Transitional bulk-solutal Marangoni instability in sessile drops

Abstract

Evaporation of sessile droplets is a method to organize suspended particles on solid substrates. Many studies have demonstrated that Marangoni flows caused by surface adsorbed molecules or temperature gradients can strongly affect the dried deposit. In the present paper, we show how transitional Marangoni instabilitiy can be triggered by bulk-diluted tensio-active ions. Thanks to PIV analysis, we identify four different flow stages. The transition between them can be understood by considering the competition between the Marangoni flow and the mass conservation flow, usually responsible for the coffee-ring pattern. We also demonstrate that the initial ionic concentration can select a coffee-ring pattern or a more homogeneous dried deposit.

Combined effects of Marangoni, sedimentation and coffee-ring flows on evaporative deposits of superparamagnetic colloids

“…Still without field but with the highest PBS concentration, a ring-shaped deposit is observed again at the edge of the droplet, but similar surface fractions of particles are also observed in the centre of the deposit. This more homogeneous deposition of particles is actually induced by solutal Marangoni flows, as described in a previous study [17]. However, when a magnetic field of intensity B = 22.5 10 −4 T is applied on the evaporating drop, the final deposits are radically different.…”

“…Notably, the presence of surface-adsorbed polymers or surfactants [10,11] or capillary interactions of ellipsoidal particles [12,13] allow a uniform coating of the particles. Temperature gradients, tensio-active agents or binary mixtures can also create surface tension gradient producing Marangoni flows [14][15][16][17][18][19]. Those flows also strongly modify the deposits' pattern, in various ways [10,[20][21][22].…”

“…In the context of evaporating droplets, it has been shown recently that these assemblies, in the case of micron-sized particles, could be deposited in the dried deposit [27]. However, it has been highlighted that such a deposition requires the right concentration of PBS, and that this latter parameter also has an impact on the dried deposit [17].…”

Remote-controlled deposit of superparamagnetic colloidal droplets

“…Marangoni flows created by temperature gradients, tensio-active agents or in binary mixtures can also strongly modify the deposit pattern. [11][12][13][14][15] More recently, some researchers have shown that more complex deposit structures can be achieved thanks to various concentrations of liquid crystals [16]. In this paper, we demonstrate that magnetic interactions between superparamagnetic colloidal particles [17][18][19][20][21] can be used to control the properties of colloidal droplets deposit, as illustrated in Fig.1.…”

“…This almost homogeneous deposition of particles is actually induced by solutal Marangoni flows, due to the presence of PBS, as described in a previous study. [13] However, when a magnetic field B is applied on the evaporating drop, the eventually dried deposit is radically different. The coffee-ring trend is even more reduced, as quantitatively described further, and structures resulting from the magnetic self-assembly of the particles are observed throughout the deposit.…”