Solutal Marangoni flow as the cause of ring stains from drying salty colloidal drops

Abstract

Evaporating salty droplets are ubiquitous in nature, in our home and in the laboratory. Interestingly, the transport processes in such apparently simple systems differ strongly from evaporating "freshwater" droplets since convection is partly inverted due to Marangoni stresses. Such an effect has crucial consequences to the salt crystallization process and to the deposits left behind. In this work we show unprecedented measurements that, not only confirm clearly the patterns of the flow inversion, but also elucidate their impact on the distribution of non-volatile solutes. Contrary to what has been often reported in the literature, such a flow reversal does not prevent the formation of ring-shaped stains: particles accumulate at the contact line driven solely by the interfacial flow. We can therefore conclude that the classical "coffee-stain effect" is not the only mechanism that can generate ring-shaped stains in evaporating droplets.

Physicochemical hydrodynamics of droplets out of equilibrium

“…The reason lies in a combination of the (in general) different volatility of the components and the singularity of the evaporation (or dissolution) rate at the rim of the droplet [8,110,112,121] (provided the contact angle is smaller than 90 o ), leading to a concentration gradient at the interface of the droplet. The resulting surface tension gradient drives a Marangoni flow inside the droplet [17,38,40,[122][123][124][125][126][127][128][129][130][131][132], see figure 4a. This effect is very similar to what leads to the so-called tears of wine [133] inside a partially filled wine-glass: In this case, selective evaporation of ethanol at the edge of the meniscus leads to larger surface tension, which thus pulls part of the remaining wine upwards along the alcohol-wetting glass, finally leading to an instability of the film and droplets sinking down the glass wall.…”

“…Also on the substrate on which the droplets are jetted Marangoni flow within one droplet [17,38,40,122,[125][126][127][128][129][130][131][132] (see figures 4a,b,c) or in between different droplets [73] (figure 2) can lead to unwanted effects, in particular as these flows transport pigments. Or does the emerging Marangoni flow between the droplets perhaps even help in mixing the droplets ("bleeding of mixed colors")?…”

Particle monolayer assembly in evaporating salty colloidal droplets

“…For the classical coffee-stain flow, Deegan et al [1] successfully predicted that the total number of particles deposited should scale as N ∼t 4/3 , which is robustly found in experiments whenever this deposition mechanism dominates. In our case of salty droplets, where the deposition is dominated by an interfacial solutal Marangoni flow [12], a completely different scaling is expected to be found. Our visualization technique allows to count particle-by-particle with good resolution, and the results are shown in Fig.…”

“…This induces a surface tension gradient along the droplet of the surface, and a Marangoni flow sets up directed from the apex of the droplet toward the contact line. In a recent paper, Marin et al [12] showed both experimentally and numerically, that this surface flow is strong enough to overcome the bulk capillary flow. They also showed that the ring-shaped deposit originates from particles adsorbed at the liquid-air interface (from now on, droplet's surface) and convects toward the contact line region by the surface flow.…”

“…Interestingly, certain type of compositional changes also result in ring-shaped stains despite a substantial change in the internal flow profile: The addition of tiny amounts of NaCl to a water droplet induces an internal flow that looks as an inverted coffee-stain flow but also leads to a ring-shaped stain [12]. The effect is due to a solutal Marangoni flow caused by the presence of NaCl which increases the surface tension of water (as other similar chaotropic electrolytes, it effectively acts as an antisurfactant [13,14]).…”