The influence of substrate's elasticity and thermal properties on drop evaporation

• Main Author: Camarinha Lopes, Marcus
• Format: Others
• Language: German; en
• Published: 2013
• Online Access: http://tuprints.ulb.tu-darmstadt.de/3701/13/Thesis%20Lopes_Ver%C3%B6ffentlichung.pdf; Camarinha Lopes, Marcus <http://tuprints.ulb.tu-darmstadt.de/view/person/Camarinha_Lopes=3AMarcus=3A=3A.html> : The influence of substrate's elasticity and thermal properties on drop evaporation. Technische Universität, Darmstadt [Ph.D. Thesis], (2013)

This work contributes to expand the scientific knowledge on sessile drop evaporation by presenting experimental investigations on the influence of substrate’s elasticity, thermal conductivity, thermal diffusivity and wettability on the evaporation of a sessile drop. Performed experimental investigations show that soft substrates (Young’s modulus below 10 MPa) are able to directly influence the dynamics of the triple-phase-contact-line (TPCL) of a sessile drop by influencing its receding contact angle. A model to predict the change of the receding contact angle in dependence of the Young’s modulus and maximal deformation height of the substrate was developed and reproduced experimental values very well. The influence on the receding contact angle has consequences for the evaporation mode of the drop. Softer substrates delay the transition from constant contact radius (CCR) to constant contact angle (CCA) evaporation mode. Because evaporation rate of a drop during the CCR mode is higher than during CCA mode, evaporation is faster on softer substrates. By finetuning the Young’s modulus of a polydimethylsiloxane elastomer (PDMS), the control of evaporation mode and, consequently, evaporation time was demonstrated. In this work, investigation on particle deposition during the evaporation of water-silica suspension drops showed that the TPCL velocity increases with decreasing Young’s modulus of the substrate. A model for the contact line velocity considering capillary forces, viscoelastic dissipation, and contact line friction was developed and reinforced experimental findings. With increasing TPCL velocity, the thickness of the liquid film (Landau-Levich film) trailing behind the drop rim increases. Particles smaller than this film were more likely to be deposited. Particles larger than the thickness of the film moved together with the contact line and were accumulated until the contact line was pinned. Particle accumulation at the TPCL is thus a result not only of the evaporation driven flow, like described in previous work, but also of the movement of the TPCL. Fine-tuning of Young’s modulus of PDMS substrates allowed the control of particle deposition without having to necessarily modify the used suspension Thermal conductivity controlled the evaporation rate by affecting the temperature underneath the drop. Substrates with higher thermal conductivity have higher temperatures underneath the drop because enough heat could be transported to the cooling drop. For substrates with low thermal diffusivity the temperature decreases steadily during the course of evaporation. Consequently, differently than mostly assumed, thermal properties need to be considered for a precise prediction of evaporation rate and time.