For our experimental observations we used monolayer (1L), 2 layers (2L) and 4 layers (4L) of graphene and multilayer graphite (ML) supported and suspended over holes and trenches. For simplicity, we henceforth refer to suspended graphene with the superscript sus and to supported graphene with the subscript sup. Surface characterization of the fabricated 1L substrate is shown in Fig. 2. More details and description of the surface fabrication and characterization on the rest of the fabricated substrates are given in Section 4.1 Substrate Fabrication and 4.2. Sample Characterization of 1L Suspended Graphene within Section 4 Experimental Section/Methods and within the accompanying Supplementary Information SI-1 and SI-2, respectively.
Direct contact angle measurements of sessile droplets on suspended graphene (1Lsus) have not been reported to date as a consequence of the impossibility to create large suspended regions, by our current fabrication and cleaning procedures, large enough to allow for macroscopic sessile droplet deposition or microgoniometry direct wettability measurements. Note that the minimum size of droplets generated via microgoniometry is at least one order of magnitude greater than the fabricated area of 1Lsus . Thus, to date, the wettability of graphene has been assessed exclusively by indirect methods such as measuring the contact angle of a bubble immersed in water in contact with monolayer graphene or rolling a droplet of water over monolayer graphene powder. Hereafter, to allow for the immediate and intimate visualization of the interactions between droplets with sizes in the order of micrometers in diameter or smaller (femtoliter droplets) and the different suspended and supported graphene substrates, we make use of the excellent spatial resolution provided by Environmental Scanning Electron Microscopy (ESEM) (FEI Versa 3D™, Hillsboro, Oregon, U.S.A.). Note that Zhang et al. recently reported on the wettability of suspended monolayer graphene over a holey substrate atop a TEM grid, i.e., closed holes, making use of ESEM at the same time of this submission. ESEM experimental observations on suspended monolayer graphene over a holey substrate atop a TEM grid with closed holes similar to those reported by Zhang et al. were also carried out in our work and can be seen in Figure S8 in the accompanying Supporting Information SI-5. To highlight is the remarkable agreement on the contact angles reported by Zhang et al.  and those reported in Figure S8 for droplets sitting fully on the suspended graphene droplets resting partially on both suspended and supported graphene with closed holes (See Fig. 4 providing a direct comparison between the contact angle measurements for monolayer graphene suspended over closed holes of this work and that of Zhang et al. [29).
Figure 3 | ESEM experimental observations of condensation on (a-c) 1Lsus, (d) 2Lsus, (e) 4Lsus, and (f) MLsus in time (see Supplementary Videos 1 to 6). Non-wetting water droplets are only observed on suspended graphene (1Lsus). We note here that experimental observations on (a-b) and (c) correspond to 2 different samples prepared by the same fabrication procedure reported in section 4.1 Sample Fabrication and in the accompanying Supplementary Information SI-1. Relative condensation time with respect to the first frame, environmental pressure and scale bars reported from ESEM experimental observations are included for each frame (more details on the ESEM experimental procedure and observations can be found in Section 4.3 ESEM Experimental Observations and within the Supplementary Information SI-3).
Figure 3a,b,c- show droplets growing in the non-wetting regime following the constant contact angle mode (CCA) on 1Lsus, whereas on 2Lsus, 4Lsus, and MLsus (Fig. 3d,e,f) droplets grow also in the CCA mode but in the partial wetting regime. The Supplementary Videos 1 to 6 contain the ESEM images of the 6 panels (a-f) presented in Fig. 3. Videos were captured at approximately 1 frame every 5 seconds highlighting the quasi-static state of our observations. Here, we define “non-wetting droplet” as a droplet with a measured contact angle (or advancing contact angle henceforth referred to as contact angle) larger than 120º, which was known to be the hydrophobic limit/barrier for any smooth hydrophobic surface set by Teflon™ [4, 11]. Non-wetting droplets were found to be reproducible on the same sample even after five condensation-evaporation cycles. This is a remarkable evidence of the difference in wettability between 1Lsus when compared to 1Lsup. This is further ascertained with the observations of the main droplet in Fig. 3b, which initially grows in the non-wetting regime while the base of the droplet is on 1Lsus, and as the droplet grows bigger and the triple contact line touches the supported region (1Lsup) the droplet suddenly spreads, i.e., it transitions from non-wetting on 1Lsus to partial wetting on 1Lsup. The largest contact angle of a droplet on 1Lsus approaches 180º as in the right side of Fig. 3b, which resembles the ideal shape of a free-standing droplet in vacuum and/or in air. To further confirm that the presence of extreme non-wetting droplets is solely an inherent quality of 1Lsus, Fig. 4 includes independent contact angle measurements on both 1L, 2L, 4L and ML suspended and supported graphene at different condensation intervals. In addition, Fig. 4 shows experimental observations of droplets on 1L, 2L, 4L and ML samples simultaneously on both supported and suspended regions. The suspended and supported graphene areas are marked in Fig. 4 and they can be clearly distinguished by the position of the edges.
Figure 4 shows that droplets on 1Lsus have contact angles as high as 175º while droplets on 1Lsup display contact angles close to or below 90º. In the case of 2L, 4L and ML, contact angles of droplets on both suspended and supported samples are practically the same and in the partial wetting regime with values close to 90º or below 90º, which are also similar to those reported here for water droplets on 1Lsup and consistent with the literature22. Non-wetting contact angles on 1Lsus reported in this work and in Fig. 4 differ from wetting contact angles reported on partially suspended monolayer graphene (solid surface fractions as low as 5%) in the work of Ondarçuhu et al.. This is mainly due to the complete suppression of any substrate interaction, which is entirely removed from underneath the footprint of the droplets coupled with the presence of an adsorbed layer of water above and below the 1Lsus as per the high humidity environmental conditions studied under ESEM. Note that spreading of a non-wetting droplet growing on 1Lsus was observed once the advancing contact line of the non-wetting droplet reached the 1Lsup as shown in Fig. 3b. In addition, the agreement between this work and that of Zhang et al. on the wettability of suspended monolayer graphene over closed holes is further provided within Fig. 4. In addition, the occurrence of partially wetting droplets reported on 1Lsus with open holes is attributed to the current inability of the scientific community of fabricating 100% full crystalline, wrinkle free and clean suspended graphene of the sizes required. The presence of amorphous regions, wrinkles and hydrocarbons may induce surface defects which in turn decrease the energy barrier for nucleation favoring condensation on those regions[31, 32]. This provides a plausible and reasonable explanation for the occurrence of the bimodal contact angle distribution showing both wetting and non-wetting droplets on the same 1Lsus.
In macroscopic wetting studies, to assess the extent of pinning the droplet contact line is advanced or receded by addition or withdraw of liquid to the droplet. In contrast, in the ESEM approach the contact line advances by condensation of molecules and, significantly, can overcome small irregularities on the surface. Although nanoscale defects can induce pinning, the quasi-steady droplet growth imposed where condensing molecules build up at the triple phase contact line and at the liquid-gas interface can overcome the pinning energy barrier via thermal fluctuations and/or external forces. Additionally, it is worth noting that condensation shall preferentially take place on such topological or chemical defects; however, based on the Kelvin equation, the cluster size necessary for nucleation is orders of magnitude larger than any of the defects measured on our 1Lsus reported in Fig. 2b. To estimate the extent of this we consider the critical droplet radius for nucleation, re, by making use of the Kelvin equation
where Tv is the temperature of the vapor, γlv is the liquid-vapor interfacial tension, hfg the latent heat of vapor to liquid phase-change, ρl the density of the liquid and ΔT the subcooling temperature[32, 35]. In the present conditions where the ESEM works near the liquid-vapor saturation curve, re is of the order of tens of nanometers, which is two orders of magnitude greater than the average of the defects found in 1Lsus (see calculations on re versus ΔT in Figure S10 in the accompanying Supporting Information SI-5, and AFM characterization of 1Lsus in Fig. 2b and in the accompanying Supporting Information SI-2.2). This estimation points out that the cluster size of the molecules required for nucleation is orders of magnitude greater than the defects present. This suggests that pinning of the contact line by small defects on the nm scale is unlikely to explain the observation of non-wetting droplets on 1Lsus.
Moreover, the simultaneous occurrence of spherical non-wetting and partial wetting droplets on 1Lsus and 1Lsup, respectively, shown in Fig. 4 within the same frame under the same temperature and pressure conditions provides evidence for unique non-wetting properties of 1Lsus. Further, nanoscale defects increase in size and number as the number of graphene layers increase; hence if pinning on such defects were to be the reasons for the non-wetting droplets observed, such behaviour should have been reported on the rest of supported and suspended graphene substrates with multiple layers. Despite the greater degree of nanoscopic defects and roughness of 1Lsup, 2L, 4L and ML, when compared to 1Lsus (see Fig. 2 and Figure S5 and S6), which should promote contact line pinning, the lack of non-wetting droplets on of 1Lsup, 2L, 4L and ML (0 drops out of more than 2000 observations) suggests that surface contamination, roughness and/or pinning may not be the mechanism for the extreme non-wetting behaviour reported here on 1Lsus. An intrinsic limitation in these experiments is that because of the small volume of the imaged droplets limited by the 1Lsus sample size and the quasi-steady ESEM imaging technique utilized, any receding of the triple contact line before complete evaporation cannot be captured.
Theoretical trends based on the Lennard-Jones (L-J) potential from Ref. 9 and 22 offer a reasonable explanation for the contact angles of wetting droplets reported on multilayer graphene but cannot explain the case of suspended monolayer[10, 24]. Under ESEM operating conditions, the environment is saturated with water vapor and hence the suspended graphene layer is surrounded by water vapor where the polar interactions of water molecules above and below graphene could act across this one atomic layer, as suggested in recent works. Recent ESEM experiment of water condensation on single side of graphene and additional experiments reproducing the conditions reported in Ref. 29 carried out in this investigation so to validate our approach (Figure S8 in the Supplementary Information) indicate that non-wetting droplets can be seen exclusively on suspended monolayers open to the ambient on both sides and not on monolayer graphene where either of the sides is closed to the environment. The present findings may be relevant to further understanding monolayer wettability and offer additional elements on the wetting translucency and the high slippage of graphene upon interaction with water molecules as reported recently[23,37−39].