Open Access
Add to BibSonomy
Abstract
Topography-dependent tuning of water wettability was achieved on a stainless steel surface textured by nanosecond-laser pulses at different laser fluences, with the minimal contribution of the surface chemical modification. Such differently-wet neighboring surface spots were demonstrated to drive an autonomous directional water flow. A series of elementary microfluidic devices based on the spatial wetting gradients were designed and tested as building blocks of “green”, energy-saving autonomous microfluidic circuits.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nature was always a bright and strong source of inspiration for scientists, attempting to understand and replicate its simple wisdom in different natural complex phenomena. In fluid mechanics, there are a few kinds of natural specimens with various wettability, such as rose petals [1], rice leaves [2–4] lotus leaves [2,5] Nepentes [6–8], and Cicada wings [9], which unique wetting properties are impressive and studied in different — either biocompatibility [10], or bactericidity [11] - aspects up to now. Following the simple, but very efficient principles of biomimetics, for centuries native population over the word built everyday life to close the gap in natural resources, with clean water being the most vital one. For example, stenocara beetle [12], cacti [13], and Texas horned lizard [14], living in deserts, can trap moisture from fog by collecting drops of water on alternating hydrophilic and hydrophobic rough spots of their surface back. This property served to make a prototype of fog condenser for collecting water in desert regions. During our times, scientists began to explore and imitate these important natural modalities by engineering hydrophilic and hydrophobic microstructures in terms of multi-level hierarchical topography and chemical composition [10,15,16].
Directed autonomous (passive, pump-free) diode-like capillary wetting and flow of liquids on materials surfaces is actively studied across the world for “green", energy-saving collection, removal, separation, and purification of liquids, pre-concentration of analytes on sensor surfaces and other prospective applications [17]. Up to now, directed autonomous wetting was realized via ordered texturing [18], in capillary [19] or semi-opened [20] channels, chemical modification [21–23], or simply by an inclination of chemically-modified surfaces with small rolling angles [24]. At the same time, open microfluidic systems with a wetting gradient stand out for a number of advantages compared to closed capillary ones, such as ease of manufacture, compactness, the ability to vary the size of channels for using these systems for different volumes of liquid, a significant reduction of clogging and breakage of a complex closed system. Recently, laser-fabricated wetting gradient structures were demonstrated on metallic (stainless steel [25,26], copper, brass [27] ) and binary (hydrophobic polymer film/hydrophilic metallic or glass substrate [28–31]) specimens, harnessing ultrafast large-scale laser nano- and microtexturing for spatial tuning local surface tension of structured surfaces between weakly hydrophilic and superhydrophilic, hydrophobic and superhydrophilic states, respectively. Such macroscopic laser-textured surface structures, supporting directional autonomous, self-starting fluid flows, are promising in various applications in microfluidics [29,31], chemical sensors [22,32], and biomolecular interactions [30], but direct demonstrations of simple functional elements based on gradient wetting are still very exemplary [25,31].
In this study, we report on demonstrations of autonomous directional water flows in laser-fabricated wetting gradient nano- and microtextures on stainless steel surface, which are arranged in the form of elementary surface microfluidic mini-devices, and characterization of their flow characteristics.
2. Experimental details
The experiments were carried out on AISI 430 stainless steel plates (Laser Center Co., Ltd., Russia) of 5x4.5 cm in size and with a thickness of 0.5 cm. Before structuring, these steel samples were cleaned in distilled water in an ultrasonic bath (Sapphire, Russia) for 30 minutes at the temperature of 30$\circ$C. The samples were structured in the air in a scanning mode, using a nanosecond pulsed ytterbium fiber laser at a wavelength λ= 1064 nm (Laser Center Co., Ltd., Russia). The scanning of the laser beam was conducted with two galvanic rotary mirrors, while the laser beam was focused using an F-theta lens (focal length - 22 cm) into a spot with a 1/e-diameter D1/e $\approx$ 45 ${\mathrm{\mu}}$m. Laser processing parameters for all samples are presented in Table 1. To analyze the chemical composition of the obtained modified surface spots, scanning electron microscopy (SEM) studies were performed using a JSM 7001FA microscope (JEOL, Japan) and energy dispersive X-ray microanalysis (EDS) using an INCA Energy 350XT spectrometer (Oxford Instruments Analytical, UK). Profilograms of the surface of the samples were obtained using a NanoEducator scanning probe microscope (SPM, NT-MDT, Russia) in a 100x100 ${\mathrm{\mu}}$m area with a scanning step of 1 ${\mathrm{\mu}}$m and analyzed, using the Gwyddion software.
Table 1. Laser processing parameters
To measure the wetting angle, the lying drop method was used. The measurements were carried out at the facility using an LED source and a ToupCam high-resolution CCD camera (ToupTek Photonics Co., Ltd, China). The distilled water droplet volume for measurements ranged from 1 to 5 ${\mathrm{\mu}}$l, dosing, and deposition of the droplet were carried out using a mechanical dispenser (Satorius, Germany) with a volume of 0.1-10 ${\mathrm{\mu}}$l. The obtained droplet images were processed using the Digimizer software (MedCalc Software, Ltd, Belgium) for the contact angle measurements.
3. Results and discussion
3.1 Laser tuning of steel surface wettability via texturing and oxidation
As a preceding step to the fabrication of wettability gradient structures, we explored the water wetting tunability on the steel surface, subjected to laser texturing at different conditions. As a result of the previous studies, the dependence of the contact angle on the power density of laser irradiation and on overlapping of laser spots was obtained, demonstrating that with an increase in power density and with an increase in the overlapping the contact angle of structured surface decreases [33]. A technological map of laser processing parameters was obtained, which allows to vary the surface contact angles from 65 to 0 degrees. After the laser radiation effect on the surface wetting properties was determined, a structure was produced providing directional liquid flow due to increase in surface hydrophilicity. Based on the obtained technological map, we performed nanosecond-laser texturing at the fixed exposure (N = 2.5 pulses/spot) and different laser fluences, changing in the range from 1.9 J/cm$^2$ to 18 J/cm$^2$ (Fig. 1); as a result, the samples h1-h6 were produced. Their wetting characterization indicates that the contact angle can be tuned from 65$^\circ$ (the clean unstructured steel surface) till almost 0$^\circ$ (the strongly textured surface in Fig. 1). This variation can be related to surface roughness development and formation of oxides, which are hydrophilic when clean.
Fig. 1. (A) Dependence of contact angle on fluence for laser-textured spots h1-h6 and (B) the corresponding SEM images; scale bar -40 µm. Insets in (B)-droplet images on the textures.
To analyze the potential topography (roughness) effect on the water wetting of the textured steel spots, SEM images of the samples h1-h6 were obtained (Fig. 1(B)), showing the more developed and complex surface topography at higher fluences. Similarly, atomic force microscopy scans performed over the area of 100$\times$100 ${\mathrm{\mu}}$m with a step of 1 ${\mathrm{\mu}}$m, demonstrate the increasing surface roughness versus the increasing fluence (Fig. 2(B)). Moreover, the roughness factor R, which is the ratio of the area of the real surface to the area of its projection onto the horizontal plane, was measured from the AFM scans and used as a parameter, controlling the water contact angle, tending to superhydrophilic values (<10$^\circ$) for the increasing R (Fig. 2(A)).
Fig. 2. (A) Contact angle variation for the laser textures h1-h6 versus the roughness factor R; (B) SPM images of the surface textures h1-h6 (from the left to the right, from the top to the bottom).
Generally, not only roughness, but also chemical composition of the interface layer dictates the contact angle, which then reads as a hypothetic combined Wenzel-Cassy equation [15]
where $\theta _i$ is wetting angle on chemically modified spots and $f$$_i$ is their fraction on the surface (R-roughness factor). That is why additional EDS analysis was undertaken on the laser-fabricated textures h1-h6 (Table 2) to evaluate the corresponding levels of chemical surface modification during the texturing.Table 2. Results of EDS chemical elemental analysis of the textures (mass %)
The untextured and textured spots exhibit almost constant content of oxygen (2-3%), with the typical error of EDS measurement about 0.5%, despite their high-temperature laser processing in air, as expected for the stainless steel. In contrast, more pronounced carbonization (1-3 mass %) of the more developed high-fluence topographies h4-h6 indicates Fe$_3$O$_4$ formation, which strongly adsorbs carbonaceous contaminants due to its chemical properties and increased adsorption area [33]. Although clean metal oxides are known to have hydrophilic properties [34], the almost constant surface oxidation and enhanced carbonaceous contamination at higher fluence (textures h4-h6) imply the predominating topography (roughness), rather than chemical, effect on the water wetting on the textures. These conclusions on the composition of the structured surface are supplemented by the previous studies [33]. It is also important to note that due to the formation of metal oxides on the surface, which in quick time adsorb hydrophobic organic compounds, the resulting structures with wettability gradient remain stable contact angles for a short period of time. . In order to maintain the initial hydrophilic contact angles, storing in high humidity air conditions may be helpful as H2O molecules in air may obstruct the surface adsorption of organic compounds. However, additional studies are to be conducted to determine the exact conditions for maintaining the required contact angles.
As a result of the abovementioned studies, appropriate laser-fabricated surface textures with significantly different wetting angles were chosen to design wetting gradient structures below.
3.2 Autonomous directional micro-flows on surface wetting gradient structures
A series of neighboring surface spots laser-textured in regimes h1-h6, were produced on the steel surface to realize a surface roughness gradient, implying the corresponding wettability gradient. Specifically, a sample composed of rectangular areas h1-h6 and arranged in the direction of increasing hydrophilicity (i.e., decreasing water contact angle) was fabricated (Fig. 3).
Fig. 3. Schematic of the sample with wettability gradient and a graph of a droplet spreading velocity at various areas of structure.
The size of each spot h1-h5 was 4 mm $\times$ 2 mm, the size of the spot h6 was 4 mm $\times$ 7.5 mm. The spot h6 had the largest area since it is superhydrophilic with the contact angle <10$^\circ$, serving as a water accumulator after its flow through the spots h1-h5. The 2 mm width of the spots h1-h6 was chosen experimentally, considering the fact that the 1.5 mm diameter of the water droplet with the utilized volume of 3.5-5 ${\mathrm{\mu}}$l should be approximately equal to the spot width for the directional droplet motion.
During our flow tests, water droplets were resided at the borders between the spots h1 and h2, h3 and h4, h4, and h5 (Fig. 4(A)-(C)), respectively, and moved toward the spot h6 with the minimal contact angle (minimal surface tension) in Fig. 3 (top). Actually, starting on the border between any two spots, in Fig. 4 the water droplets are displaced to the right (along the wetting angle gradient), finally-to the instant of 0.4 s-occupying both these two initial spots and other spots with the lower contact angles. However, independently on the initial step of the contact angle (surface tension) between the neighboring spots, the resided droplets never occupied the spots with higher contact angles, i.e., never moved to the left. This fact demonstrates the directional water motion, occurring spontaneously along the surface tension gradient. For this type of gradient, the droplet velocity approached 0.02 m/s, being estimated as the droplet right-edge displacement over the time. Figure 3 shows variation of a droplet spreading velocity at the various sections of the sample. It was noticed that the droplet velocity first increases significantly when the first section is moved, then slows down in the remaining sections as it moves through the sample. The velocity of droplet movement on the wetting gradient increases with the rise of the droplet driving force, which in turn increases as the difference between two contact angles grows [35]. Thus, the highest droplet velocity is located in the h1-h3 section, since there the contact angle difference is greatest, which is confirmed by our measurements of the droplet velocity.
Fig. 4. The dynamics of droplet motion, starting on the wetting gradient on the border between the spots A) h1 and h2, B) h3 and h4, C) h4 and h5. The scale bar is 3 mm.
Thus, water wetting gradient structure was successfully laser-fabricated on the steel surface and tested regarding the directionality and velocity of the water flow on the different regions of the wetting gradient.
3.3 Surface microfluidic devices based on autonomous directional flows
To ensure the possibility of liquid movement through open channels, it is necessary to create a certain trajectory, different elements which usually are a line, turning, splitting and combined “splitting/coupling" (bending around an obstacle) (Fig. 5(A)), providing the operations of transportation, separation and mixing of liquids. Dimensions slightly different in width for spots h1-h6 and in length for h6, depending on the necessary volume of water drainage.
Fig. 5. Elementary microfluidic elements turning (A,B), splitting (A,C), splitting/coupling (bending around an obstacle) (A,D) autonomous water flows on the steel surface.
These elements were tested regarding the directionality of autonomous flows top-visualized in dynamics (Figs. 5(B)-(D)). When a 3.5-${\mathrm{\mu}}$l water droplet was deposited in each element onto the border between the spot h1 and h2, the curved flows autonomously moved the liquid onto the spots h3-h6 (the instants in the range 0.2-0.3 s in Fig. 5). In this way, separation of the water flow was realized in the splitting and splitting/coupling elements (Figs. 5(C) and (D)), while merging of the flows was achieved in the last element. The observed velocities of the droplet front were in the range 0.02-0.05 m/s, consistent with our previous measurements above on the linear gradient structure. Hence, the basic pre-determined modalities of these elements, as potential building blocks for surface microfluidic circuits and devices, were successfully confirmed in these experiments.
4. Conclusion
In this work, ablative nanosecond-laser texturing of stainless steel surface was utilized to enhance its natural hydrophilicity till superhydrophylic state for the strongly textured, rough surface at the minimal chemical modification. A number of steel surface spots laser-textured at different laser fluences, resulting in increased surface roughness, demonstrated a gradual tunability of their water wetting angles. A linear stack of such differently wet spots represented a water wetting gradient structure, which exhibited an autonomous directional water flow at velocities, approaching to 20 mm/s. Finally, different microfluidic elements-splitter, turner, splitter accompanied by a coupler-were laser-fabricated based on the spatial wetting gradients, and successfully tested regarding their fast autonomous directional water motion. Hence, these studies opened a way to single-step, direct and facile high-throughput, large-scale laser fabrication of “green", energy-saving autonomous microfluidic circuits for numerous promising applications, such as bactericidal treatment, preconcentration of hydrophilic and hydrophobic analytes for chemo- and biosensorics, collection and removal of liquids, as well as their separation and purification.
Funding
Russian Science Foundation (20-62-46045).
Acknowledgments
Institute Bioengineering ITMO University is gratefully acknowledged for the help with experiments on laser-induced modification of metal surface to control its wetting properties. The reported study was supported by the Russian Science Foundation (project 20-62-46045).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia, and L. Jiang, “Petal effect: A superhydrophobic state with high adhesive force,” Langmuir 24(8), 4114–4119 (2008). [CrossRef]
2. L. Feng, S. Li, Y. Li, H. Li, L. Zhang, J. Zhai, Y. Song, B. Liu, L. Jiang, and D. Zhu, “Super-hydrophobic surfaces: from natural to artificial,” Adv. Mater. 14(24), 1857–1860 (2002). [CrossRef]
3. S. G. Lee, H. S. Lim, D. Y. Lee, D. Kwak, and K. Cho, “Tunable anisotropic wettability of rice leaf-like wavy surfaces,” Adv. Funct. Mater. 23(5), 547–553 (2013). [CrossRef]
4. Y. Lu, L. Yu, Z. Zhang, S. Wu, G. Li, P. Wu, Y. Hu, J. Li, J. Chu, and D. Wu, “Biomimetic surfaces with anisotropic sliding wetting by energy-modulation femtosecond laser irradiation for enhanced water collection,” RSC Adv. 7(18), 11170–11179 (2017). [CrossRef]
5. B. Bhushan, Y. C. Jung, A. Niemietz, and K. Koch, “Lotus-like biomimetic hierarchical structures developed by the self-assembly of tubular plant waxes,” Langmuir 25(3), 1659–1666 (2009). [CrossRef]
6. H. F. Bohn and W. Federle, “Insect aquaplaning: Nepenthes pitcher plants capture prey with the peristome, a fully wettable water-lubricated anisotropic surface,” Proc. Natl. Acad. Sci. 101(39), 14138–14143 (2004). [CrossRef]
7. H. Chen, P. Zhang, L. Zhang, H. Liu, Y. Jiang, D. Zhang, Z. Han, and L. Jiang, “Continuous directional water transport on the peristome surface of nepenthes alata,” Nature 532(7597), 85–89 (2016). [CrossRef]
8. H. Chen, L. Zhang, Y. Zhang, P. Zhang, D. Zhang, and L. Jiang, “Uni-directional liquid spreading control on a bio-inspired surface from the peristome of nepenthes alata,” J. Mater. Chem. A 5(15), 6914–6920 (2017). [CrossRef]
9. E. P. Ivanova, J. Hasan, H. K. Webb, V. K. Truong, G. S. Watson, J. A. Watson, V. A. Baulin, S. Pogodin, J. Y. Wang, and M. J. Tobin, “Natural bactericidal surfaces: mechanical rupture of pseudomonas aeruginosa cells by cicada wings,” Small 8(16), 2489–2494 (2012). [CrossRef]
10. E. Fadeeva and B. Chichkov, “Biomimetic liquid-repellent surfaces by ultrafast laser processing,” Appl. Sci. 8(9), 1424 (2018). [CrossRef]
11. E. Fadeeva, V. K. Truong, M. Stiesch, B. N. Chichkov, R. J. Crawford, J. Wang, and E. P. Ivanova, “Bacterial retention on superhydrophobic titanium surfaces fabricated by femtosecond laser ablation,” Langmuir 27(6), 3012–3019 (2011). [CrossRef]
12. S. C. Thickett, C. Neto, and A. T. Harris, “Biomimetic surface coatings for atmospheric water capture prepared by dewetting of polymer films,” Adv. Mater. 23(32), 3718–3722 (2011). [CrossRef]
13. J. Ju, K. Xiao, X. Yao, H. Bai, and L. Jiang, “Bioinspired conical copper wire with gradient wettability for continuous and efficient fog collection,” Adv. Mater. 25(41), 5937–5942 (2013). [CrossRef]
14. U. Hermens, S. Kirner, C. Emonts, P. Comanns, E. Skoulas, A. Mimidis, H. Mescheder, K. Winands, J. Krüger, and E. Stratakis, “Mimicking lizard-like surface structures upon ultrashort laser pulse irradiation of inorganic materials,” Appl. Surf. Sci. 418, 499–507 (2017). [CrossRef]
15. L. B. Boinovich and A. M. Emelyanenko, “Hydrophobic materials and coatings: principles of design, properties and applications,” Russ. Chem. Rev. 77(7), 583–600 (2008). [CrossRef]
16. A. Vorobyev and C. Guo, “Femtosecond laser modification of material wetting properties: a brief review,” Sci. Adv. Mater. 4(3), 432–438 (2012). [CrossRef]
17. J. Berthier and P. Silberzan, Microfluidics for biotechnology, 2009 (Artech House, Boston), 2003.
18. M. Blow, H. Kusumaatmaja, and J. Yeomans, “Imbibition through an array of triangular posts,” J. Phys.: Condens. Matter 21, 464125 (2009). [CrossRef]
19. C. Extrand, “Retention forces of a liquid slug in a rough capillary tube with symmetric or asymmetric features,” Langmuir 23(4), 1867–1871 (2007). [CrossRef]
20. A. Buguin, L. Talini, and P. Silberzan, “Ratchet-like topological structures for the control of microdrops,” Appl. Phys. A 75(2), 207–212 (2002). [CrossRef]
21. M. M. Weislogel, “Steady spontaneous capillary flow in partially coated tubes,” AIChE J. 43(3), 645–654 (1997). [CrossRef]
22. X. Yu, Z. Wang, Y. Jiang, and X. Zhang, “Surface gradient material: from superhydrophobicity to superhydrophilicity,” Langmuir 22(10), 4483–4486 (2006). [CrossRef]
23. Y. P. Hou, S. L. Feng, L. M. Dai, and Y. M. Zheng, “Droplet manipulation on wettable gradient surfaces with micro-/nano-hierarchical structure,” Chem. Mater. 28(11), 3625–3629 (2016). [CrossRef]
24. L. B. Boinovich and A. M. Emelyanenko, “Anti-icing potential of superhydrophobic coatings,” Mendeleev Commun. 23(1), 3–10 (2013). [CrossRef]
25. P. Comanns, G. Buchberger, A. Buchsbaum, R. Baumgartner, A. Kogler, S. Bauer, and W. Baumgartner, “Directional, passive liquid transport: the texas horned lizard as a model for a biomimetic “liquid diode",” J. R. Soc., Interface 12(109), 20150415 (2015). [CrossRef]
26. I. Krylach, S. Kudryashov, R. Olekhnovich, M. Moskvin, and M. Uspenskaya, “Tuning water wetting angle of a steel surface via nanosecond laser ablative nano/microtexturing for chemical and biomedical microfluidic applications,” Laser Phys. Lett. 16(10), 105602 (2019). [CrossRef]
27. J. Wu, K. Yin, S. Xiao, Z. Wu, Z. Zhu, J.-A. Duan, and J. He, “Laser fabrication of bioinspired gradient surfaces for wettability applications,” Adv. Mater. Interfaces 8(5), 2001610 (2021). [CrossRef]
28. I. Paradisanos, C. Fotakis, S. Anastasiadis, and E. Stratakis, “Gradient induced liquid motion on laser structured black si surfaces,” Appl. Phys. Lett. 107(11), 111603 (2015). [CrossRef]
29. D. Wu, Z. Zhang, Y. Zhang, Y. Jiao, S. Jiang, H. Wu, C. Li, C. Zhang, J. Li, Y. Hu, G. Li, J. Chu, and L. Jiang, “High-performance unidirectional manipulation of microdroplets by horizontal vibration on femtosecond laser-induced slant microwall arrays,” Adv. Mater. 32(48), 2005039 (2020). [CrossRef]
30. I. Krylach, S. Kudryashov, M. Fokina, V. Sitnikova, R. Olekhnovich, M. Moskvin, N. Shchedrina, S. Gonchukov, G. Odintsova, and M. Uspenskaya, “Directional autonomous water flow in laser-engineered microfluidic gradient structures on polymethylmetacrylate-coated steel surface,” Laser Phys. Lett. 17(8), 085602 (2020). [CrossRef]
31. I. Krylach, S. Kudryashov, R. Olekhnovich, M. Fokina, V. Sitnikova, M. Moskvin, N. Shchedrina, and M. Uspenskaya, “Fabrication of a functional relief on the surface of a polyvinyl chloride film by nanosecond laser microtexturing,” Opt. Spectrosc. 128(8), 1251–1255 (2020). [CrossRef]
32. V. D. Ta, A. Dunn, T. J. Wasley, J. Li, R. W. Kay, J. Stringer, P. J. Smith, E. Esenturk, C. Connaughton, and J. D. Shephard, “Laser textured surface gradients,” Appl. Surf. Sci. 371, 583–589 (2016). [CrossRef]
33. N. Shchedrina, Y. Karlagina, T. Itina, A. Ramos, D. Correa, A. Tokmacheva-Kolobova, S. Manokhin, D. Lutoshina, R. Yatsuk, and I. Krylach, “Wetting angle stability of steel surface structures after laser treatment,” Opt. Quantum Electron. 52(3), 163–212 (2020). [CrossRef]
34. S. Takeda, M. Fukawa, Y. Hayashi, and K. Matsumoto, “Surface oh group governing adsorption properties of metal oxide films,” Thin Solid Films 339(1-2), 220–224 (1999). [CrossRef]
35. M. Liu, L. Huang, Y. Yao, Z. Peng, and S. Chen, “Dynamic behavior of a droplet across a hydrophobic and hydrophilic boundary,” J. Phys. Chem. C 123(38), 23505–23510 (2019). [CrossRef]
