Marangoni Bursting: Evaporation-Induced Emulsification of Binary Mixtures on a Liquid Layer

Adjusting the wetting properties of water through the addition of a miscible liquid is commonly used in a wide variety of industrial processes involving interfaces. We investigate experimentally the evolution of a drop of water and volatile alcohol deposited on a bath of oil: The drop spreads and spontaneously fragments into a myriad of minute droplets whose size strongly depends on the initial concentration of alcohol. Marangoni flows induced by the evaporation of alcohol play a key role in the overall phenomenon. The intricate coupling of hydrodynamics, wetting, and evaporation is well captured by analytical scaling laws. Our scenario is confirmed by experiments involving other combinations of liquids that also lead to this fascinating phenomenon.

Figure 1

(a) Image sequence of a drop of a mixture of isopropyl alcohol (IPA) with water spreading and fragmenting on a bath of sunflower oil. The initial IPA mass fraction is ${\varphi }_0=0.4$, and the volume of the drop is ${\mathrm{\Omega }}_0=100\mu \mathrm{L}$. Methyl blue dye has been added to improve the optical contrast. (b) Sketch of the experiment. The drop of water and alcohol mixture (light gray) spreads over a bath of oil of centimetric depth $H$. The thickness of the drop $h(r,t)$ depends on time $t$ and on the distance $r$ from the center of the drop. We define the drop radius $R(t)$, the minimal drop thickness $h_m$, the apparent width $d$ of the peripheral rim, the characteristic velocity $V$ of the flow, and the alcohol fractions ${\varphi }_0$ at the center and ${\varphi }_c$ at the boundary of the drop. $j_v$ is the volume of alcohol evaporated from the drop per unit time and surface area. (c) Evolution of $R$ as a function of time for various initial IPA fractions ${\varphi }_0$ and volumes ${\mathrm{\Omega }}_0$. The maximal spreading radius $R_{\max }$ and total experiment time $t_{\exp }$ can be defined on each data set.

Figure 2

Measured interfacial tensions: oil-air (${\gamma }_{\text{oa}}\simeq 32\mathrm{mN}/\mathrm{m}$, dashed line), mixture-air (${\gamma }_{\text{ma}}$, open circle), and mixture-oil (${\gamma }_{\text{mo}}$, open square) as a function of the initial IPA mass fraction ${\varphi }_0$. The spreading parameter $S={\gamma }_{\text{oa}}-{\gamma }_{\text{ma}}-{\gamma }_{\text{mo}}$ (filled square) of the mixture on the oil becomes positive for $\varphi >{\varphi }_c\simeq 0.35$, the critical IPA fraction above which we observe spontaneous drop spreading and fragmentation [see Fig. 1 in Supplemental Material for the plot corresponding to ethanol-water mixtures 35].

Figure 3

(a) Thickness profiles $h(r)$ of a drop of volume ${\mathrm{\Omega }}_0=5\mu \mathrm{L}$ and IPA fraction ${\varphi }_0=0.6$ at successive times in the experiment. The typical thickness is $10\mu \mathrm{m}$, with a thick central region surrounded by a submicron annular base. The dashed line indicates the minimal thickness $h_m$ of the drop close to its periphery. For this experiment, $h_m\simeq 300\mathrm{nm}$. (b) The wavelength $\lambda$ of the observed instability seems to diverge for ${\varphi }_0={\varphi }_c$ and strongly decreases with increasing ${\varphi }_0$, concomitantly with the radius of ejected droplets. The inset plot displays the variation of $\lambda$ and the apparent rim diameter $d$ with the minimal thickness of the film $h_m$. The lines of equation $\lambda =533h_m$ and $d=190h_m$ are linear fits to the data.

Figure 4

(a) Normalized plots $R/R_{\max }$ versus $t/t_{\exp }$. Data obtained with drops of different ${\mathrm{\Omega }}_0$ and ${\varphi }_0$ and nature of oil (castor oil is about 10 times more viscous than sunflower oil) collapse on a single master curve. (b) Measured spreading radius $R_{\max }$ and (c) duration of the experiment $t_{\exp }$, respectively, as a function of the characteristic radius $R^{*}$ and time scale $\tau$ as predicted by Eqs. (1) and (2). The experimental parameters span the range $0.4<{\varphi }_0<0.8$, $1\mathrm{mm}<H<20\mathrm{mm}$, $2{\mathrm{mm}}^3<{\mathrm{\Omega }}_0<500{\mathrm{mm}}^3$. Blue dots (circle) correspond to IPA-water mixtures in a still atmosphere ($j_v=115\mathrm{nm}/\mathrm{s}$). Experiments represented by red squares (square) have been performed under a moderate wind ($450\mathrm{nm}/\mathrm{s}<j_v<1240\mathrm{nm}/\mathrm{s}$). Green triangles (triangle) correspond to ethanol-water mixtures in a still atmosphere ($j_v=130\mathrm{nm}/\mathrm{s}$). Black lines are linear fits to the data: $R_{\max }=0.28R^{*}$ and $t_{\exp }=1.5\tau$.