HM acknowledges funding from King Abdullah University of Science and Technology (KAUST).

NOTE: Arrays of reentrant and doubly reentrant cavities and pillars were microfabricated by adapting the multistep protocol for pillars reported by Liu and Kim27. Precautions were taken to minimize the formation of pin residues or particles on our surfaces that could interfere with wetting transitions36.
MICROFABRICATION OF CAVITIES 
Broadly, the protocols for the microfabrication of reentrant and doubly reentrant cavities (RCs and DRCs) consist of two-dimensional layout designing, photolithography, general silica etching, and specific silicon etching, depending on the final feature required37,38,39,40,41.
1. Design
<ol>
	<li>Start the microfabrication process by designing the required pattern in a layout software42. An example of such a software is listed in the Materials List.</li>
	<li>Using the software, create a new file. Draw a unit cell comprising a circle of diameter, D = 200 &#181;m. Copy and paste this circle with a center-to-center distance (pitch) of L = 212 &#181;m to create an array of circles in a square patch of area 1 cm2 (Figure 2).</li>
	<li>Draw a circle of diameter 100 mm (4 inches). Place the 1 cm2 square array inside the circle and replicate it to create a 4 x 4 grid of square arrays. Features inside the circle will be transferred onto the 4-inch wafers (Figure 2).</li>
	<li>Export the design file to the desired format for the mask writing system (e.g., the GDSII format).</li>
</ol>
2. Cleaning of Wafers
<ol>
	<li>Clean a silicon wafer 4 inches in diameter, &#60;100&#62; orientation, and with a 2.4 &#181;m thick thermal oxide layer (see the Materials List), in piranha solution for 10 min. Piranha solution comprises sulfuric acid (H2SO4, 96%): hydrogen peroxide (H2O2, 30%) in a 3:1 volumetric ratio and is maintained at T = 388 K.</li>
	<li>Rinse the wafer with deionized water and spin-dry under nitrogen (N2) environment.</li>
</ol>
3. Photolithography
<ol>
	<li>Coat the wafer with hexamethyldisilazane (HMDS) using vapor-phase deposition to improve adhesion with the photoresist. Refer to Table 1 for the process details.</li>
	<li>Mount the wafer on a 4-inch vacuum chuck in the spin coater. Cover the wafer with the AZ-5214E photoresist. Use the spin coater to spread the photoresist uniformly on the surface as a 1.6 &#181;m-thick layer. Refer to Table 2 for spin coating parameters.</li>
	<li>Bake the photoresist-coated wafer on a hot-plate maintained at 110 &#176;C for 120 s.</li>
	<li>Transfer the wafer to a direct-writing system and expose the wafer to UV radiation for 55 ms (defocus: +5). This step transfers the desired design on the AZ-5214E (used in the positive tone; see Materials List) (Figure 2).</li>
	<li>Place the UV-exposed wafer in a glass Petri dish containing the AZ-726 developer for 60 s for the features to develop. See Materials List for details.</li>
	<li>Remove the wafer from the developer solution and rinse with deionized (DI) water gently to remove excess developer. Spin dry the wafer in a N2 environment. These steps are presented in Figure 3A&#8211;C.</li>
</ol>
NOTE: At the end of this step, design patterns on the wafer can be seen under a standard optical microscope.
4. Anisotropic Etching of Silica (SiO2) Layer
NOTE: The goal of this step is to completely etch away the silica layer (2.4 &#181;m-thick) that was exposed during photolithography to expose the silicon layer underneath.
<ol>
	<li>After photolithography, transfer the wafer to an inductively coupled plasma (ICP) reactive-ion etching (RIE) system that employs a mixture of octafluorocyclobutane (C4F8) and oxygen (O2) gases to etch silica vertically downward (anisotropic etching).</li>
	<li>Run the ICP-RIE process for approximately 13 min to etch the exposed silica layer. Refer to the ICP-RIE parameters in Table 3. During this step, the photoresist layer also gets completely etched away (Figure 3C&#8211;D).</li>
	<li>To ensure that the silica layer thickness inside the desired patterns is reduced to zero, so that the silicon layer is exposed, measure the thickness of the remaining silica using a reflectometer. Adjust the duration of the subsequent etching period based on the thicknesses of the silica layers (especially in and around the patterns).</li>
</ol>
NOTE: A reflectometer was used to measure the thickness of the remaining silica layer43. Alternatively, other tools, such as ellipsometer or an interactive color chart to predict the color of SiO2 and thickness can also be used44,45.
The procedures detailed in steps 1 and 4 are common for both reentrant and doubly reentrant cavities. However, the etching protocols for the silicon layer are different and are described below:
5. Reentrant Cavities
<ol>
	<li>Anisotropic silicon etching
	<ol>
		<li>After etching the silica layer, transfer the wafer to a deep ICP-RIE system to etch silicon. The first step consists of a fluorine-based anisotropic etching method known as the Bosch process that etches silicon vertically downward, creating a straight wall. 
		NOTE: The Bosch process uses C4F8 and sulfur hexafluoride (SF6) gases in the reaction chamber: the C4F8 deposition creates a passivation layer, while the SF6 etches silicon vertically downward. Thus, the Bosch process enables the microfabrication of deep trenches in silicon with high-aspect ratios.</li>
		<li>Run this process for five cycles, which corresponds to an etching depth for silicon equivalent to &#8776; 2 &#181;m. Process parameters are listed in Table 4.</li>
		<li>Clean the wafer in piranha solution for 10 min to remove any remnants of the Bosch process. Rinse the wafer with DI water and spin-dry in a N2 environment (Figure 3E).</li>
	</ol>
	</li>
	<li>Isotropic silicon etching: In order to create the reentrant feature, perform isotropic etching that would create an undercut beneath the silica layer. A 5 &#181;m overhang can be achieved by etching the silicon layer with SF6 for 2 min 45 s (Figure 3F). Refer to Table 5 for the process parameters.</li>
	<li>Anisotropic silicon etching: Once the reentrant features are created, tune the depth of the cavities by the Bosch process (step 5.1). 
	NOTE: To microfabricate cavities with a depth of hc &#8776; 50 &#181;m, 160 cycles of the Bosch process are required (Figure 3G, Table 4).</li>
	<li>Wafer cleaning and storage
	<ol>
		<li>Clean the wafer using piranha solution as described in step 2. After this step, the wafer becomes superhydrophilic, characterized by contact angles of water, &#952;o &#8776; 0&#176;.</li>
		<li>Store the wafer in a glass Petri dish and place inside a clean vacuum oven maintained at T = 323 K and vacuum pressure PVac = 3.3 kPa for 48 h, after which the intrinsic contact angle of the silica layer stabilizes to &#952;o &#8776; 40&#176;.</li>
		<li>Store the samples in a clean cabinet equipped with an outward nitrogen (99%) flow, ready for further characterization.</li>
	</ol>
	</li>
</ol>
6. Doubly Reentrant Cavities
<ol>
	<li>Anisotropic silicon etching: To create doubly reentrant cavities, follow steps 1, 2, 3, 4, and 5.1 (see Figure 4A&#8211;E).</li>
	<li>Isotropic silicon etching 
	In order to create doubly reentrant features, reentrant features must be created first. To achieve that, perform isotropic etching to create an undercut beneath the silica layer. Etch the silicon layer with SF6 for 25 s (Figure 4F). Refer to Table 5 for the process parameters. Subsequently, clean the wafer using piranha solution as described in step 2.</li>
	<li>Thermal oxide growth
	<ol>
		<li>To achieve doubly reentrant features, grow a 500 nm layer of thermal oxide on the wafer, using a high temperature furnace system (Figure 4G).</li>
		<li>Measure the thickness of the oxide layer using a reflectometer. 
		NOTE: The oxidation was carried out by exposing the samples to an environment comprising oxygen (O2) and water vapor, leading to the wet oxidation of silicon in an enclosed environment at temperatures ranging from 800&#8211;1,200 &#176;C.</li>
	</ol>
	</li>
	<li>Silica etching: Carry out the same process as described in the step 4 to etch silica vertically downward for 3 min. As a result of the anisotropic etching, the thermal oxide (500 nm thick silica layer) is etched away from the cavity, but it leaves an &#34;overhang&#34; along the sidewalls that would form the doubly reentrant edge eventually (Figure 4H, Table 3).</li>
	<li>Anisotropic silicon etching: Repeat five cycles of the Bosch process to deepen of the cavities by &#8776; 2 &#181;m (Figure 4I, Table 5). This step is necessary to remove the silicon behind the doubly reentrant feature in the next step. Clean the wafer using piranha solution.</li>
	<li>Isotropic silicon etching: Perform the isotropic etching of silicon for 2 min and 30 s using the process parameters described in Table 4. This step creates an empty space (&#8776;2 &#181;m) behind the thermally-grown oxide at the mouth of the cavity, leading to the doubly reentrant edge (Figure 4J).</li>
	<li>Anisotropic silicon etching: Use the Bosch process recipe (step 5.1) for 160 cycles to increase the depth of the cavities to hc &#8776; 50 &#181;m, (Figure 4K, Table 5).</li>
	<li>Wafer cleaning and storage: Clean the wafer using piranha solution and store as described in step 5.4 above.</li>
</ol>
MICROFABRICATION OF PILLARS 
The design protocol for fabricating reentrant and doubly reentrant pillars and &#34;hybrids&#34; (comprising doubly reentrant pillars surrounded by walls) consists of three key steps: wafer preparation, silica etching, and specific silicon etching. Figure 5A&#8211;C show the top-view of the layout design for reentrant and doubly reentrant pillars, while Figure 5D&#8211;F represent the layout of the hybrid arrays. Select the dark-field option of the UV exposure in order to expose the whole wafer except for the pattern using the same photoresist (AZ5214E) (Figure 6A&#8211;C and Figure 7A&#8211;C). Besides these specificities, the processes for cleaning the wafer (step 2) and etching silica (step 4) are identical.
7. Reentrant Pillars
<ol>
	<li>Anisotropic silicon etching: After photolithography, UV exposure, development, and etching silica with the specificities for pillars described above (steps 1&#8211;4), transfer the wafer to a deep ICP-RIE system to etch the silicon layer using the Bosch process. This step controls the height of the pillars. Use 160 cycles of the Bosch process to achieve pillars of height, hP &#8776; 30 &#181;m (Figure 6E, Table 5). Clean the wafer as described in step 2.</li>
	<li>Isotropic silicon etching: Perform isotropic etching using SF6 for 5 min to create the reentrant edge on the pillars (Figure 6F, Table 4). The resulting length of the overhang is 5 &#181;m.</li>
	<li>Piranha cleaning and storage: Clean the wafer using piranha solution and store as described in step 5.4 above.</li>
</ol>
8. Doubly Reentrant Pillars and Hybrids
<ol>
	<li>Anisotropic silicon etching: After etching SiO2, transfer the wafer to a deep ICP-RIE system to etch the Si under the SiO2 layer. Perform five cycles of the Bosch process that corresponds to an etching depth of &#8776; 2 &#181;m (Figure 7E, Table 4). Subsequently, clean the wafer as described in step (2).</li>
	<li>Isotropic silicon etching: Carry out isotropic etching using SF6 for 16 s to create the reentrant edge (Table 5, Figure 7F). Clean the wafer as described in step 2.</li>
	<li>Thermal oxide growth: Grow 500 nm layer of thermal oxide all over the wafer using a high temperature furnace system as described in step 6.3 (Figure 7G).</li>
	<li>Silica etching: Etch the thermally-grown oxide layer (500 nm thick) for 3 min as described in step 6.4 (Figure 7H, Table 3).</li>
	<li>Anisotropic silicon etching: Repeat 160 cycles of the Bosch process (Table 4) to increase the height of the pillars (Figure 7I). Clean the wafer as described in step 2 above.</li>
	<li>Isotropic silicon etching: Perform isotropic etching of silicon for 5 min using the process parameters as described in Table 4. This step creates the doubly reentrant edge (Figure 7J). The space between pillar stem and doubly reentrant edge is &#8776;2 &#181;m.</li>
	<li>Wafer cleaning and storage: Clean the wafer using piranha solution and store as described in step 5.4 above.</li>
</ol>
Figure 8 represents the list of processes used in microfabricating reentrant and doubly reentrant cavities and pillars.

The authors declare that they have no competing interests.

Here we discuss additional factors and design criteria to help the reader in applying these microfabrication protocols. For cavity microtextures (RCs and DRCs) the choice of pitch is crucial. Thinner walls between adjacent cavities would lead to low liquid-solid interfacial area and high liquid-vapor interfacial area, leading to high apparent contact angles34. However, thin walls could compromise the mechanical integrity of the microtexture, for instance, during handling and characterization; a little over-etching with thin walls (e.g., in step 6.6) could destroy the entire microtexture; under-etching with thin walls could also prevent the development of doubly reentrant features. If the DRC features are not fully developed, their ability to entrap air for long-term might suffer, especially if the liquid condenses inside the cavities26. For this reason, we chose the pitch in our experiments to be L = D + 12 &#956;m (i.e., the minimum wall thickness between the cavities was 12 &#181;m). We also fabricated doubly reentrant cavities with a smaller pitch of L = D + 5 &#956;m, but the resulting surfaces were not homogeneous due to structural damage during microfabrication.
During the etching of the silica layer with C4F8 and O2 in step 4, the prior history of usage or the cleanliness of the reaction chamber could give variable results, despite following the same steps, for instance, in a common user facility such as in most universities. Thus, it is recommended that this step is performed in short time periods, for instance, no more than 5 min each and monitored the thickness of the silica layer by an independent technique, such as reflectometry. For our wafers with a 2.4 &#956;m-thick silica layer, a typical etching routine took 13 min to remove silica completely from the targeted areas (Table 3). Because the photoresist was also etched during the process, this step removed 1 &#956;m of the silica layer that was initially masked by the photoresist. Furthermore, to ensure that the etching rate was as expected, and to avoid cross-contamination from previous etch processes (a common issue in multiuser facilities), silica etching was always preceded by etching a sacrificial wafer as a precautionary step. During the development of the photoresist, the exposed surface might get contaminated with the photoresist&#39;s traces/particles, which could act as (microscopic) masks leading to the formation of pin residues. To avoid this, rigorous cleaning and storage protocols should be followed throughout the microfabrication process36.
Similarly, during the Bosch process, even though the SiO2 layer acts as a mask for the Si-layer underneath, it does get etched during long etching cycles, albeit at slower rates. Thus, the depth of the cavities or the height of the pillars is limited up to the point that the reentrant features will not be compromised. The passivation and etching times during the Bosch process should be tuned to obtain smooth walls. This can be achieved by testing recipes iteratively and observing their effects on samples, for instance, using electron microscopy.
In the case of RPs and DRPs, the longer the duration of isotropic etching, the smaller the diameter of the stem. If the diameter is less than 10 &#956;m, it might lead to mechanical fragility. This limitation should inform the design at the beginning of the microfabrication procedure.
Dry-etching tools commonly available in universities do not have industrial-grade tolerances, leading to spatial non-uniformities in terms of the rate of etching inside the chamber. Thus, the features obtained in the center of the wafer might not be the same as those at the boundary. To overcome this limitation, we used four-inch wafers and concentrated only in the central region.
We also recommend using direct-writing systems instead of using hard-contact masks for photolithography, allowing for rapid changes in design parameters, including feature diameters, pitches and shapes (circular, hexagonal and square), etc.
Obviously, neither SiO2/Si wafers nor photolithography are the desired materials or processes for the mass production of omniphobic surfaces. However, they serve as an excellent model system to explore innovative microtextures for engineering omniphobic surfaces, for instance by biomimetics26,27,34,35,46,47, which can be translated to low-cost and scalable materials systems for applications. It is expected that in the near future, the design principles for GEMs might be scaled up using techniques such as 3-D printing48, additive manufacturing49, and laser micromachining50, among others. Microtextured SiO2/Si surfaces could also be used for templating soft materials29,51. Currently, we are investigating applications of our gas-entrapping surfaces for mitigating cavitation damage47, desalination46,52, and reducing hydrodynamic drag.

Solid surfaces that exhibit apparent contact angles, &#952;r &#62; 90&#176; for polar and nonpolar liquids, such as water and hexadecane, are referred to as omniphobic1. These surfaces serve numerous practical applications, including water desalination2,3, oil-water separation4,5, antibiofouling6, and reducing hydrodynamic drag7. Typically, omniphobicity necessitates perfluorinated chemicals and random topographies8,9,10,11,12. However, the cost, non-biodegradability, and vulnerability of those materials/coatings pose a myriad of constraints, e.g., perfluorinated desalination membranes degrade as the feed-side temperatures are raised, leading to pore-wetting13,14, and perfluorinated/hydrocarbon coatings also get abraded15,16 and degraded by silt particles in the flow streams and cleaning protocols. Thus, there is a need for alternative strategies for achieving the functions of perfluorinated coatings (i.e., entrapping air on immersion in liquids without using water repellent coatings). Therefore, researchers have proposed surface topographies comprised of overhanging (reentrant) features that could entrap air on immersion by microtexturing alone17,18,19,20,21,22,23,24,25. These microtextures come in three types: cavities26, pillars27, and fibrous mats8. Hereafter, we will refer to reentrant features with simple overhangs as reentrant (Figure 1A&#8211;B and Figure 1E&#8211;F) and reentrant features with overhangs that make a 90&#176;-turn towards the base as doubly reentrant (Figure 1C&#8211;D and Figure 1G&#8211;H).
In their pioneering work, Werner et al.22,28,29,30,31 characterized cuticles of springtails (Collembola), soil-dwelling arthropods, and explained the significance of mushroom-shaped (reentrant) features in the context of wetting. Others have also investigated the role of mushroom-shaped hairs in sea-skaters32,33 towards facilitating extreme water repellence. Werner and coworkers demonstrated the omniphobicity of intrinsically wetting polymeric surfaces by carving biomimetic structures through reverse imprint lithography29. Liu and Kim reported on silica surfaces adorned with arrays of doubly reentrant pillars that could repel drops of liquids with surface tensions as low as &#947;LV = 10 mN/m, characterized by apparent contact angles, &#952;r &#8776; 150&#176; and extremely low contact angle hysteresis27. Inspired by these amazing developments, we followed the recipes of Liu and Kim to reproduce their results. However, we discovered that those microtextures would catastrophically lose their superomniphobicity, i.e. &#952;r &#8594; 0&#176;, if wetting liquid drops touched the edge of the microtexture or if there was localized physical damage34. These findings demonstrated that pillar-based microtextures were unfit for applications that required omniphobicity on immersion, and they also questioned the criteria for assessing omniphobicity (i.e., should they be limited to contact angles alone, or if additional criteria are needed).
In response, using the SiO2/Si wafers, we prepared arrays of microscale cavities with doubly reentrant inlets and, and using water and hexadecane as the representative polar and nonpolar liquids, we demonstrated that (i) these microtextures prevent liquids from entering them by entrapping air, and (ii) the compartmentalized architecture of the cavities prevents the loss of the entrapped air by localized defects34. Thus, we have termed these microtextures as &#34;gas-entrapping microtextures&#34; (GEMs). As the next step, we microfabricated GEMs with varying shapes (circular, square, hexagonal) and profiles (simple, reentrant, and doubly reentrant) to systematically compare their performance under immersion in wetting liquids26. We also created a hybrid microtexture comprising arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles, which prevented liquids from touching the stems of the pillars and robustly entrapped air on immersion35. Below, we present detailed protocols for manufacturing GEMs on SiO2/Si surfaces through photolithography and etching techniques along with design parameters. We also present representative results of characterizing their wetting by contact angle goniometry (advancing/receding/as-placed angles) and immersion in hexadecane and water.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>AZ-5214 E photoresist</td>
			<td>Merck</td>
			<td>DEAA070796-0W59</td>
			<td>Photoresist, flammable liquid</td>
		</tr>
		<tr>
			<td>AZ-726 MIF developer</td>
			<td>Merck</td>
			<td>10055824960</td>
			<td>To develop photoresist</td>
		</tr>
		<tr>
			<td>Confocal microscopy</td>
			<td>Zeiss</td>
			<td>Zeiss LSM710</td>
			<td>Upright confocal microscope to visualize liquid meniscus shape</td>
		</tr>
		<tr>
			<td>Deep ICP-RIE</td>
			<td>Oxford Instruments</td>
			<td>Plasmalab system100</td>
			<td>Silicon etching tool</td>
		</tr>
		<tr>
			<td>Direct writer</td>
			<td>Heidelberg Instruments</td>
			<td>&#181;PG501</td>
			<td>Direct-writing system</td>
		</tr>
		<tr>
			<td>Drop shape analyzer</td>
			<td>KRUSS</td>
			<td>DSA100</td>
			<td>To measure contact angle</td>
		</tr>
		<tr>
			<td>Hexadecane</td>
			<td>Alfa Aesar</td>
			<td>544-76-3</td>
			<td>Test liquid</td>
		</tr>
		<tr>
			<td>Highspeed imaging camera</td>
			<td>Phantom vision research</td>
			<td>v1212</td>
			<td>To image droplet bouncing</td>
		</tr>
		<tr>
			<td>HMDS vapor prime</td>
			<td>Yield Engineering systems</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Hot plate</td>
			<td>Cost effective equipments</td>
			<td>Model 1300</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide 30%</td>
			<td>Sigma Aldrich</td>
			<td>7722-84-1</td>
			<td>To prepare piranha solution</td>
		</tr>
		<tr>
			<td>Imaris software</td>
			<td>Bitplane</td>
			<td>Version 8</td>
			<td>Post process confocal microscopy images</td>
		</tr>
		<tr>
			<td>Nile Red</td>
			<td>Sigma Aldrich</td>
			<td>7385-67-3</td>
			<td>Fluorescent dye for hexadecane</td>
		</tr>
		<tr>
			<td>Nitrogen gas</td>
			<td>KAUST lab supply</td>
			<td></td>
			<td>To dry the wafer</td>
		</tr>
		<tr>
			<td>Petri dish</td>
			<td>VWR</td>
			<td>HECH41042036</td>
			<td></td>
		</tr>
		<tr>
			<td>Reactive-Ion Etching (RIE)</td>
			<td>Oxford Instruments</td>
			<td>Plasmalab system100</td>
			<td>Silica etching tool</td>
		</tr>
		<tr>
			<td>Reflectometer</td>
			<td>Nanometrics</td>
			<td>Nanospec 6100</td>
			<td>To check remaining oxide layer thickness</td>
		</tr>
		<tr>
			<td>Rhodamine B (Acros)</td>
			<td>Fisher scientific</td>
			<td>81-88-9</td>
			<td>Fluorescent dye for water</td>
		</tr>
		<tr>
			<td>SEM stub</td>
			<td>Electron Microscopy Sciences</td>
			<td>75923-19</td>
			<td></td>
		</tr>
		<tr>
			<td>SEM-Quanta 3D</td>
			<td>FEI</td>
			<td>Quanta 3D FEG Dual Beam</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicon wafer</td>
			<td>Silicon Valley Microelectronics</td>
			<td></td>
			<td>Single side polished, 4&#34; diameter, 500 &#181;m thickness, 2.4 &#181;m thick oxide layer</td>
		</tr>
		<tr>
			<td>Spin coater</td>
			<td>Headway Research,Inc</td>
			<td>PWM32</td>
			<td></td>
		</tr>
		<tr>
			<td>Spin rinse dryer</td>
			<td>MicroProcess technology</td>
			<td>Avenger Ultra -Pure 6</td>
			<td>Dry the wafers after piranha clean</td>
		</tr>
		<tr>
			<td>Sulfuric acid 96%</td>
			<td>Technic</td>
			<td>764-93-9</td>
			<td>To prepare piranha solution</td>
		</tr>
		<tr>
			<td>Tanner EDA L-Edit software</td>
			<td>Tanner EDA, Inc.</td>
			<td>version15</td>
			<td>Layout design</td>
		</tr>
		<tr>
			<td>Thermal oxide growth</td>
			<td>Tystar furnace</td>
			<td></td>
			<td>To grow thermal oxide in patterned silicon wafer</td>
		</tr>
		<tr>
			<td>Tweezers</td>
			<td>Excelta</td>
			<td>490-SA-PI</td>
			<td>Wafer tweezer</td>
		</tr>
		<tr>
			<td>Vacuum oven</td>
			<td>Thermo Scientific</td>
			<td>13-258-13</td>
			<td></td>
		</tr>
		<tr>
			<td>Water</td>
			<td>Milli-Q</td>
			<td>Advantage A10</td>
			<td>Test liquid</td>
		</tr>
	</tbody>
</table>

rendering sio2/si surfaces omniphobic by carving gas-entrapping microtextures comprising reentrant and doubly reentrant cavities or pillars

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping microtextures (GEMs) on them comprising cavities and pillars with reentrant and doubly reentrant features. Specifically, we use SiO2/Si as the model system and share protocols for two-dimensional (2D) designing, photolithography, isotropic/anisotropic etching techniques, thermal oxide growth, piranha cleaning, and storage towards achieving those microtextures. Even though the conventional wisdom indicates that roughening intrinsically wetting surfaces (&#952;o &#60; 90&#176;) renders them even more wetting (&#952;r &#60; &#952;o &#60; 90&#176;), GEMs demonstrate liquid repellence despite the intrinsic wettability of the substrate. For instance, despite the intrinsic wettability of silica &#952;o &#8776; 40&#176; for the water/air system, and &#952;o &#8776; 20&#176; for the hexadecane/air system, GEMs comprising cavities entrap air robustly on immersion in those liquids, and the apparent contact angles for the droplets are &#952;r &#62; 90&#176;. The reentrant and doubly reentrant features in the GEMs stabilize the intruding liquid meniscus thereby trapping the liquid-solid-vapor system in metastable air-filled states (Cassie states) and delaying wetting transitions to the thermodynamically-stable fully-filled state (Wenzel state) by, for instance, hours to months. Similarly, SiO2/Si surfaces with arrays of reentrant and doubly reentrant micropillars demonstrate extremely high contact angles (&#952;r &#8776; 150&#176;&#8211;160&#176;) and low contact angle hysteresis for the probe liquids, thus characterized as superomniphobic. However, on immersion in the same liquids, those surfaces dramatically lose their superomniphobicity and get fully-filled within &#60;1 s. To address this challenge, we present protocols for hybrid designs that comprise arrays of doubly reentrant pillars surrounded by walls with doubly reentrant profiles. Indeed, hybrid microtextures entrap air on immersion in the probe liquids. To summarize, the protocols described here should enable the investigation of GEMs in the context of achieving omniphobicity without chemical coatings, such as perfluorocarbons, which might unlock the scope of inexpensive common materials for applications as omniphobic materials. Silica microtextures could also serve as templates for soft materials.

Watch this Scientific Journal Video about Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars at JoVE.com

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

This work presents microfabrication protocols for achieving cavities and pillars with reentrant and doubly reentrant profiles on SiO2/Si wafers using photolithography and dry etching. Resulting microtextured surfaces demonstrate remarkable liquid repellence, characterized by robust long-term entrapment of air under wetting liquids, despite the intrinsic wettability of silica.

Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

We present microfabrication protocols for rendering intrinsically wetting materials repellent to liquids (omniphobic) by creating gas-entrapping ...

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Research

JoVE Journal

Engineering

8.8K Views.  King Abdullah University of Science and Technology (KAUST). This work presents microfabrication protocols for achieving cavities and pillars with reentrant and doubly reentrant profiles on SiO2/Si wafers using photolithography and dry etching. Resulting microtextured surfaces demonstrate remarkable liquid repellence, characterized by robust long-term entrapment of air under wetting liquids, despite the intrinsic wettability of silica.

Video: Rendering SiO2/Si Surfaces Omniphobic by Carving Gas-Entrapping Microtextures Comprising Reentrant and Doubly Reentrant Cavities or Pillars

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

High-speed Particle Image Velocimetry Near Surfaces

Multi-dimensional and transient flows play a key role in many areas of science, engineering, and health sciences but are often not well understood. The complex nature of these flows may be studied using particle image velocimetry (PIV), a laser-based imaging technique for optically accessible flows. Though many forms of PIV exist that extend the technique beyond the original planar two-component velocity measurement capabilities, the basic PIV system consists of a light source (laser), a camera, tracer particles, and analysis algorithms. The imaging and recording parameters, the light source, and the algorithms are adjusted to optimize the recording for the flow of interest and obtain valid velocity data. 
Common PIV investigations measure two-component velocities in a plane at a few frames per second. However, recent developments in instrumentation have facilitated high-frame rate (&gt; 1 kHz) measurements capable of resolving transient flows with high temporal resolution. Therefore, high-frame rate measurements have enabled investigations on the evolution of the structure and dynamics of highly transient flows. These investigations play a critical role in understanding the fundamental physics of complex flows.
A detailed description for performing high-resolution, high-speed planar PIV to study a transient flow near the surface of a flat plate is presented here. Details for adjusting the parameter constraints such as image and recording properties, the laser sheet properties, and processing algorithms to adapt PIV for any flow of interest are included.

A procedure for studying transient flows near boundaries using high-resolution, high-speed particle image velocimetry (PIV) is described here. PIV is a non-intrusive measurement technique applicable to any optically accessible flow by optimizing several parameter constraints such as the image and recording properties, the laser sheet properties, and analysis algorithms.

Multi-dimensional and transient flows play a key role in many areas of science, engineering, and health sciences but are often not well understood. The ...

Monolayer Contact Doping of Silicon Surfaces and Nanowires Using Organophosphorus Compounds

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra shallow and sharp doping profiles at the nanometer scale. In MLCD process the dopant source is a monolayer containing dopant atoms.
In this article a detailed procedure for surface doping of silicon substrate as well as silicon nanowires is demonstrated. Phosphorus dopant source was formed using tetraethyl methylenediphosphonate monolayer on a silicon substrate. This monolayer containing substrate was brought to contact with a pristine intrinsic silicon target substrate and annealed while in contact. Sheet resistance of the target substrate was measured using 4 point probe. Intrinsic silicon nanowires were synthesized by chemical vapor deposition (CVD) process using a vapor-liquid-solid (VLS) mechanism; gold nanoparticles were used as catalyst for nanowire growth. The nanowires were suspended in ethanol by mild sonication. This suspension was used to dropcast the nanowires on silicon substrate with a silicon nitride dielectric top layer. These nanowires were doped with phosphorus in similar manner as used for the intrinsic silicon wafer. Standard photolithography process was used to fabricate metal electrodes for the formation of nanowire based field effect transistor (NW-FET). The electrical properties of a representative nanowire device were measured by a semiconductor device analyzer and a probe station.

A detailed procedure for surface doping of Silicon interfaces is provided. The ultra-shallow surface doping is demonstrated by using phosphorus containing monolayers and rapid annealing process. The method can be used for doping of macroscopic area surfaces as well as nanostructures.

Monolayer Contact Doping (MLCD) is a simple method for doping of surfaces and nanostructures1. MLCD results in the formation of highly controlled, ultra ...

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments3, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.
Exceptionally flat, 5 cm diameter, 375 &#181;m thick Si wafers with about 1 &#181;m variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned2 in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water4. The wafers are bonded at RT and then annealed at ~1,100 &#176;C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%1,2. The resulting geometry allows for measurements of confined liquids and gasses.

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned ...

Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

Here, we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. Optical property evaluation of thin films and the surfaces of bulk materials is an important step in understanding new optical material systems and their applications. The method presented can estimate thickness, refractive index, and use absorptive properties of materials for detection. This metrology system uses evanescent field-based photoacoustics (EFPA), a field of research based upon the interaction of an evanescent field with the photoacoustic effect. This interaction and its resulting family of techniques allow the technique to probe optical properties within a few hundred nanometers of the sample surface. This optical near field allows for the highly accurate estimation of material properties on the same scale as the field itself such as refractive index and film thickness. With the use of EFPA and its sub techniques such as total internal reflection photoacoustic spectroscopy (TIRPAS) and optical tunneling photoacoustic spectroscopy (OTPAS), it is possible to evaluate a material at the nanoscale in a consolidated instrument without the need for many instruments and experiments that may be cost prohibitive.

Here we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. This technique evanescent field-based photoacoustics can be used to create a photoacoustic metrology system to estimate materials&#39; thicknesses, bulk and thin film refractive indices, and explore their optical properties.

Here, we present a protocol to estimate material and surface optical properties using the photoacoustic effect combined with total internal reflection. ...

The Evolution of Silica Nanoparticle-polyester Coatings on Surfaces Exposed to Sunlight

Corrosion of metallic surfaces is prevalent in the environment and is of great concern in many areas, including the military, transport, aviation, building and food industries, amongst others. Polyester and coatings containing both polyester and silica nanoparticles (SiO2NPs) have been widely used to protect steel substrata from corrosion. In this study, we utilized X-ray photoelectron spectroscopy, attenuated total reflection infrared micro-spectroscopy, water contact angle measurements, optical profiling and atomic force microscopy to provide an insight into how exposure to sunlight can cause changes in the micro- and nanoscale integrity of the coatings. No significant change in surface micro-topography was detected using optical profilometry, however, statistically significant nanoscale changes to the surface were detected using atomic force microscopy. Analysis of the X-ray photoelectron spectroscopy and attenuated total reflection infrared micro-spectroscopy data revealed that degradation of the ester groups had occurred through exposure to ultraviolet light to form COO&#903;, -H2C&#903;, -O&#903;, -CO&#903; radicals. During the degradation process, CO and CO2 were also produced.

Two types of surfaces, polyester-coated steel and polyester coated with a layer of silica nanoparticles, were studied. Both surfaces were exposed to sunlight, which was found to cause substantial changes in the chemistry and nanoscale topography of the surface.

Corrosion of metallic surfaces is prevalent in the environment and is of great concern in many areas, including the military, transport, aviation, ...

Pool-Boiling Heat-Transfer Enhancement on Cylindrical Surfaces with Hybrid Wettable Patterns

In this study, pool-boiling heat-transfer experiments were performed to investigate the effect of the number of interlines and the orientation of the hybrid wettable pattern. Hybrid wettable patterns were produced by coating superhydrophilic SiO2 on a masked, hydrophobic, cylindrical copper surface. Using de-ionized (DI) water as the working fluid, pool-boiling heat-transfer studies were conducted on the different surface-treated copper cylinders of a 25-mm diameter and a 40-mm length. The experimental results showed that the number of interlines and the orientation of the hybrid wettable pattern influenced the wall superheat and the HTC. By increasing the number of interlines, the HTC was enhanced when compared to the plain surface. Images obtained from the charge-coupled device (CCD) camera indicated that more bubbles formed on the interlines as compared to other parts. The hybrid wettable pattern with the lowermost section being hydrophobic gave the best heat-transfer coefficient (HTC). The experimental results indicated that the bubble dynamics of the surface is an important factor that determines the nucleate boiling.

Pool-boiling heat-transfer experiments were carried out to observe the effects of hybrid wettable patterns on the heat-transfer coefficient (HTC). The parameters of investigation are the number of interlines and the pattern orientation of the modified wettable surface.

In this study, pool-boiling heat-transfer experiments were performed to investigate the effect of the number of interlines and the orientation of the ...

A Soft Tooling Process Chain for Injection Molding of a 3D Component with Micro Pillars

The purpose of this paper is to present the method of a soft tooling process chain employing Additive Manufacturing (AM) for fabrication of injection molding inserts with micro surface features. The Soft Tooling inserts are manufactured by Digital Light Processing (vat photo polymerization) using a photopolymer that can withstand relatively high temperaturea. The part manufactured here has four tines with an angle of 60&#176;. Micro pillars (&#216;200 &#181;m, aspect ratio of 1) are arranged on the surfaces by two rows. Polyethylene (PE) injection molding with the soft tooling inserts is used to fabricate the final parts. This method demonstrates that it is feasible to obtain injection-molded parts with microstructures on complex geometry by additive manufactured inserts. The machining time and cost is reduced significantly compared to conventional tooling processes based on computer numerical control (CNC) machining. The dimensions of the micro features are influenced by the applied additive manufacturing process. The lifetime of the inserts determines that this process is more suitable for pilot production. The precision of the inserts production is limited by the additive manufacturing process as well.

A protocol for fabricating injection molding inserts for complex geometry with micro features on surfaces employing Additive Manufacturing (AM) is presented.

The purpose of this paper is to present the method of a soft tooling process chain employing Additive Manufacturing (AM) for fabrication of injection ...