux, and the actual contact area. The experiments were conducted on human subjects, testing different temperature gradients and normal forces, while also monitoring skin hydration. The acquired data were analyzed using a custom-built pipeline, which includes an exponential fit for thermal transients and image segmentation for epidermal ridges. The comparison between the experimental data and the theoretical predictions clearly demonstrates that the time constant 𝜏 decreases as the force and contact area increase: in practice, as the applied pressure increases, the fingertip deforms and heat is exchanged more rapidly. The other parameters, such as the initial peak and the final asymptote of the heat flux, are not influenced by force but rather by the initial temperature gradient and the underlying physiological blood perfusion, respectively. Finally, regarding skin moisture, the evaluation of this measure highlighted its high variability, confirming that it remains very difficult to model and interpret. These results demonstrate that thermomechanical interactions depend on the microscopic topography of the skin, successfully validating the proposed model. This thesis, therefore, provides a foundation for the future development of closed-loop haptic displays. In this way, devices could dynamically adjust their thermal output based on real-time force feedback, thereby providing a more realistic sensation for the user.