On the effect of substrate viscoelasticity on the evaporation kinetics and deposition patterns of nanosuspension drops

Chen, Yuhong and Askounis, Alexandros and Koutsos, Vasileios and Valluri, Prashant and Takata, Yasuyuki and Wilson, Stephen K. and Sefiane, Khellil (2020) On the effect of substrate viscoelasticity on the evaporation kinetics and deposition patterns of nanosuspension drops. Langmuir, 36 (1). pp. 204-213. ISSN 0743-7463

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    Abstract

    This study investigates the evaporation of sessile pure water and nano-suspension drops on viscoelastic polydimethylsiloxane (PDMS) films. We varied the viscoelasticity of the PDMS films by controlling the curing ratio, and categorized them into three types: stiff (10:1, 20:1, 40:1), soft (60:1, 80:1), and very soft (100:1, 120:1, 140:1, 160:1). On stiff surfaces, pure water drops initially evaporate in a constant contact radius (CCR) mode, followed by a constant contact angle (CCA) mode, and finally in a mixed mode of evaporation. Nano-suspension drops follow the same trend as water drops but with a difference towards the end of their lifetimes, when a short second CCR mode is observed. Complete evaporation of nano-suspension drops on stiff substrates leads to particle deposition patterns similar to a coffee ring with cracks and deposition tails. On soft surfaces, the initial spreading is followed by a pseudo-CCR mode. Complete evaporation of nano-suspension drops on soft substrates leads to deposits in the form of a uniform ring with a sharp ox-horn profile. Unexpectedly, the initial spreading is followed by a mixed mode on very soft substrates, on which wetting ridges pulled up by the vertical component of surface tension are clearly observed in the vicinity of the contact line. As the evaporation proceeds, the decreasing contact angle breaks the force balance in the horizontal direction at the contact line and gives rise to a net horizontal force, which causes the contact line to recede, transferring the horizontal force to the wetting ridge. Due to the viscoelastic nature of the very soft substrate, this horizontal force acting on the wetting ridge cannot be completely countered by the bulk of the substrate underneath. As a result, the wetting ridge moves horizontally in a viscous-flow way, which also enables the contact line to be continuously anchored to the ridge and to recede relative to the bulk of the substrate. Consequently, a mixed mode of evaporation occurs. Complete evaporation of nano-suspension drops on very soft substrates leads to finger-like deposits.