Femtosecond laser controlled wettability of solid surfaces

Femtosecond laser microfabrication is emerging as a hot tool for controlling the wettability of solid surfaces. This paper introduces four typical aspects of femtosecond laser induced special wettability: superhydrophobicity, underwater superoleophobicity, anisotropic wettability


Introduction
2][13][14][15][16][17][18][19][20] Four typical aspects of wettability are widely studied by researchers, namely superhydrophobicity, superoleophobicity, anisotropic wettability, and smart wettability.2][23][24] Superhydrophobicity is the earliest one studied.If the liquid object is another very popular liquid in daily life: oil, the oil-repellent surface is called a superoleophobic surface.0][31] This anisotropic property is caused by the ordered arrangement of the rough micro-papillas on rice leaves.The static characteristic (anisotropic wetting) and the dynamic feature (anisotropic sliding) are two forms of anisotropic wettability.In addition, some intelligent surfaces show ''smart wettability''; that is, the wettability of these surfaces can be reversibly switched through responding to external stimuli, such as light, 32,33 pH, 34,35 temperature, 36,37 electricity, 38,39 force, 40,41 etc. Smart wettability has been artificially achieved but is not observed in nature. 14Up to now, many techniques have been developed to obtain the above-mentioned wettabilities, such as template imprinting, 42,43 self-assembly methods, 44,45 chemical etching, 46,47 photolithography, 48,49 plasma etching, 50,51 and electrospinning. 52,535][56][57][58][59][60][61][62][63] This technology can directly create micro/ nanoscale hierarchical structures on a wide variety of materials by a simple one-step scanning method. 54,57,58,61,64By combining a femtosecond laser system with a computer controlled threedimensional translation stage, the laser processing position, scanning speed and scanning track can be precisely controlled by computer programs.Various pre-designed patterns have been prepared by this technology without the need for a harsh machining environment and expensive masks (Fig. 1).][67][68] This paper provides an introduction to employing a femtosecond laser to realize special wettabilities.According to the characteristics of different wettabilities, we divide this article into four parts, which respectively focus on superhydrophobicity, underwater superoleophobicity, anisotropic wettability, and smart wettability.The formation mechanism of these properties is from simple to complex, while the function of these surfaces develops from single to multiple.With the price of femtosecond laser systems decreasing year by year, more and more laboratories have their own femtosecond laser equipment.This article should provide a useful guide for the integrated field of femtosecond laser microfabrication and wettability, and should attract more researchers to work on this interesting research area.

Superhydrophobicity
The most famous superhydrophobic surface is a lotus leaf whose surface is covered with micro/nanoscale hierarchical papillas and a hydrophobic wax crystal layer. 29,69The combination of rough microstructures and hydrophobic chemistry endows the lotus leaf with water repellent ability because the real contact area between a water droplet and the lotus leaf is greatly reduced.][67][68] Mazur et al. and Stratakis et al., respectively, prepared superhydrophobic Si surfaces by femtosecond laser irradiation under reactive SF 6 gas. 70,71The laser induced structure is composed of microscale conical spikes, arranging as a spike forest, as shown in Fig. 2a and b.The average size of the spikes is 10 mm and the aspect ratio is about 4.There are abundant nanoprotrusions with the size of a few hundred nanometers covering every spike.After fluoroalkylsilane modification, the two length-scale rough surface shows a WCA of 1541 AE 11 and a very small SA of 51 AE 21.Both the artificial microstructure and the water-repellent characteristics are very similar to a natural lotus leaf. 71However, SF 6 gas results in complex experimental equipment and a complicated fabrication process.It is of great significance to develop a simple way to prepare superhydrophobic surfaces in an ambient environment using a femtosecond laser.
We recently reported a self-organized rough micro-mountain array structure which was fabricated using a femtosecond laser on Si surface in an atmospheric environment, as shown in Fig. 2c-f. 72,73he diameter and height of the rough micro-mountains are 6 mm and 2.9 mm, respectively.The micro-mountains arrange neatly and form a square array with a period of 10 mm.The surface roughness is about 2.46 mm.The inset in Fig. 2d shows the image of a 9 mL water droplet lying on the fluoroalkylsilane modified rough surface, with a WCA of 1581 AE 11.The water droplet on the surface can roll off easily as soon as the sample is tilted 41.The surface shows superhydrophobicity and ultralow water adhesion, agreeing well with the Cassie contact state. 74,75The water droplet only contacts the peak of the rough microstructures.An air cushion is trapped between the droplet and the rough surface.This air cushion endows the femtosecond laser induced surface with excellent water-repellent properties.
The as-prepared superhydrophobic surfaces possess excellent self-cleaning ability. 72If the surfaces are polluted by calcium carbonate powder and slightly tilted, the water droplet located on the surface will roll off easily and take away the pollutants, leaving a clean path behind.Interestingly, the as-prepared surfaces still have superhydrophobicity for strong acid and alkaline solutions.This feature endows the surfaces with a self-cleaning function even in acid or alkaline rain.In addition, this pH stability makes the superhydrophobic surfaces promising for application in biological and chemical fields.
Superhydrophobicity can be directly achieved on hydrophobic materials without low surface energy modification compared with intrinsic hydrophilic substrates. 62Fig. 3a shows the SEM image of the femtosecond laser irradiated polydimethylsiloxane (PDMS) surface.There are many flower-like particles with a size of several micrometers covering the surface.The particles are decorated with abundant nanoscale protrusions.The water droplet on the surface shows a static WCA of 1621 AE 11, and can roll off with the sample being tilted 11.The superhydrophobicity with ultralow adhesion indicates that the droplet on the sample is in the Cassie state. 74,75e further weaved patterned PDMS surfaces consisting of a periodic square array based on selective femtosecond laser irradiation technology (Fig. 3b). 62The femtosecond laser scanned domain is a micro/nanoscale hierarchical rough structure and shows ultralow adhesive superhydrophobicity.The rest of the non-irradiated flat PDMS surface exhibits intrinsic hydrophobicity (WCA = 1101).Fig. 3c shows the variation of WCA/SA with the length (L) of the side of the unstructured square when the period (T) of the square array is set at 200 mm.For a 7 mL water droplet, superhydrophobicity is obtained with L r 180 mm.For the patterned surface with a greater L, there is not enough laser irradiated rough area to realize superhydrophobicity; that is, the average roughness of the overall pattern surface is too low.The dynamic properties of a water droplet on the structured surface were also investigated via its sliding behavior.The water droplet moves easily on the surface with L r 140 mm as soon as the surface is only slightly tilted or shaken (Fig. 3d).As the L increases from 140 mm to 180 mm, the SA increases from 101 AE 1.51 to 541 AE 31 and then to 901 (Fig. 3e-g).SA = 901 means that the water droplet firmly pins on the sample even though the substrate is erected or turned upside down.The SA results reveal that the water adhesion of the superhydrophobic surfaces can be tuned from ultralow to ultrahigh.This adhesion controllability is realized by adjusting the area proportion of laser structured and non-structured regions, which show ultralow adhesion and ultrahigh adhesion, respectively.A water droplet on the heterogeneous patterned surfaces contacts both the flat and laserinduced rough domains.These two contact states are in Young and Cassie wetting models, respectively. 62As the L increases, the area fraction of the laser structured domain (Cassie contact) will decrease.The decrease of the average surface roughness causes a decrease of the WCA.Whereas, the area fraction of the flat domain (Young contact) will increase, resulting in the increase of the water adhesion or SA value.Using these as-prepared surfaces, droplet rapid stop, small water droplet transfer, and controllable droplet bounce are achieved. 62In addition, many other microstructures and patterns are also prepared and showed controllable adhesive superhydrophobicity. 61,63,76

Underwater superoleophobicity
Fish can swim freely in oil-spill water and avoid being polluted by oil because of the underwater superoleophobicity of their skin. 77The unique ability is found from the synergy between the hierarchical rough microstructures and the hydrophilic chemistry of fish scales.The characteristic of fish scales suggests that surfaces that are superhydrophilic in air are generally superoleophobic in water.
Femtosecond laser ablated micro-mountain array structures on an Si surface (without fluoroalkylsilane modification) can also be used to obtain underwater superoleophobicity. 56A flat Si surface shows weak hydrophilicity in air with a WCA of 601 (Fig. 4a).If the flat sample is immersed in water, more than half of an oil droplet (1,2-dichloroethane) on the surface will retain a nearly spherical shape (Fig. 4c).The OCA is 124.61AE 11, showing ordinary oleophobicity in water.After femtosecond laser ablation, the wettability of the sample is enhanced by the hierarchical rough microstructures.The water droplet can spread out quickly on the laser ablated surface, exhibiting superhydrophilicity with a WCA of 41 in air (Fig. 4b).Interestingly, the laser induced surface shows superoleophobicity in water.The oil droplet can retain a spherical shape on the sample surface in water medium (Fig. 4d).The OCA reaches up to 159.41 AE 11.The surface also achieves ultralow oil adhesion in water since the oil droplet can roll easily on a 0.51 tilted structured surface (Fig. 4e).According to the underwater version of the Cassie model, the rough microstructure is fully wetted by water. 56,77The underwater oil droplet is supported by the micro-mountains and only touches the tip of the rough microstructures.A water cushion is trapped below the oil droplet.The trapped water endows the surface with underwater superoleophobicity because water naturally repels oil.Ultralow oil adhesion is a result of a small contact area between the underwater oil droplet and the rough substrate.Hydrophilic silicon in air becomes oleophobic in water, while the oleophobicity is greatly amplified into underwater superoleophobicity by femtosecond laser induced rough microstructures.
Transparent underwater superoleophobicity was also realized on a silica glass surface. 57Achieving underwater superoleophobicity and light transmission are inspired by fish scales and Diphylleia grayi, respectively.The petals of Diphylleia grayi are white in air, but upon contact with water (such as in the rain) they become transparent, as shown in Fig. 5a and b. 57 Because of the loose cell structure, water can enter the intercellular space of Diphylleia grayi petals on rainy days.A water-water interface displaces the initial water-air interface (in air), increasing light transmission so the petals turn transparent.Fig. 5c shows the microstructure of the femtosecond laser ablated silica glass surface.Abundant irregular nano-particles disorderly distribute on the surface.When the laser ablated silica glass is immersed in water, the oil droplet on it retains a spherical shape (Fig. 5d).The OCA reaches up to 160.21 AE 11, while the SA is only 11, revealing an underwater superoleophobicity with ultralow oiladhesion.
The transmittance of rough silica glass is improved by shifting from the atmosphere to water medium.When we put paper with black letters behind the laser induced glass, the letters look obscure in air.After putting the sample in water, the letters ''XJTU'' become very clear (Fig. 5e).The underwater oil droplet on the sample remains a small sphere (inset of Fig. 5e).The remarkable transparency is confirmed by UV-vis spectra (Fig. 5f).The underwater structured surface has a similar transmittance to bare flat silica glass in the visible region.The water environment plays an important role in obtaining this good transparency.Similar to the Diphylleia grayi petals becoming transparent in the rain, the rough nanoscale structure of silica glass is wetted by water by immersing in water, forming a glass-water interface instead of a glass-air interface.Since the refractive index of water is closer to that of silica glass than to that of air, the degree of Mie scattering is effectively weakened, resulting in excellent transparency. 78,79ontrollable underwater oil-adhesion was realized on femtosecond laser ablated ordinary glass surfaces. 80The resultant surfaces show a micro/nanoscale binary rough topography and underwater superoleophobicity.By increasing the average distance of the laser pulse focus, the underwater oil-adhesion increases from ultralow to ultrahigh because the oil droplet on the structured surfaces changes from the underwater Cassie state to the metastable state, and then to the underwater Wenzel state.The superoleophobic surface with low oil-adhesion is an ideal anti-oil-contamination layer in water.The superoleophobic surface with ultrahigh oil-adhesion can be applied in oil droplet transfer and even in the fusion of oil/organic microdroplets.

Anisotropic wettability
A slow femtosecond laser scan leaves a microgroove on a substrate surface.Using the line-by-line scanning mode, we obtained a periodic microgroove array on a PDMS substrate, as shown in Fig. 6a. 55Every microgroove has a width of 12.17 mm and a depth of 8.57 mm.In addition, many irregular nanoscale particles randomly decorate on the inner wall and outer rim of the microgrooves.
A water droplet on the microgroove array structure is stretched along the microgrooves (Fig. 6b).2][83] The WCAs (SAs) perpendicular to and parallel to the microgrooves, respectively, are defined as WCA > and WCA J (SA > and SA J ). Fig. 6c shows the parallel view of a water droplet on the microgroove array surface with a period (D) of 150 mm.The WCA J is 116.51AE 2.51, whereas the droplet along the perpendicular direction has a higher contact angle and the WCA > is 134.81AE 2.51 (Fig. 6d).The degree of wetting anisotropy, WCA > À WCA J , is about 18.31.Three main factors cause this anisotropic wetting phenomenon.Firstly, an energy barrier usually forms at the boundary between the laser-induced microgroove and the non-irradiated flat PDMS domain. 73,83,84his energy barrier can prevent the water droplet from spreading along the perpendicular direction, while there is no energy barrier along the parallel direction.6][87][88][89][90] Finally, the three-phase contact line (TCL) is continuous along the microgrooves. 91It is better for the droplet spreading and moving forward.
Fig. 6e and f describe the rolling process of a water droplet on the microgroove array surface (D = 25 mm) along the parallel and perpendicular directions, respectively.The SA J is 311 AE 31, while the SA > is about 761 AE 51.The results show that the droplet prefers to move along the groove direction.The SA difference reaches up to 451, but the value of a rice leaf is only 61.A continuous short TCL always endows the surfaces with excellent water-shedding properties. 91The water droplet on the microgroove structure has a continuous TCL along the parallel direction.However, the TCL along the perpendicular direction is very discontinuous and looks longer.A stronger hysteresis effect forms in the perpendicular direction, resulting in the water droplet rolling along this direction being more difficult than along the parallel direction.
A femtosecond laser induced microgroove array structure can also be immersed in water to control oil wettability. 92We fabricated a similar microgroove array on a Si substrate and realized underwater anisotropic oil-wetting.For the sample with D = 450 mm, the OCAs parallel to and perpendicular to the grooves are 135.71AE 1.51 and 155.51 AE 1.61, respectively.The contact angle along the parallel direction is significantly smaller than that along the perpendicular direction, revealing that the oil droplet tends to spread along the microgrooves.The degree of wetting anisotropy can be controlled from 01 to 19.81 by the period of the microgroove arrays.
A butterfly avoids being ''shot down'' by raindrops due to the directional adhesion of its wings. 93Inspired by butterfly wings, a kind of triangle array pattern was constructed on PDMS material based on selective femtosecond laser irradiation. 94he untreated triangle array is flat hydrophobic PDMS, while the surrounding domain is induced by a femtosecond laser and shows superhydrophobicity.The droplet on the sample rolls along one direction distinctly easier than its opposite direction when the droplet and a single hydrophobic triangle have a comparable size.This special anisotropic wettable phenomenon is usually called ''directional adhesion''. 93The obtained sliding anisotropy can even reach up to 211.The directional adhesion results from the direction-dependent arrangement of hydrophobic triangles because the water droplets rolling along those two opposite directions have different TCL situations.

Smart wettability
Switchable wettability has been realized on femtosecond laser ablated Ti surfaces. 58As shown in Fig. 7a and b, the laser induced morphology is typical rough micro-mountain arrays.The laser ablated sample consists of both Ti and a new element O instead of pure Ti.Their atomic proportions are 51.81% and 48.19%, respectively.Oxidation and hierarchical rough microstructure formation synchronously happen during the femtosecond laser ablation, constructing a rough TiO 2 layer covering on the original substrate.
The laser-induced surface initially shows superhydrophobicity with a WCA of 154.51 AE 2.51 (Fig. 7c).A mirror-like interface can be seen by immersing the sample in water, revealing that the laser-irradiated Ti is a Cassie surface. 95After UV irradiation for 40 min, the WCA decreases to 2.51 (Fig. 7d).At present, the sample submerged in water does not show a silver mirror.The wettability is switched from superhydrophobicity to superhydrophilicity in air.Furthermore, the sample can re-obtain superhydrophobicity through dark storage for 2 days.
The switching trend of oil wettability in water is opposite to that of water wettability in air.The laser-induced sample incipiently presents underwater superoleophilicity (Fig. 7e).The underwater oil droplet will spread out quickly with an OCA of nearly 41 once it contacts the submerged rough TiO 2 surface.After UV light irradiation, the wettability of the sample switches to underwater superoleophobicity (Fig. 7f).The underwater oil droplet on the sample has an OCA of 160.51 AE 21 and can easily roll away if the sample is titled 11.The underwater superoleophilicity can also be recovered by keeping the sample in the dark for 2 days.Both the water wettability and the underwater oil wettability can be cycled many times.A similar property is also seen on the femtosecond laser treated Zn surface. 59s shown in Fig. 7g, femtosecond laser irradiation not only generates hierarchical rough microstructures but also oxidizes the Ti material.The laser induced rough TiO 2 layer shows ultralow water adhesive superhydrophobicity.5][76] An air cushion forms between the water droplet and the rough sample surface by submerging the as-prepared sample in water.When an underwater oil droplet contacts the surface, the oil will spread along and displace the trapped air due to the pressure and capillary action, resulting in underwater superoleophilicity (Fig. 7k).][98][99][100][101] The oxygen vacancies further turn into hydroxyl groups by dissociative adsorption of atmospheric water (Fig. 7h).The hydroxyl groups make the rough TiO 2 surface exhibit superhydrophilicity (Fig. 7j).Upon immersing the sample in water, the microstructures are completely wetted and occupied by water.If an oil droplet is placed on the sample surface, a water cushion will be trapped between the oil droplet and the substrate (Fig. 7l).The water cushion endows the sample with underwater superoleophobicity based on the underwater version of the Cassie contact model. 56,77For the dark storage process, the implanted hydroxyl moieties tend to be replaced by ambient oxygen. 96,97As the hydrophobicity grows stronger, the sample recovers its original superhydrophobic and underwater superoleophilic state (Fig. 7i and k).
With the aid of underwater superoleophobicity and ultralow oil-adhesion of femtosecond laser induced rough Si surfaces, an in situ ''mechanical hand'' for oil droplet transportation was realized in water through switching the density (r) of the water solution. 60As shown in Fig. 8a, the setup mainly consists of one water container and two underwater superoleophobic samples (femtosecond laser irradiated Si surfaces).The whole transportation process (Fig. 8b) was performed in a water environment.A 15 mL oil droplet (r = 1.26 g cm À3 ) was firstly located on the below sample (Step 1).The above sample was lowered down until it just touched the oil droplet (Step 2).Next, sugar water (r = 1.52 g cm À3 ) was slowly poured into the glass container to increase the r of the water solution, switching the oil droplet from heavier to lighter than the surrounding water solution (Step 3).The oil droplet became brighter because sugar water has a refractive index closer to the oil droplet than deionized water.If we lifted the above sample up, the oil droplet would be picked up since the gravity was less than the buoyancy acting on the oil droplet (Step 4).Interestingly, we can also put the oil droplet back onto the below sample by taking an opposite approach.Firstly, the above sample was lowered down so that the hanging oil droplet touched the below sample again (Step 5).Then, the water solution in the container was diluted by deionized water to become much lighter than the oil droplet until it again turned very transparent (Step 6).The underwater oil droplet also recovered its original brightness.Finally, by lifting the above sample up, the oil droplet detached from the above sample and stayed on the surface below (Step 7).The oil droplet can be picked up when the oil is lighter than the water environment, while it can be put down when the oil is heavier than the water environment (Fig. 8c).This reversible transportation can be repeated over many cycles and has almost no oil loss (Fig. 8d).

Conclusions
This paper introduces some recent developments in achieving special wettability through femtosecond laser microfabrication.Using different materials and machining methods, we can design the rough microstructure, pattern, and even chemistry on solid substrates.Superhydrophobicity, underwater superoleophobicity, anisotropic wettability, and smart wettability have been obtained.A micro-mountain array structure is built on a Si surface by femtosecond laser irradiation.After fluoroalkylsilane modification, the sample shows superhydrophobicity and ultralow water adhesion.Superhydrophobicity can also be directly obtained on hydrophobic materials, such as PDMS, without modification by a low surface energy.In addition to superhydrophobicity, the femtosecond laser induced rough Si surface also exhibits superoleophobicity when the sample is immersed in water.A microgroove array structure is prepared by a line-by-line scanning process, and the resultant surface achieves anisotropic wetting and anisotropic sliding, being similar to the natural rice leaf.For some materials like Ti, an oxidation process also happens with rough microstructure formation during femtosecond laser ablation.The generated rough TiO 2 layer exhibits smart wettability; that is, superhydrophobicity-superhydrophilicity, as well as underwater superoleophilicity-superoleophobicity, can be reversibly switched by alternate UV irradiation and dark storage.We believe that the combination of wettability and a femtosecond laser will have important potential applications in droplet/fluid manipulation, microfluidics, biomedicine, chemical and biological sensing, etc.

Fig. 1
Fig. 1 Various patterns prepared through femtosecond laser microfabrication on a Ti sheet.(reproduced from ref. 58 with the permission of RSC)

Fig. 2
Fig. 2 Typical SEM images of the Si surfaces irradiated by a femtosecond laser under SF 6 gas (a and b) and in an air environment (c-f), respectively.The insets show a water droplet on the corresponding surfaces.(Reproduced from ref. 71 and 72 with the permission of Wiley and Springer.)

Fig. 3
Fig. 3 (a) SEM image of the PDMS microstructure ablated by a femtosecond laser.(b) SEM image of the structured square array pattern.(c) WCA and SA values as a function of the L. (d-g) The shape and movement of a droplet on the square array surfaces: (d) L = 0 mm, (e) L = 140 mm, (f) L = 170 mm, and (g) L = 180 mm.(Reproduced from ref. 62 with the permission of ACS.)

Fig. 5
Fig. 5 (a and b) Petals of Diphylleia grayi: (a) on a sunny day; (b) in the rain.(c) SEM image of the femtosecond laser irradiated silica glass surface.(d) An underwater oil droplet on femtosecond laser irradiated silica glass.(e) High transparency of the laser induced surface in water medium.(f) UV-vis spectra of various silica glasses.(Reproduced from ref. 57 with the permission of RSC.)

Fig. 6
Fig. 6 (a) SEM image of a femtosecond laser induced microgroove array with D = 40 mm.(b) A blue-dyed water droplet on a microgroove array surface.(c and d) A water droplet on a microgroove patterned surface (D = 150 mm) viewed from the parallel (c) and perpendicular (d) directions.(e and f) Droplet rolling on the microgroove array (D = 25 mm) along the parallel and perpendicular directions.The sample is titled 311 (e) and 761 (f), respectively.(Reproduced from ref. 55 with the permission of RSC.)

Fig. 8
Fig. 8 Underwater oil droplet transportation by underwater superoleophobic surfaces.(a) A schematic of the setup.(b) The operational process of ''picking up'' and ''putting down'' an oil droplet in water medium.(c and d) Reversibility and repeatability of the no-loss oil droplet transportation.(Reproduced from ref. 60 with the permission of Wiley.)