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Using high-intensity femtosecond laser pulses, we create a novel surface pattern that transforms regular silicon to superwicking. Due to the created surface structure, water sprints vertically uphill in a gravity defying way. Our study of the liquid motion shows that the fast self-propelling motion of water is due to a supercapillary effect from the surface structures we created. The wicking dynamics in the produced surface structure is found to follow the classical square root of time dependence.
©2010 Optical Society of America
1. Introduction
Silicon is widely used in microfluidics [1,2], lab-on-chip technologies [3], fluidic microreactors [4], and electronics cooling [5]. In these applications, the possibility of altering the surface wetting property of silicon is of paramount importance. It is known that the wetting properties of solids can be altered through surface structuring [6–10]. Recently, we demonstrated a laser surface structuring technique that turns metals hydrophilic for volatile liquids [11]. In this work, by using high-intensity femtosecond laser pulses, we create a novel surface pattern that transforms a regular silicon surface to superwicking for water and other liquids. In a gravity defying way, water sprints vertically uphill along the structured silicon surface at a high velocity. Our study of the fluid dynamics shows that the extraordinarily strong self-propelling motion of water is due to a supercapillary effect from the surface structures we created.
2. Experimental setup
To structure silicon surfaces, we use an amplified Ti:sapphire laser system that generates 65-fs pulses with energy around 1.5 mJ/pulse at a maximum repetition rate of 1 kHz with a central wavelength of 800 nm. The laser beam is horizontally polarized and is focused normally onto the sample mounted vertically on a translation stage. The samples used in our experiments are single-crystal phosphorus-doped silicon [(100)-oriented, resistivity of 1 - 30 Ω⋅cm] with a dimension of 25×25×0.65 mm3. By scanning the sample across the laser beam, we produce a 22-mm-long micro-groove along the vertical direction. The micro-groove is produced at fluence of 8.5 J/cm2 with a scanning speed of 1 mm/s, and the focused laser spot diameter is 100 μm. After a horizontal shift of the sample by 100 μm we produce next micro-groove and this process is repeated to create an extended array of parallel micro-grooves with an area of 22×11 mm2. The micro-grooves are fabricated in air of atmospheric pressure. A scanning electron microscope (SEM) is used to examine the surface structures following femtosecond laser treatments. We test the wetting property of our samples with various liquids, including distilled water, acetone, and methanol.
3. Results and discussion
A photograph of the laser treated silicon sample is shown in Fig. 1(a) . First of all, we notice a dramatic change in optical property of the structured sample, where the processed area appears pitch black. SEM images of the surface structures we created on the silicon surface are shown in Figs. 1(b)–1(d). Figure 1(b) shows that the treated surface has multiple parallel micro-grooves with a period of 100 μm, corresponding to the horizontal step between two vertical scanning lines. Magnified views of typical surface structures are shown in Figs. 1(c) and 1(d), where we can see that microgrooves are covered with nano- and fine micro-structures. The nanostructures include nanoprotrusions and nanocavities, while fine microstructures include microcavities and microscale aggregates from nanoparticles that fuse onto each other and onto the silicon surface. An average depth of micro-grooves is measured to be about 40 μm.
Fig. 1 (a) Photograph of the treated silicon sample. (b) SEM image of parallel microgrooves. (c) and (d) Micro- and nano-structural features of the surface pattern.
We study the processed silicon surface wetting properties by examining the spreading dynamics of liquids in various volumes, in the range of 1-5 μl. A camera is used to record the spreading dynamics at a speed of 5 frames per second. Figures 2(a) –2(f) show the spreading dynamics of a 3-μl water droplet pipetted on a horizontally-placed silicon surface, where we see that the water droplet spreads highly anisotropically on the treated area and it flows preferentially along the micro-grooves. As shown in Figs. 2(a)–2(f), the water rapidly travels through the entire 22-mm treated area along the micro-grooves in about 0.6 s, while the waterspreading perpendicular to the grooves is much slower and the long stripe of the wetting trace only widens by about 1.5 mm each side in about 4.2 s. As seen from Figs. 2(a) and 2(d), the velocity of water spreading along the micro-grooves decreases with time. For comparison, the behavior of a water droplet pipetted on an untreated silicon surface is also shown in Figs. 2(g) and 2(h). A comparison between Fig. 2(h) and 2(f) shows that the ultrafast laser treatment turns silicon superhydrophilic. Next, we stand the silicon sample vertically by pointing the grooves perpendicular to the table. Strikingly, when we pipette a water droplet on the bottom of the groove area, the water immediately sprints vertically uphill against the gravity, as shown in Figs. 3(a) –3(f) as well as in a supplementary video [12]. As seen in Figs. 3(a)–3(f), the water rapidly travels vertically uphill through the entire 22-mm treated area in about 1 s and the spreading velocity of water decreases with time, similar to the case of the horizontal orientation of the sample. We also experiment with other liquids, including methanol and acetone, and we see similar liquid spreading behaviors as with water. Oxidation is possible for surface structures formed in air. To understand the possible effect of the chemical change on the water spreading, we prepare a sample in a vacuum at a base pressure of 7.0 × 10−3 Torr. The experiment with this sample shows similar rapid vertically uphill flow of water as on the sample prepared in air. Therefore the effect of chemical composition change on water spreading is insignificant in our study.
Fig. 2 Spreading dynamics of water on a horizontal silicon sample. (a)–(d) Water spreading along microgrooves. (e) and (f) Water spreading across microgrooves. (g) and (h) Water behavior on the untreated surface.
Fig. 3 (a)–(f) Dynamics of water running uphill on a vertically standing silicon sample with vertically oriented microgrooves.
To understand the spreading dynamics of liquids in our study, we consider capillary effects in various capillary systems. It is known that the equation of motion for liquids in a closed capillary is governed by the classical Washburn equation [13]
where z is the distance traveled by the liquid, t is the time, γ and μ are the surface tension and viscosity of the liquid, r is the capillary radius, and θ is the contact angle between thewall and the meniscus. The Eq. (1) shows that capillary-driven liquid advances with a t 1/2 dependence, i.e. . The Washburn dynamics was observed in a wide range of capillary dimensions [14,15] and also under microgravity conditions [16]. The dimension of our structures falls within the verified capillary size range for the Washburn dynamics [14]. The Washburn-type dynamics has also been observed on a horizontally placed surface with open surface grooves [17–19] and two-dimensional arrays of pillars [8,20]. Therefore, the Washburn t 1/2 dynamics appears to be universally followed by various types of surface structures [8,10,19–22] with , where D is the diffusion constant, and a structured surface with a complex geometry can be viewed as a network of open capillaries [19]. To determine the type of wicking dynamics in our experiment, we plotted the distance z of the wetting front versus t 1/2 for the vertically-standing sample, as shown in Fig. 4 . We cansee that the spreading distance linearly depends on t 1/2 even for such a complex geometry as the grooves with superimposed nano- and fine micro-structures. The linear t 1/2 dependence is also observed on the horizontally-placed sample. The t 1/2 dynamics observed here leads us to believe that the liquid flowing on silicon surfaces results indeed from the capillary effect. The extremely rapid self-propelling uphill motion of liquids indicates that the capillary force in our experiment is extremely strong, and we essentially transform a regular silicon surface to superwicking.Fig. 4 Plot of distance traveled by wetting front versus t 1/2 for vertical orientation of the silicon sample.
4. Conclusions
In summary, we transform regular silicon surfaces to superwicking through direct high-intensity femtosecond laser surface structuring. Due to superwicking, the structured surface renders liquids to sprint vertically uphill against the gravity over an extended distance. We show that the driving force of the self-propelling liquid motion is the supercapillary effect from the surface structures we created. The wicking dynamics in the produced surface structure is found to follow the classical square root of time dependence. The unique wetting and wicking properties demonstrated here on the ultrafast laser-structured silicon may find a wide range of applications in nano/microfluidics, optofluidics, lab-on-chip technology, fluidic microreactors, chemical and biological sensors, biomedicine, and heat transfer devices (e.g., heat pipes for cooling of electronic devices, high-power light-emitting diode arrays, and microreactors for exothermic chemical reactions).
Acknowledgments
This work was supported by the US Air Force Office of Scientific Research and the National Science Foundation.
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