Evaporation of Dilute Sodium Dodecyl Sulfate Droplets on a Hydrophobic Substrate

Abstract

Evaporation of surfactant-laden sessile droplets is omnipresent in nature and industrial applications such as inkjet printing. Soluble surfactants start to form micelles in an aqueous solution for surfactant concentrations exceeding the critical micelle concentration (CMC). Here, the evaporation of aqueous sodium dodecyl sulfate (SDS) sessile droplets on hydrophobic surfaces was experimentally investigated for SDS concentrations ranging from 0.025 to 1 CMC. In contrast to the constant contact angle of an evaporating sessile water droplet, we observed that, at the same surface, the contact angle of an SDS laden droplet with concentration below 0.5 CMC first decreases, then increases, and finally decreases, resulting in a local contact angle minimum. Surprisingly, the minimum contact angle was found to be substantially lower than the static receding contact angle and decreased with decreasing initial SDS concentration. Furthermore, the bulk SDS concentration at the local contact angle minimum was found to decrease with decrease in the initial SDS concentration. The location of the observed contact angle minimum relative to the normalized evaporation time and its minimum value proved to be independent of both the relative humidity and droplet volume and thus of the total evaporation time. We discuss the observed contact angle dynamics in terms of the formation of a disordered layer of SDS molecules on the substrate at concentrations below 0.5 CMC. The present work underlines the complexity of the evaporation of sessile liquid-surfactant droplets and the influence of surfactant–substrate interactions on the evaporation process.

Competing Marangoni and Rayleigh convection in evaporating binary droplets

“…For solutal Marangoni flow, the droplet must consist of more than one component, e.g. a solvent and one or more surfactants (Still, Yunker & Yodh 2012;Marin et al 2016;Kwieciński et al 2019) or a solvent and possibly multiple co-solvents (Sefiane, Tadrist & Douglas 2003;Christy, Hamamoto & Sefiane 2011;Tan et al 2016;Li et al 2018) or dissolved salts (Soulié et al 2015;Marin et al 2019). For a recent perspective review on droplets consisting of more than one component, we refer to Lohse & Zhang (2020).…”

“…2016; Kwieciński et al. 2019) or a solvent and possibly multiple co-solvents (Sefiane, Tadrist & Douglas 2003; Christy, Hamamoto & Sefiane 2011; Tan et al. 2016; Li et al.…”

Feedback-controlled microbubble generator producing one million monodisperse bubbles per second

“…Variations in bubble size can be caused by external parameters such as vibrations, dislodging dirt (a common issue in microfluidics) or aging of the chip which potentially results in a change in wetting properties of the channel walls resulting from, e.g., contact angle changes of plasma-treated PDMS [61] or surfactant adsorption to the channel walls. [62] These disturbances decrease the production stability and thereby degrade the bubble size distribution both on short time scales as well as on the long-term. In the experiments presented before (see Fig.…”

Precise crystal regulation and harvest by constructing confined flexible droplet crystallizer on 3D-printed sessile platform

“…Generally, the regulation on droplet crystallization was focused on the modulation of antisolvent, heterogeneous interface, and evaporative atmosphere (temperature and wettability, etc.) [11][12][13][14][15][16][17][18][19] . The patterned heterogeneous interface was designed to control the nucleation location of insulin and lysozyme crystals, facilitating the crystal harvest 20 .…”