# The Effect of Adsorbed Volatile Organic Compounds on an Ultrathin Water Film Measurement
## 4 Conclusions  
The existence and thickness of a thin film between droplets during the dropwise condensation process has been under debate for decades. In a recent article published by the authors, we shared our observation (using SPRi) of a monolayer-thick water film that exists between the droplets on a smooth hydrophilic condensing surface. As the thickness of this ultrathin film can be similar to the thickness of the adsorbed VOCs layer that forms on the surface during the dropwise condensation experiment, we used two surface-treatment processes-SCP and air plasma cleaning-and four surface characterization techniques-AFM, optical microscopy, XPS, and SPRi-to quantify the effect of adsorbed VOCs during the dropwise condensation experiment. The results indicate that (1) both surface cleaning methods have negligible effects on surface homogeneity, (2) neither method fully removes the adsorbed VOCs from the surface, (3) the rate of adsorption of VOCs after plasma cleaning is high and it can cause up to 5% change in the SPR signal, (4) the SCP is recommended prior to sensitive measurement such as ultrathin film (<1 nm) measurement using SPRi due to its minimal effect on the SPR reflectance, and (5) the thickness of adsorbed VOCs is more than 70% smaller than the thickness of the monolayer film that is observed between the droplets during the dropwise condensation on a gold-coated glass. This finding verifies our claim [5; 6] that a thin water film exists during dropwise condensation.  
**Author Contributions:** Conceptualization, S.B.A.; methodology, S.B.A.; software, S.B.A.; validation, S.B.A.; formal analysis, S.B.A.; investigation, S.B.A.; resources, J.S.A., S.H.L., and C.K.C.; data curation, S.B.A.; writing--original draft preparation, S.B.A.; writing--review and editing, S.B.A., C.H.J., J.S.A., S.H.L., and C.K.C.; visualization, S.B.A. and F.L.; supervision, S.H.L. and C.K.C.; project administration, C.K.C.; funding acquisition, J.S.A. and S.H.L.; All authors have read and agreed to the published version of the manuscript.  
**Funding:** This work was partially supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), USA under award number DE-EE0007650 and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF-2020R1F1A1072600).  
**Acknowledgments:** The authors would like to thank Dr. [PERSON] from Michigan Technological University for the XPS imaging and discussion of the XPS results. The authors also appreciate support from Multi-Scale Technologies Institute (MuSTI), Michigan Technological University.  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**List of Symbols**  
\begin{tabular}{l l l} \(C\) & Offset & _mit_ & Minimum (at the resonance angle) \\ \(d\) & Thickness of a layer (nm) & _over_ & Overlayer \\ \(I\) & Intensity value & _SPR_ & Associated with SPRi \\ \(K\) & Wave vector (1/nm) & _sub_ & Sublayer \\ \(n\) & Refractive index & \(t\) & Theoretical \\ \(r\) & Coefficient of reflection between layers & _XPS_ & Associated with XPS microscopy \\ \(R\) & Reflectance & \(0\) & First measurement \\ \(S\) & Scaling factor & \(1\) & Layer 1 \\ \(\alpha\) & Incident angle (\({}^{\circ}\)) in SPRi & \(2\) & Lauer 2 \\ \(\theta\) & XPS angle relative to the surface (\({}^{\circ}\)) & \(3\) & Layer 3 \\ \(\lambda\) & Inelastic mean free path in gold (nm) & \(4\) & Layer 4 \\ subscript & \(\infty\) & Thick Au 4f layer with no overlayer \\ _max_ & Maximum (at the total internal reflection angle) & & \\ \end{tabular}