Fabrication of Nanostructured Omniphobic and Superomniphobic

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Fabrication of Nanostructured Omniphobic and Superomniphobic Surfaces with Inexpensive CO2 Laser Engraver Anudeep Pendurthi, Sanli Movafaghi, Wei Wang, Soran Shadman, Azer Yalin, and Arun K. Kota ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06924 • Publication Date (Web): 21 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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ACS Applied Materials & Interfaces

Fabrication of Nanostructured Omniphobic and Superomniphobic Surfaces with Inexpensive CO2 Laser Engraver Anudeep Pendurthi1‡, Sanli Movafaghi1‡, Wei Wang1, Soran Shadman1, Azer P. Yalin1 and Arun K. Kota1,2,3* 1

Department of Mechanical Engineering, Colorado State University, Fort Collins, CO, USA;

2

School of Biomedical Engineering, Colorado State University, Fort Collins, CO, USA;

3

Department of Chemical & Biological Engineering, Colorado State University, Fort Collins, CO,

USA.

Keywords: superomniphobic, laser ablation, CO2 laser, re-entrant texture, nanostructure ABSTRACT Superomniphobic surfaces (i.e. surfaces that are extremely repellent to both high surface tension liquids like water and low surface tension liquid like oils) can be fabricated through a combination of surface chemistry that imparts low solid surface energy with a re-entrant surface texture. Recently, surface texturing with lasers has received significant attention because laser texturing is scalable, solvent-free, and can produce a monolithic texture on virtually any material. In this work, we fabricated nanostructured omniphobic and superomniphobic surfaces with a variety of materials using a simple, inexpensive and commercially available CO2 laser engraver. Further, we demonstrated that the nanostructured omniphobic and superomniphobic surfaces fabricated using our laser texturing technique can be used to design patterned surfaces, surfaces with discrete domains of the desired wettability and on-surface microfluidic devices.

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Super-repellent surfaces can be broadly classified as superhydrophobic surfaces (i.e. surfaces that are extremely repellent to high surface tension liquids like water) and superomniphobic surfaces (i.e. surfaces that are extremely repellent to both high surface tension liquids like water and low surface tension liquid like oils).1-9 Super-repellent surfaces can be fabricated by combining a surface chemistry that imparts low solid surface energy with an appropriate surface texture.10-16 Recently, fabrication of superhydrophobic surfaces via surface texturing with lasers has received significant attention17-29 because laser texturing is scalable, solvent-free, and can produce a monolithic texture (i.e. texture which is an integral part of the surface unlike a coating that is deposited on the underlying substrate) on virtually any material. However, to the best of our knowledge, there are no reports of superomniphobic surfaces fabricated via laser texturing. Further, most reports of superhydrophobic surfaces fabricated via laser texturing have employed expensive nanosecond or femtosecond lasers.17, 19-20, 23-24, 27-28, 30-33 In this work, we present nanostructured superomniphobic surfaces fabricated via laser texturing with an inexpensive CO2 laser engraver. We demonstrate that our simple, inexpensive, scalable and solvent-free laser texturing technique allows fabrication of superomniphobic (or omniphobic) surfaces, gradient wettability surfaces, and droplet manipulation tracks with a wide variety of materials. The primary measure of wetting of a liquid on a non-textured (i.e., smooth) solid surface is the equilibrium contact angle θ (i.e. Young’s contact angle).34 When a liquid droplet contacts a textured (i.e., rough) solid surface, it displays an apparent contact angle θ*, and it can adopt one of the following two configurations to minimize its overall free energy – the fully wetted Wenzel35 state or the Cassie-Baxter36 state. The Cassie-Baxter state is preferred for designing * superomniphobic surfaces because it leads to high θ* and low contact angle hysteresis ∆θ* = θ adv

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* * * – θ rec , where θ adv and θ rec are the apparent advancing and apparent receding contact angles,

respectively.10-11, 14, 37 Low ∆θ* leads to low roll-off angle ω (i.e., the minimum angle by which the surface must be tilted relative to the horizontal for the droplet to roll-off).10-11 A surface is considered superhydrophobic if it displays θ* > 150° and ω < 10° with water, and superoleophobic if it displays θ* > 150° and ω < 10º with low surface tension (typically γlv < 30 mN/m) liquids.8, 10-11 Recent work4-7, 9-11, 38 has explained the importance of re-entrant texture and low solid surface energy (typically γsv < 15 mN/m) in designing superomniphobic surfaces. In this work, we fabricated nanostructured and re-entrant textured surfaces using an inexpensive, commercially available, CO2 laser with a central wavelength of 10.6 µm. Upon modifying the re-entrant textured surfaces with a fluorinated silane to impart low surface energy (γsv ≈ 10 mN/m),4, 7, 39 the surfaces displayed superomniphobicity towards liquids with surface tension ≥ 27.5 mN/m. The key parameters influencing the surface texture (and hence superomniphobicity) are the laser power and the laser raster speed.40-43 In order to systematically investigate the influence of laser power on the surface texture and surface wettability, we ablated the surface of stainless steel 430 (a widely used industrial material) with different laser powers at a constant laser raster speed of 2 cm/s and subsequently modified the laser-ablated surfaces with a fluorinated silane (see the Supporting information, Section 1). As received surfaces of stainless * steel 430 displayed low surface roughness (Rrms = 0.53 µm, Figure 1a) and were omniphilic ( θ adv * * * = 68°, θ rec = 49° for water, γlv = 72.1 mN/m; and θ adv = 38°, θ rec = 16° for n-hexadecane, γlv =

27.5 mN/m). When the stainless steel 430 was subjected to laser ablation, the surface absorbed the laser energy, which resulted in a series of intertwined physical phenomena such as melting, vaporization, sublimation, splashing and re-solidification.19, 40-41, 44 These complex laser-material interactions resulted in nanostructured (or rough) surfaces. As the laser power increased, the 3



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surface roughness increased (see Figures 1a-1c) and resulted in enhanced liquid repellency (i.e., higher contact angles and lower roll off angles) after surface fluorination to impart low solid surface energy (Figures 1d-1e). For example, at an intermediate laser power of 84 W, a bump-like nanostructure with a feature size of ~50 nm (Rrms = 0.85 µm, Figure 1b) formed on the surface. * * = 151°, θ rec = 144°, ω = Upon surface fluorination, the textured surface was omniphobic ( θ adv * * 13° for water and θ adv = 136°, θ rec = 122°, ω = 34° for n-hexadecane, Figure 1d and 1e).

Apparent contact angles higher than the corresponding contact angles on fluorinated non-textured surfaces (θadv = 89°, θrec = 74° for water and θadv = 68°, θrec = 43° for n-hexadecane, see the Supporting information, Section 1) and finite roll off angles for water and n-hexadecane indicate that both the liquid droplets have adopted the Cassie-Baxter state on this omniphobic surface. The apparent contact angles of n-hexadecane were lower than that of water due to its lower surface tension. As the laser power was increased further to 120 W (the maximum power of the CO2 laser system employed in this work), a densely-packed bead-like nanostructure with a feature size of ~10 nm (Rrms = 1.43 µm, Figure 1c) formed on the surface. Upon surface fluorination, the surface * * * * = 169°, θ rec = 164°, ω = 1° for water and θ adv = 167°, θ rec = 161°, ω was superomniphobic ( θ adv

= 3° for n-hexadecane, Figure 1d and 1e) with droplets of water and n-hexadecane bouncing on the surface (see Movies S1 and S2). The very high apparent contact angles and the very low roll off angles for water and n-hexadecane indicate that both the liquid droplets have adopted the Cassie-Baxter state on this superomniphobic surface.

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Figure 1. Scanning electron microscope (SEM) images showing the morphology of a) asreceived stainless steel 430, b) surface textured at an intermediate laser power of 84 W, and c) surface textured at a high laser power of 120 W. d) and e) Apparent contact angles and roll off angles, respectively, of water and n-hexadecane droplets (~8 µL) on the laser textured stainless steel 430 surfaces fabricated with different laser powers.

In addition to laser power, the laser raster speed is another key parameter influencing the surface texture (and hence superomniphobicity).40-41, 45 In order to systematically investigate the influence of laser raster speed on the surface texture and surface wettability, we ablated the surface of stainless steel 430 with different laser raster speeds at a constant laser power of 120 W and subsequently modified the laser-ablated surfaces with a fluorinated silane. As the laser raster speed decreased, the amount of laser energy absorbed by the surface per unit area increased,40, 46 resulting in higher surface roughness (Figures 2a-2c) and consequently enhanced liquid repellency (i.e., higher contact angles and lower roll off angles) upon surface fluorination (Figures 2d and 2e). For example, at a high laser raster speed of 210 cm/s, the amount of laser 5



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energy absorbed by the surface per unit area was insufficient to increase the surface roughness * (Rrms = 0.58 µm, Figure 2a). Upon surface fluorination, the surface was omniphilic ( θ adv = 84°, * * * = 69° for water and θ adv = 54°, θ rec = 38° for n-hexadecane, Figure 2d and 2e). At an θ rec

intermediate laser raster speed of 80 cm/s, a sparsely packed bead-like nanostructure with a feature size of ~30 nm (Rrms = 0.77 µm, Figure 2b) formed on the surface. Upon surface * * fluorination, the textured surface was omniphobic ( θ adv = 129°, θ rec = 117°, ω = 70° for water * * and θ adv = 73°, θ rec = 54°, no roll off for n-hexadecane, Figure 2d and 2e). Apparent contact

angles higher than the corresponding contact angles on fluorinated non-textured surfaces for water and finite roll off angles for water indicate that the water droplets adopted the CassieBaxter state on this omniphobic surface, while the no roll off for n-hexadecane indicates that it adopted the Wenzel state on this surface. At a low laser raster speed of 2 cm/s, the increased laser energy absorbed by the surface per unit area led to the formation of densely packed bead-like nanostructures with a feature size of ~10 nm (Rrms = 1.43 µm, Figure 2c). Upon surface * * = 169°, θ rec = 164°, ω = 1° for water and fluorination, this surface was superomniphobic ( θ adv * * = 167°, θ rec = 161°, ω = 3° for n-hexadecane, Figure 2d and 2e). θ adv

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Figure 2. SEM images showing the morphology of stainless steel 430 surfaces textured at laser raster speeds of a) 210 cm/s, b) 80 cm/s, and c) 2 cm/s. d) and e) Apparent contact angles and roll off angles, respectively, of water and n-hexadecane droplets (~8 µL) on the laser textured stainless steel 430 surfaces fabricated with different laser raster speeds.

In a similar manner, we determined the optimal combination of laser power and laser raster speed that resulted in the maximum liquid repellency for a variety of materials (see the Supporting information, Section 2). Utilizing our laser texturing technique, we fabricated * * superomniphobic surfaces with stainless steel 316 ( θ adv = 168°, θ rec =164°, ω = 1° for water, and * * = 162°, θ rec =154°, ω = 4° for n-hexadecane), omniphobic surfaces with stainless steel 304 ( θ adv * * * * = 159°, θ rec =153°, ω = 8° for water, and θ adv = 116°, θ rec =84°, ω = no roll off for nθ adv * * hexadecane), omniphobic surfaces with titanium ( θ adv = 158°, θ rec = 152°, ω = 7° for water and * * = 112°, θ rec = 92°, ω = no roll off for n-hexadecane), omniphobic surfaces with aluminum ( θ adv

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* * * * = 164°, θ rec = 159°, ω = 5° for water and θ adv = 120°, θ rec = 98°, ω = no roll off° for nθ adv * * hexadecane), and omniphobic surfaces with glass ( θ adv = 161°, θ rec = 156°, ω = 6° for water and * * = 119°, θ rec = 76°, ω = no roll off° for n-hexadecane) (see Table 1 and Figures 3a-3f). While θ adv

all the surfaces were superhydrophobic (i.e., θ* > 150° and ω < 10° with water), only the textured stainless steel 430 and 316 surfaces were superomniphobic (i.e., θ* > 150° and ω < 10° with both water and n-hexadecane). Consequently, jets of water easily bounced on all surfaces (see Movie S3) and jets of n-hexadecane easily bounced on the superomniphobic surfaces (see Movie S4). The different surface wettability obtained with different materials is due to the different surface textures induced by the complex laser-material interactions, which in turn depend on the material properties (e.g., melting point, thermal conductivity, reflectivity etc.) and the laser properties (e.g., laser focal spot size, temporal output profile etc.).40-41, 47-48 On the surfaces that are only omniphobic and not superomniphobic (i.e., stainless steel 304, titanium, aluminum and glass), we believe that the n-hexadecane droplets have adopted a hybrid wetting state with partial Cassie-Baxter state and partial Wenzel state underneath the droplet. The partial Cassie-Baxter state of n-hexadecane is evident from the higher apparent contact angles compared to contact angles on fluorinated non-textured surfaces. The partial * Wenzel state of n-hexadecane is evident from the low θ rec and no roll off of n-hexadecane on the

surface. Knowing that superomniphobicity is a strong function of surface chemistry, surface roughness, re-entrant texture and breakthrough pressure from prior extensive work,4, 6-7, 11, 49-50 we have attempted to better identify (or eliminate) the possible parameters that are leading to the difference in wettability. We conducted X-ray photo-electron spectroscopy (XPS) to assess the surface chemistry for all the laser ablated and silanized surfaces. Our high-resolution C1s XPS 8


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spectra indicate that the ratio of –CF3 groups to –CF2 groups (a measure of surface fluorination)5153

is not significantly different for all the surfaces studied (see Table 1 and the Supporting

information, Section 3). Consequently, we infer that surface chemistry is not the primary factor leading to the differences in wettability. We also conducted RMS roughness measurements for all the laser ablated and silanized surfaces to assess the surface roughness (see Table 1). Our RMS roughness measurements for all the metal and metal alloy surfaces are not significantly different from each other. The RMS roughness for the glass surfaces is slightly higher, and yet glass surfaces do not display superomniphobicity. Consequently, we infer that surface roughness is not the primary factor leading to the differences in wettability. Having eliminated surface chemistry and surface roughness as the primary factors, we speculate that the difference in wettability arises from: (i) lack of re-entrant texture (i.e., lack of convex topography) locally on the surface, and/or (ii) high inter-feature spacing, leading to low breakthrough pressure (i.e., the pressure at which the droplet transitions from the Cassie-Baxter state to the Wenzel state)49-50 locally on the surface (see Supporting information, Section 4).

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Figure 3. Images showing water droplets (dyed blue, on the left) and n-hexadecane droplets (dyed red, on the right) beading up on the laser textured and fluorinated (SEM images, in the middle) a) stainless steel 430, b) stainless steel 316, c) stainless steel 304, d) titanium, e) aluminum, and f) glass surfaces fabricated with optimal combinations of laser power and laser raster speed. 10


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Table 1. Apparent advancing contact angles, apparent receding contact angles, roll off angles of water and n-hexadecane droplets, RMS roughness and ratio of –CF3 to –CF2 groups for all the laser ablated and silanized surfaces. SS – stainless steel and NR – no roll off. Water

n-Hexadecane

ω (°)

* θadv (°)

* θ rec (°)

ω (°)

RMS roughness (µm)

Substrate * * θ adv θ rec (°) (°)

Ratio of -CF3 to -CF2 groups

SS 430 SS 316

169 168

164 164

1 1

167 162

161 154

3 4

1.43±0.17 1.19±0.18

0.6 0.5

SS 304 Titanium

159 158 164 161

153 152 159 156

8 7 5 6

116 112 120 119

84 92 98 76

NR NR NR NR

1.82±0.20 1.00±0.15 1.94±0.23 4.90±0.22

0.5 0.5 0.4 0.4

Aluminum Glass

Our laser texturing technique enables simple, inexpensive, scalable and solvent-free fabrication of nanostructured surfaces with patterned wettability on a wide range of materials via selective laser ablation. In order to demonstrate this, we fabricated patterned surfaces (“CSU” pattern as an example) with significant wettability contrast on glass by texturing a glass surface everywhere except the “CSU” pattern. Upon fluorination, the textured surface was superhydrophobic and the non-textured surface (i.e., “CSU” pattern) was hydrophobic. When water droplets (dyed blue) were deposited everywhere on this patterned surface, they selectively adhered to the hydrophobic surface and rolled off easily from the superhydrophobic surface to result in a “CSU” liquid pattern (Figure 4a). In addition, our laser texturing technique also allows easy fabrication of a single nanostructured surface with discrete domains of the desired wettability by tuning the surface texture. To illustrate this, we fabricated a titanium surface with multiple, discrete domains with different surface textures by using the same laser power, but different laser raster speeds (Figure 4b). Upon fluorination, as anticipated, each discrete domain displayed different wettability (Figure 4c-4f) due to the different surface texture. Further, our 11



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laser texturing technique can be used to fabricate on-surface microfluidic devices with a wide variety of patterns. To illustrate this, we fabricated a circular guiding track by laser texturing a stainless steel 430 surface everywhere except the circular track. Upon fluorination, the textured surface was superhydrophobic and the non-textured surface (i.e., the circular track) was hydrophobic. When a water droplet was placed on the circular track, it was confined to the hydrophobic track due to the wettability contrast induced by the surrounding superhydrophobic surface. As the surface was tilted, when the work done by gravity overcame the work expended due to adhesion between water and the hydrophobic surface, the water droplet moved along the circular track (Figure 4g-4j and Movie S5). Further, we assessed the chemical resistance of our superomniphobic surfaces (see Supporting Information, section 1). Our surfaces retained their superomniphobicity (i.e., no change in apparent contact angles or roll off angles for water and n-hexadecane) when immersed in corrosive (i.e., acidic and basic) liquids with a wide pH range. We also assessed the mechanical durability of our superomniphobic surfaces (see Supporting Information, section 1). While our surfaces retained their superomniphobicity (i.e., no change in apparent contact angles or roll off angles for water and n-hexadecane) against liquids flowing past the surface for a short time, they immediately lost their superomniphobicity upon abrasion with solids.

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Figure 4. a) Image illustrating the “CSU” liquid pattern (with water, dyed blue) on the textured glass surface with wettability contrast. b) A titanium surface with four discrete domains with different surface textures obtained by using the same laser power, but different laser raster speeds. c), d), e) and f) Water droplets on the four discrete domains with different wettability. g), h), i) and j) Series of images showing water droplet (dyed blue) moving along the circular guiding tracking on a stainless steel 430 surface.

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In conclusion, we demonstrated the fabrication of nanostructured omniphobic and superomniphobic surfaces with a wide variety of materials using a simple, inexpensive, scalable and solvent-free CO2 laser texturing technique. In order to obtain the appropriate surface texture necessary for maximum liquid repellency, we investigated the influence of laser power and laser raster speed on the surface texture and surface wettability. Our laser texturing technique can be used to fabricate patterned surfaces, surfaces with discrete domains of the desired wettability and on-surface microfluidic devices. While our superomniphobic surfaces display poor mechanical durability, they display good short-term chemical resistance.

Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Section 1 – Experimental details. Section 2 – Optimal laser ablation parameters. Section 3 – Characterization of surface chemical composition with XPS. Section 4 – Re-entrant texture and inter-feature spacing. Movie S1 – Water droplet bouncing on a superomniphobic stainless steel 430 surface. Movie S2 – n-hexadecane droplet bouncing on a superomniphobic stainless steel 430 surface. Movie S3 – Water jets bouncing on superomniphobic surfaces (i.e., stainless steel 430 and stainless steel 316) and omniphobic surfaces (i.e., stainless steel 304, titanium, aluminum and glass). Movie S4 – n-hexadecane jets bouncing on superomniphobic surfaces (i.e., stainless steel 430 and stainless steel 316). Movie S5 – Water droplet (dyed blue) moving along the hydrophobic circular guiding tracking on a superhydrophobic stainless steel 430 surface. Corresponding Author * E-mail: [email protected] Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally to the work. Acknowledgement We thank Colorado Office of Economic Development and International Trade for financial support under the Advanced Industry Accelerator Grant Program. References (1) (2) (3)

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Table of Contents Graphic

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Figure 1 588x402mm (96 x 96 DPI)


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Fabrication of Nanostructured Omniphobic and Superomniphobic

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