The story begins a day about seven years ago, when I was a PhD candidate and did the ice nucleation temperature measurement (it is an experiment about observing the freezing process of water droplets on substrate surface through an optical microscope. See ‘Nature 2019, 576, 437-441’ for more details if interested). When I re-measured the surfaces which had been measured for ice nucleation temperature, I observed that the traces of the water droplets in the previous measurement appeared through a distinguished pattern of condensed water droplets (Figure 1a). It looked like a fascinating phenomenon; however, I was busy with my PhD project and set it aside. Until to the year of 2020 when my student observed the phenomenon again, I picked it up and carried out systematic research.

Figure 1. a, Reappear of the water droplet traces during the ice nucleation temperature measurement. The scale bars are 500, 500, and 100 µm, respectively, from left to right. b, Schematic diagram of the LCICD phenomenon. The areas in contact with and without liquid are referred to as “in” and “out”, respectively. The scale bar is 100 µm. c, Schematic diagram of the mechanisms for the LCICD phenomenon and the ethanol (EtOH) soaking-induced condensation rate/density recovery effect.

As shown in Figure 1b, when the surface is contacted simply and quickly by a droplet, the condensation behaviors such as the condensation rate and density and the size of droplet on the contacted area can be significantly changed. We abbreviate this liquid contact-induced condensation rate/density decrease phenomenon to LCICD. At first, I doubted the significance of studying this phenomenon like everyone else, considering that it might probably be caused by the contamination of the contacted droplet. Until we conducted a series of experiments to exclude the possible reasons such as the change of surface chemistry/topography, the adsorption of water molecules/volatile organic compounds induced by water contact, and the mutual condensation interferences between the two areas that were in contact with and without the liquid, we were convinced to continue the work. The preliminary progress was made when we found that the LCICD phenomenon doesn’t apply to the unmodified silicon surface but remains on self-assembled monolayers (SAMs) irrespective of whether the substrate is flat or nanostructured, hard or soft. That suggests that the SAM molecules on the surface play a key role in the LCICD phenomenon. Given that the conformation of SAM molecules can be influenced by liquid, we investigated whether the conformation of SAM molecules on surfaces changed upon liquid contact. Finally, the mechanism behind LCICD is revealed (Figure 1c)—under liquid contact, the molecular chains on surface become more tilted, making some favorable nucleation sites covered and ultimately resulting in decreased condensation rate/density. Based on this, we find that LCICD is universal for different kinds of contacted liquid and also for various surfaces on condition that there are flexible segments capable of shielding at least part of nucleation sites through changing the segment conformation under liquid contact induction. The detailed influences of contacted liquid and surface properties on LCICD degree are elucidated—both increasing liquid polarity and extending contact duration within limits contribute to the response of surface molecule conformation and thus the LCICD degree; with the increase of flexibility of surface segment, the LCICD degree also increases.

Water condensation on surfaces is not only a common phenomenon responsible for the daily clouds, morning dews, and fogging of glasses, but also has a critical effect on many industrial processes such as steam cycle energy production, condensation-controlled heat transfer, and chemical compound fractionation. However, during the past century, understandings on influencing/controlling water condensation have been remained at the level of palpable surface chemical and topographical properties. Herein, we demonstrate that water condensation can be influenced/controlled by more subtle molecule conformation heterogenization. Therefore, our findings are meaningful for gaining insights into the surface properties influencing water condensation and offering new strategies for controlling condensation on surfaces. Other significances of this study are shown below:

1)      Without designing any surface chemical and topographical patterns, we achieve spatial control of water condensation by ultrafast liquid contact with and removal from surfaces based on the LCICD phenomenon;

2)      The strategy using the LCICD phenomenon to control condensation can be extended to systems beyond water.

Looking ahead, the LCICD phenomenon may evoke more research interests in the fields of surface science, fluid dynamics, and chemical analysis. Here, I just list two critical areas to break the ice—interfacial characterizations and phenomena on the microscopic level. Specifically, the molecular structure information characterizations on interfaces (e.g., molecular conformations) are usually not easy and accessible. The discovered LCICD phenomenon may offer an easy and accessible signal for characterizing molecular conformation variation, if the conformation features can be calibrated by the LCICD degree. In addition, based on the liquid polarity-related surface molecule conformation response, some interesting interfacial phenomena such as in interfacial friction/lubrication and electrification may be explained and further applied.

More details can be found in our paper "Controlled condensation by liquid contact-induced adaptations of molecular conformations in self-assembled monolayers" published in Nature Communications.