Slippery damper of an overlay for arresting and manipulating droplets on nonwetting surfaces

In diverse processes, such as fertilization, insecticides, and cooling, liquid delivery is compromised by the super-repellency of receiving surfaces, including super-hydro-/omni-phobic and superheated types, a consequence of intercalated air pockets or vapor cushions that promote droplet rebounds as floating mass-spring systems. By simply overlaying impacting droplets with a tiny amount of lubricant (less than 0.1 vol% of the droplet), their interfacial properties are modified in such a way that damper-roller support is attached to the mass-spring system. The overlayers suppress the out-of-plane rebounds by slowing the departing droplets through viscous dissipation and sustain the droplets’ in-plane mobility through self-lubrication, a preferential state for scenarios such as shedding of liquid in spray cooling and repositioning of droplets in printing. The footprint of our method can be made to be minimal, circumventing surface contamination and toxification. Our method enables multifunctional and dynamic control of droplets that impact different types of nonwetting surfaces.

1. Blanken et al. focused exactly on the mechanism of drop rebound, describing how an oil layer can PROMOTE drop rebound on a smooth hydrophilic and hydrophobic surface, instead of SUPPRESING it, as reported in this study for a superhydrophobic surface. The results appear, at first sight, contradictory. However, they offer the opportunity to better understand the mechanism of drop rebound. Clearly, the authors should discuss why the opposite phenomena are observed and make a comparative discussion. In particular, two questions arise: is suppression given by a different viscosity of the layer? Is roughness causing a breakup of the lubricating oil layer, so that rebound occurs on a smooth surface, and not on a rough surface? I think the key role is the breakup of the oil film, which roughness may cause. Still, on a smooth hydrophobic surface rebound occurs even when the oil layer breaks up, differently from what reported by the authors for superhydrophobic surfaces. All these questions and issues need to be properly addressed in the revised version of the manuscript.
2. "The critical value is the same for pure and overlaid droplets, an evidence showing that thin overlayers have negligible impacts on the fluid dynamics except for the out-of-plane rebounding suppressions". This statement is too generic and essentially incorrect. Splashing, a relevant mechanism upon drop impact, is strongly affected by the presence of an oil layer. Spreading, as well, is affected. The authors should thus highlight the contribution of Tran et al. exactly on these two points, discussing the in the text.
Other minor comments: • Figure 2c, y-axis. The scale should be changed, to appreciate if 0.25 is the correct slope.

Reviewer #2 (Remarks to the Author):
This manuscript studies the dynamics of a droplet coated by a layer of another immiscible liquid. The authors demonstrate that the coating can suppress rebound of the droplet from a non-wetting surface. On the other hand, the presence of the coating fluid around the droplet can enhance its mobility on the surface. The manuscript appears as a list of observations without much analysis of the physics observed. Some of these observations could have been interesting if a systematic study had been conducted, together with clear physical modelling. However, the authors only provide isolated observations and hypothesized explanations not supported by quantitative data. I therefore do not recommend publication of this manuscript.
Detailed remarks: 1. It would have been interesting for example to explore the range of parameters for which the addition of the coating layer suppresses bounding, changing systematically the impact velocity, the layer thickness or the coating liquid viscosity. 2. The discussions on the velocity gradient are not supported by any quantitative data, or modelling arguments. 3. The numerical simulations have quite low resolution. At this refinement level, how many cells capture the oil film? 4. It is not clear which liquid density is used in the definition of the Weber number on line 108. 5. Line 184: "Expectedly, L is proportional to the V_o/D^2": I do not see why there should be such proportionality relationship? 6. Lines 198-199: "the magnetic controlled motion leaves a more homogeneous lubricant trails ( Fig.  4i)": This observation is representative of the whole manuscript: no quantitative data, no attempt to give any physical interpretation of the observation and no systematic variation of the control parameters. 7. Many figures are missing key information on the data presented to understand it: Weber number, coating layer thickness, drop size, … 8. Many affirmations are not supported by quantitative data, such as the statement on the coating thickness on lines 80 to 82. 9. Line 178: The authors make a claim about a transition at 500 nm, while the figure cited (Fig. 4c) suggest a transition at 5 µm, so 10 times larger.

Response to the Comments by Referee #1
Comment 1: The authors present an experimental study on the impact of compound drops (water-in-oil and hexadecane-in-oil) on non-wetting surfaces. The paper focuses specifically on the drop rebound suppression after drop impact on non-wetting surface, in presence of an oil layer covering the core drop surface, i.e. water or hexadecane. The paper is well written and structured, and the topic is relevant for the journal community, since compound drops represent a clear emerging trend.

Answer:
We thank gratefully the reviewer for his/her appreciation of this work. Answer: In accordance with the reviewer's comment 6, we have changed the scale of y-axis in Fig. 2c and added context in Paragraph 2 (Page 6, Lines 130-134) to confirm the slope.
We wish to take this opportunity to thank the Referee for his/her critical review and constructive comments/suggestions.

Response to the Comments by Referee #2
Comment 1: This manuscript studies the dynamics of a droplet coated by a layer of another immiscible liquid. The authors demonstrate that the coating can suppress rebound of the droplet from a non-wetting surface. On the other hand, the presence of the coating fluid around the droplet can enhance its mobility on the surface. The manuscript appears as a list of observations without much analysis of the physics observed. Some of these observations could have been interesting if a systematic study had been conducted, together with clear physical modelling.
However, the authors only provide isolated observations and hypothesized explanations not supported by quantitative data. I therefore do not recommend publication of this manuscript.