Marangoni spreading and contracting three-component droplets on completely wetting surfaces

Abstract

SignificanceThe shape and dynamics of small sessile droplets are dictated by capillary forces. For liquid mixtures, evaporation adds spatio-temporal modulation to these forces that can either enhance or inhibit droplet spreading, depending on the direction of the resulting Marangoni flow. This work experimentally and numerically demonstrates the coexistence of two antagonistic Marangoni flows in a ternary mixture. Played against each other, they can choreograph a boomerang-like wetting motion: Droplets initially rapidly spread, then contract into a compact cap shape. While such a behavior has been impossible in wetting scenarios of simple liquids, it enables spread-retract-remove surface processing with the addition of a single small liquid volume, demonstrated here in a surface-cleaning experiment.

Keywords: Marangoni stress; evaporating droplet; multicomponent droplet; surface cleaning; surface wetting.

Fig. 1.

Behaviors of ternary droplets on high-energy surfaces. (A–C) Time-series images (t $=$ 0, 1, 2, 3, 4, and 10 s, at 52% RH, in $V/V_{\mathit{tot}},\hspace{0.17em}{V}_{\mathit{droplet}}=200 \text{nL}$) of (A) enhanced spreading 10% H₂O/70% PG/20% EtOH, (B) contracting 70% H₂O/25% PG/5% EtOH, and (C) spreading and contracting 50% H₂O/30% PG/20% EtOH. (D and E) Radius vs. time plots of the mixtures in A–C with D linear and E log axes (n $=$ 10; SD shaded; solid colored lines correspond to the lubrication simulation). (F) The observable behaviors across the ternary parameter space at four different relative humidities. Overlaid are the theoretical considerations from Raoult’s law (24) (dashed) and the net rate of change of the surface tension of the starting concentration (dotted). Avg., average.

Fig. 2.

Comparison between a binary droplet (A–C; 66% PG/34% EtOH at 52% RH) and a ternary droplet (D–F; 50% H₂O/35% PG/15% EtOH at 52% RH). Simulation data (C and F) are computed at 50% RH. (A) BOS reconstruction of the binary droplet shows a ridge of minimal height during spreading. (B) Particles move outward for the spreading binary droplet. (C) Lubrication model (66% PG/34% EtOH at 50% RH): PG enriches near the contact line (indicated by color code); all streamlines point outward (white lines). H₂O content stems from condensation on the droplet. (D) BOS reconstruction of the ternary droplet shows a prominent ridge during spreading. (E) During spreading, particles move outward at the center of the ternary droplet, but recirculate before reaching the edge. (F) Lubrication model (52% H₂O/32% PG/16% EtOH at 50% RH) around spreading reversal: Competing Marangoni flows collect liquid in a ridge, leading to counterrotating convection rolls. See also SI Appendix, Fig. S4 and Movie S2.

Fig. 3.

Different forms of instabilities arising in ternary droplets. (A) Spreading instabilities of a droplet of 50% H₂O/20% PG/30% EtOH at 52% RH. (B) Vigorous instabilities with chaotic satellite droplet formation of a droplet of 50% H₂O/5% PG/45% EtOH at 44% RH. (C) Rivulet formation during the contracting phase of a droplet of 30% H₂O/10% PG/60% EtOH at 22% RH. (D) Observed instability formation across the ternary phase space at different humidities. Time lapses of A–C are shown in SI Appendix, Fig. S5 and Movie S4. $V_{\mathit{droplet}}=200 \text{nL}$ for A and C and 100 nL for B, respectively.

Fig. 4.

Cleaning ability of binary and ternary mixtures. (A–C) All binary mixtures show poor cleaning ability. The contracting droplet in A only cleans a small part where it sits, while the droplets in B and C spread the contamination across the surface. (D) A ternary droplet has the ability to spread and dissolve the contaminant and then contract and collect all the debris within the droplet, before rolling off on an angle. See Movie S5. Downward migration results from a deliberately tilted slide. (E) Normalized visible contamination before and after cleaning with different mixtures (n $=$ 10), errors bars indicate standard deviation.