Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

Abstract

The wetting and evaporation behavior of droplets of aqueous solutions of mixtures of poly(diallyldimethylammonium chloride) solution, PDADMAC, with two different anionic surfactants, sodium laureth sulfate, SLES, and sodium N-lauroyl N-methyl taurate, SLMT, were studied in terms of the changes of the contact angle θ and contact length L of sessile droplets of the mixtures on silicon wafers at a temperature of 25 °C and different relative humidities in the range of 30–90%. The advancing contact angle θa was found to depend on the surfactant concentration, independent of the relative humidity, with the mixtures containing SLES presenting improved wetting behaviors. Furthermore, a constant droplet contact angle was not observed during evaporation due to pinning of the droplet at the coffee-ring that was formed. The kinetics for the first evaporation stage of the mixture were independent of the relative humidity, with the evaporation behavior being well described in terms of the universal law for evaporation.

Pattern Formation upon Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

“…The above-discussed differences in d γ /d c provide a justification for the tendency of the Marangoni flow to move the polymer–surfactant complexes towards the three-phase contact line at the beginning of the drop evaporation when the salt concentration is relatively small. This is possible because the instantaneous volume of the droplets V is not much smaller than their initial volume V 0 as was demonstrated in our previous paper [ 6 ]. Despite that the d γ /d c is very small, thus leading to values of γ ≈ γ 0 (with γ 0 being surface tension of water) for the initial salt solution, the salt concentration becomes very high during the latter stage of evaporation, so that γ is several mN/m −1 higher than γ 0 .…”

“…Furthermore, as was stated in our previous publication the γ vs. surfactant concentration curves for the polymer–surfactant mixtures have the same shape as that of the surfactant solutions [ 40 ], and it is expected that d γ /d c ≈ 0 for high surfactant concentrations as the evaporation proceeds. For the sake of example, for the lowest initial surfactant concentration (2.6 μM), the surfactant concentration, 26 μM at which γ becomes constant is reached after an evaporation time t = 750 s [ 6 ]. At this time the only contribution to the Marangoni flow is that associated with the NaCl formed upon the binding of surfactant molecules to PDADMAC chains.…”

“…The solid lines represent the surface tension of the pure Milli-Q water used in this work. Adapted from Akanno et al [ 6 ], with permission under attribution license Creative Commons 4.0 (2021).…”

“…In practical applications, mixtures instead of pure liquids are commonly used, which leads to time-dependent interfacial tensions associated with a differential adsorption of the components at the interfaces [ 4 , 6 ]. This leads to surface tension gradients which originate Marangoni stresses during the evaporation process.…”

Polyelectrolyte Multilayers on Soft Colloidal Nanosurfaces: A New Life for the Layer-By-Layer Method

“…The fabrication of nanomaterials with controlled structure and specific functionalities is currently a challenge for several industrial and technological fields [1][2][3][4][5][6]. This requires tools enabling a precise control of the self-assembly and self-organization of molecules and colloidal objects at the molecular level (nanoscale), and the possibility to modulate such processes by external fields/stimuli [7][8][9][10]. The Layer-by-Layer (LbL) assembly technology offers many possibilities for fulfilling the above-mentioned requirements by the combination of a broad range of functional building blocks into macromolecular devices [11,12].…”