Silicone Oil-Grafted Low-Hysteresis Water-Repellent Surfaces

Wetting plays a major role in the close interactions between liquids and solid surfaces, which can be tailored by modifying the chemistry as well as the structures of the surfaces’ outermost layer. Several methodologies, such as chemical vapor deposition, physical vapor deposition, electroplating, and chemical reactions, among others, have been adopted for the alteration/modification of such interactions suitable for various applications. However, the fabrication of low-contact line-pinning hydrophobic surfaces via simple and easy methods remains an open challenge. In this work, we exploit one-step and multiple-step silicone oil (5–100 cSt) grafting on smooth silicon substrates (although the technique is suitable for other substrates), looking closely at the effect of viscosity as well as the volume and layers (one to five) of oil grafted as a function of the deposition method. Remarkably, the optimization of grafting of silicone oil fabrication results in non-wetting surfaces with extremely low contact angle hysteresis (CAH) below 1° and high contact angles (CAs) of ∼108° after a single grafting step, which is an order of magnitude smaller than the reported values of previous works on silicone oil-grafted surfaces. Moreover, the different droplet–surface interactions and pinning behavior can additionally be tailored to the specific application with CAH ranging from 1 to 20° and sliding angles between 1.5 and 60° (for droplet volumes of 3 μL), depending on the fabrication parameters adopted. In terms of roughness, all the samples (independent of the grafting parameters) showed small changes in the root-mean-square roughness below 20 nm. Lastly, stability analysis of the grafting method reported here under various conditions shows that the coating is quite stable under mechanical vibrations (bath ultrasonication) and in a chemical environment (ultrasonication in a bath of ethanol) but loses its low-pinning characteristics when exposed to saturated steam at T ∼ 99 °C. The findings presented here provide a basis for selecting the most appropriate and suitable method and parameters for silicone oil grafting aimed at low pinning and low hysteresis surfaces for specific applications.


SI-1. Effect of plasma cleaning on wettability
To understand the necessity of plasma cleaning step in fabrication process, some samples are prepared following the same procedure as detailed in section 2.3. of main manuscript but with exclusion of plasma cleaning step. These samples prepared without plasma cleaning are 1 and 5 layers of 5 cSt oil grafted as well as 1 layer of 100 cSt oil grafted. The resulting CA and CAH are shown

SI-2. Effect of temperature
Temperature is an important parameter for oil grafting as it controls both the oil evaporation as well as the grafting. It has been noticed that the samples deposited with 100 cSt viscosity oil and grafted at 300 o C showed preferential evaporation of oil and the surface was very rough visibly after complete evaporation of the oil. Hence, samples were grafted at 250 o C, which also showed uneven features. One of the cause of this non-homogenous surface grafting seemed to be the oil volume deposited onto it. So, to accommodate these two parameters, the samples were coated with 100 cSt viscosity oil via dip-coating method which deposited approximately 0.185 µL oil for one layer [1] and they were heated at 200 o C which left behind a visible homogenous surface. Lower viscosity oil

SI-3. Wettability analysis on copper substrate
Besides silicon as the base substrate, we prepared some samples using polished copper as substrates.
We grafted 5 cSt oil (up to 5 layers) and 100 cSt oil (up to 3 layers) on the substrate following the same steps as describe in the main manuscript for silicon substrate. The results in terms of apparent contact angle and contact angle hysteresis are given below in Error! Reference source not found.. It can be observed that the 5 cSt oil grafted samples showed similar apparent contact angles when compared to silicon substrate, while the contact angle hysteresis increased considerably by one order of magnitude.
Whereas, in case of 100 cSt oil grafted copper substrates showed only 1° or 2° increase in contact angle hysteresis. On these substrates, presumably lower wetting and more roughness of the copper substrates (as compare to atomically smooth silicon wafer) would slow down or hinder the motion of the silicone oil contact line during grafting, which may result in less uniform and less homogeneous film deposition during grafting with the consequent droplet pinning enhancement. In addition, the different affinity of PDMS brushes attachment (especially short chain length when grafting low viscosity oil) with the base substrate also may play a role in the motion of the water droplet contact line when characterising the CAH.

SI-4. Stability Tests
The prepared samples are tested under different conditions in order to check the robustness and stability of the coatings. These tests included sonication in ultrasonic bath to observe their mechanical stability, sonication in ethanol using ultrasonic bath to check their robustness under harsh organic solvent conditions, soaking in ethanol for longer time period (hours) to see the effect of aging in chemical environment and subjecting to high temperature steam to predict their behaviour under high temperature humid conditions. The sketches of these setups can be found in Figure SI   all with values above 100 o in the hydrophobic regime. When looking into the CAH, an increase in the CAH is observed for all cases after the first and second cycles with increases as high as 14 o in the case of 5 cSt and 1 layer to as low as 3 o for 100 cSt and 1 layer. In all cases, samples seem to reach a plateau after 5 cycles ( Figure SI-7 c&d). After 10 cycles an average CAH of 10 o is reported for 5cSt 1 and 5 layers, while an average CAH of 6 o can be established for 100 cSt independently of the number of layers. To note is the high increase in the CAH for 5 cSt oil grafted sample with 2 layers (Figure SI-7c). The schematic for the steam setup is shown in Figure SI-3d. By not subjecting the sample directly to the steam jet exiting the nozzle, the changes observed on the sample surface are uniform throughout. For lower viscosity oil, i.e., 5 cSt, the apparent CA decreased marginally after 1 cycle but the considerable decrease is observed in 1 layer grafted sample after 3 cycles with apparent CA below 100 o as in Figure   SI   and 5 layers coated sample also exhibited high CAH (roughly around 60 o ) ( Figure SI-8c). On the other hand, for high viscosity oil grafted samples, the increase is CAH is very small below 10 o for 2 and 3 layer grafted samples until 5 cycles of steam exposure. Thereafter, the CAH increases sharply after that with CAH equal or above 20 o as represented in Figure SI-8d. 1 layer and 3 layers grafted sample showed highest and lowest hysteresis respectively at each point. The 100 cSt oil grafted samples showed very good results in terms of wettability (high apparent CA) adhesion (low SA as well as low CAH) and stability with low change of apparent CA, CAH and SA throughout the different samples.