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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 7 — Mar. 29, 2010
  • pp: 6455–6460
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Laser turns silicon superwicking

A. Y. Vorobyev and Chunlei Guo  »View Author Affiliations


Optics Express, Vol. 18, Issue 7, pp. 6455-6460 (2010)
http://dx.doi.org/10.1364/OE.18.006455


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Abstract

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 OSA

1. Introduction

Silicon is widely used in microfluidics [1

1. P. Gravesen, J. Branebjerg, and O. S. Jensen, “Microfluidics – a review,” J. Micromech. Microeng. 3(4), 168–182 (1993). [CrossRef]

,2

2. G. M. Whitesides, “The origins and the future of microfluidics,” Nature 442(7101), 368–373 (2006). [CrossRef] [PubMed]

], lab-on-chip technologies [3

3. P. Abgrall and A. M. Gue, “Lab-on-chip technologies: making a microfluidic network and coupling it into a complete microsystem—a review,” J. Micromech. Microeng. 17(5), R15–R49 (2007). [CrossRef]

], fluidic microreactors [4

4. K. F. Jensen, “Silicon-based microchemical systems: characteristics and applications,” MRS Bull. 31, 101–107 (2006). [CrossRef]

], and electronics cooling [5

5. D. Erickson and D. Li, “Integrated microfluidic devices,” Anal. Chim. Acta 507(1), 11–26 (2004). [CrossRef]

]. 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

6. R. N. Wenzel, “Surface roughness and contact angle,” J. Phys. Colloid Chem. 53(9), 1466–1467 (1949). [CrossRef]

10

10. K. M. Hay, M. I. Dragila, and J. Liburdy, “Theoretical model for the wetting of a rough surface,” J. Colloid Interface Sci. 325(2), 472–477 (2008). [CrossRef] [PubMed]

]. Recently, we demonstrated a laser surface structuring technique that turns metals hydrophilic for volatile liquids [11

11. A. Y. Vorobyev and C. Guo, “Metal pumps liquid uphill,” Appl. Phys. Lett. 94(22), 224102 (2009). [CrossRef]

]. 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

3. Results and discussion

A photograph of the laser treated silicon sample is shown in Fig. 1(a)
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.
. 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.

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)
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.
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)
Fig. 3 (a)–(f) Dynamics of water running uphill on a vertically standing silicon sample with vertically oriented microgrooves.
3(f) as well as in a supplementary video [12

12. Supplementary video of water running vertically uphill on the surface of femtosecond laser-structured silicon as in Fig. 3(a)–(f) of the main article.

]. 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.

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

13. E. W. Washburn, “The dynamics of capillary flow,” Phys. Rev. 17(3), 273–283 (1921). [CrossRef]

]
z(t)=(γrcosθ2μ)1/2t1/2
(1)
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. z(t)t1/2. The Washburn dynamics was observed in a wide range of capillary dimensions [14

14. L. R. Fisher and P. D. Lark, “An experimental study of the Washburn equation for liquid flow in very fine capillaries,” J. Colloid Interface Sci. 69(3), 486–492 (1979). [CrossRef]

,15

15. N. R. Tas, J. Haneveld, H. V. Jansen, M. Elwenspoek, and A. van den Berg, “Capillary filling speed of water in nanochannels,” Appl. Phys. Lett. 85(15), 3274–3276 (2004). [CrossRef]

] and also under microgravity conditions [16

16. M. Stange, M. E. Dreyer, and H. J. Rath, “Capillary driven flow in circular cylindrical tubes,” Phys. Fluids 15(9), 2587–2601 (2003). [CrossRef]

]. The dimension of our structures falls within the verified capillary size range for the Washburn dynamics [14

14. L. R. Fisher and P. D. Lark, “An experimental study of the Washburn equation for liquid flow in very fine capillaries,” J. Colloid Interface Sci. 69(3), 486–492 (1979). [CrossRef]

]. The Washburn-type dynamics has also been observed on a horizontally placed surface with open surface grooves [17

17. L. A. Romero and F. G. Yost, “Flow in an open channel capillary,” J. Fluid Mech. 322(-1), 109–129 (1996). [CrossRef]

19

19. R. R. Rye, J. A. Mann, and F. G. Yost, “The flow of liquids in surface grooves,” Langmuir 12(2), 555–565 (1996). [CrossRef]

] and two-dimensional arrays of pillars [8

8. J. Bico, C. Tordeux, and D. Quere, “Rough wetting,” Europhys. Lett. 55(2), 214–220 (2001). [CrossRef]

,20

20. L. Courbin, E. Denieul, E. Dressaire, M. Roper, A. Ajdari, and H. A. Stone, “Imbibition by polygonal spreading on microdecorated surfaces,” Nat. Mater. 6(9), 661–664 (2007). [CrossRef] [PubMed]

]. Therefore, the Washburn t 1/2 dynamics appears to be universally followed by various types of surface structures [8

8. J. Bico, C. Tordeux, and D. Quere, “Rough wetting,” Europhys. Lett. 55(2), 214–220 (2001). [CrossRef]

,10

10. K. M. Hay, M. I. Dragila, and J. Liburdy, “Theoretical model for the wetting of a rough surface,” J. Colloid Interface Sci. 325(2), 472–477 (2008). [CrossRef] [PubMed]

,19

19. R. R. Rye, J. A. Mann, and F. G. Yost, “The flow of liquids in surface grooves,” Langmuir 12(2), 555–565 (1996). [CrossRef]

22

22. A. D. Dussaud, P. M. Adler, and A. Lips, “Liquid Transport in the Networked Microchannels of the Skin Surface,” Langmuir 19(18), 7341–7345 (2003). [CrossRef]

] with z(t)(Dt)1/2, where D is the diffusion constant, and a structured surface with a complex geometry can be viewed as a network of open capillaries [19

19. R. R. Rye, J. A. Mann, and F. G. Yost, “The flow of liquids in surface grooves,” Langmuir 12(2), 555–565 (1996). [CrossRef]

]. 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
Fig. 4 Plot of distance traveled by wetting front versus t 1/2 for vertical orientation of the silicon sample.
. 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.

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.

References and links

1.

P. Gravesen, J. Branebjerg, and O. S. Jensen, “Microfluidics – a review,” J. Micromech. Microeng. 3(4), 168–182 (1993). [CrossRef]

2.

G. M. Whitesides, “The origins and the future of microfluidics,” Nature 442(7101), 368–373 (2006). [CrossRef] [PubMed]

3.

P. Abgrall and A. M. Gue, “Lab-on-chip technologies: making a microfluidic network and coupling it into a complete microsystem—a review,” J. Micromech. Microeng. 17(5), R15–R49 (2007). [CrossRef]

4.

K. F. Jensen, “Silicon-based microchemical systems: characteristics and applications,” MRS Bull. 31, 101–107 (2006). [CrossRef]

5.

D. Erickson and D. Li, “Integrated microfluidic devices,” Anal. Chim. Acta 507(1), 11–26 (2004). [CrossRef]

6.

R. N. Wenzel, “Surface roughness and contact angle,” J. Phys. Colloid Chem. 53(9), 1466–1467 (1949). [CrossRef]

7.

A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Trans. Faraday Soc. 40, 546–551 (1944). [CrossRef]

8.

J. Bico, C. Tordeux, and D. Quere, “Rough wetting,” Europhys. Lett. 55(2), 214–220 (2001). [CrossRef]

9.

G. McHale, N. J. Shirtcliffe, S. Aqil, C. C. Perry, and M. I. Newton, “Topography driven spreading,” Phys. Rev. Lett. 93(3), 036102 (2004). [CrossRef] [PubMed]

10.

K. M. Hay, M. I. Dragila, and J. Liburdy, “Theoretical model for the wetting of a rough surface,” J. Colloid Interface Sci. 325(2), 472–477 (2008). [CrossRef] [PubMed]

11.

A. Y. Vorobyev and C. Guo, “Metal pumps liquid uphill,” Appl. Phys. Lett. 94(22), 224102 (2009). [CrossRef]

12.

Supplementary video of water running vertically uphill on the surface of femtosecond laser-structured silicon as in Fig. 3(a)–(f) of the main article.

13.

E. W. Washburn, “The dynamics of capillary flow,” Phys. Rev. 17(3), 273–283 (1921). [CrossRef]

14.

L. R. Fisher and P. D. Lark, “An experimental study of the Washburn equation for liquid flow in very fine capillaries,” J. Colloid Interface Sci. 69(3), 486–492 (1979). [CrossRef]

15.

N. R. Tas, J. Haneveld, H. V. Jansen, M. Elwenspoek, and A. van den Berg, “Capillary filling speed of water in nanochannels,” Appl. Phys. Lett. 85(15), 3274–3276 (2004). [CrossRef]

16.

M. Stange, M. E. Dreyer, and H. J. Rath, “Capillary driven flow in circular cylindrical tubes,” Phys. Fluids 15(9), 2587–2601 (2003). [CrossRef]

17.

L. A. Romero and F. G. Yost, “Flow in an open channel capillary,” J. Fluid Mech. 322(-1), 109–129 (1996). [CrossRef]

18.

J. A. Mann Jr, L. Romero, R. R. Rye, and F. G. Yost, “Flow of simple liquids down narrow ssV grooves,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 52(4), 3967–3972 (1995). [CrossRef] [PubMed]

19.

R. R. Rye, J. A. Mann, and F. G. Yost, “The flow of liquids in surface grooves,” Langmuir 12(2), 555–565 (1996). [CrossRef]

20.

L. Courbin, E. Denieul, E. Dressaire, M. Roper, A. Ajdari, and H. A. Stone, “Imbibition by polygonal spreading on microdecorated surfaces,” Nat. Mater. 6(9), 661–664 (2007). [CrossRef] [PubMed]

21.

S. Gerdes, A. M. Cazabat, and G. Strom, “The spreading of silicone oil droplets on a surface with parallel V-shaped grooves,” Langmuir 13(26), 7258–7264 (1997). [CrossRef]

22.

A. D. Dussaud, P. M. Adler, and A. Lips, “Liquid Transport in the Networked Microchannels of the Skin Surface,” Langmuir 19(18), 7341–7345 (2003). [CrossRef]

OCIS Codes
(000.2690) General : General physics
(140.3390) Lasers and laser optics : Laser materials processing
(160.0160) Materials : Materials
(160.6000) Materials : Semiconductor materials
(230.4000) Optical devices : Microstructure fabrication
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Materials

History
Original Manuscript: December 17, 2009
Revised Manuscript: February 5, 2010
Manuscript Accepted: February 11, 2010
Published: March 15, 2010

Citation
A. Y. Vorobyev and Chunlei Guo, "Laser turns silicon superwicking," Opt. Express 18, 6455-6460 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-7-6455


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References

  1. P. Gravesen, J. Branebjerg, and O. S. Jensen, “Microfluidics – a review,” J. Micromech. Microeng. 3(4), 168–182 (1993). [CrossRef]
  2. G. M. Whitesides, “The origins and the future of microfluidics,” Nature 442(7101), 368–373 (2006). [CrossRef] [PubMed]
  3. P. Abgrall and A. M. Gue, “Lab-on-chip technologies: making a microfluidic network and coupling it into a complete microsystem—a review,” J. Micromech. Microeng. 17(5), R15–R49 (2007). [CrossRef]
  4. K. F. Jensen, “Silicon-based microchemical systems: characteristics and applications,” MRS Bull. 31, 101–107 (2006). [CrossRef]
  5. D. Erickson and D. Li, “Integrated microfluidic devices,” Anal. Chim. Acta 507(1), 11–26 (2004). [CrossRef]
  6. R. N. Wenzel, “Surface roughness and contact angle,” J. Phys. Colloid Chem. 53(9), 1466–1467 (1949). [CrossRef]
  7. A. B. D. Cassie and S. Baxter, “Wettability of porous surfaces,” Trans. Faraday Soc. 40, 546–551 (1944). [CrossRef]
  8. J. Bico, C. Tordeux, and D. Quere, “Rough wetting,” Europhys. Lett. 55(2), 214–220 (2001). [CrossRef]
  9. G. McHale, N. J. Shirtcliffe, S. Aqil, C. C. Perry, and M. I. Newton, “Topography driven spreading,” Phys. Rev. Lett. 93(3), 036102 (2004). [CrossRef] [PubMed]
  10. K. M. Hay, M. I. Dragila, and J. Liburdy, “Theoretical model for the wetting of a rough surface,” J. Colloid Interface Sci. 325(2), 472–477 (2008). [CrossRef] [PubMed]
  11. A. Y. Vorobyev and C. Guo, “Metal pumps liquid uphill,” Appl. Phys. Lett. 94(22), 224102 (2009). [CrossRef]
  12. Supplementary video of water running vertically uphill on the surface of femtosecond laser-structured silicon as in Fig. 3(a)–(f) of the main article.
  13. E. W. Washburn, “The dynamics of capillary flow,” Phys. Rev. 17(3), 273–283 (1921). [CrossRef]
  14. L. R. Fisher and P. D. Lark, “An experimental study of the Washburn equation for liquid flow in very fine capillaries,” J. Colloid Interface Sci. 69(3), 486–492 (1979). [CrossRef]
  15. N. R. Tas, J. Haneveld, H. V. Jansen, M. Elwenspoek, and A. van den Berg, “Capillary filling speed of water in nanochannels,” Appl. Phys. Lett. 85(15), 3274–3276 (2004). [CrossRef]
  16. M. Stange, M. E. Dreyer, and H. J. Rath, “Capillary driven flow in circular cylindrical tubes,” Phys. Fluids 15(9), 2587–2601 (2003). [CrossRef]
  17. L. A. Romero and F. G. Yost, “Flow in an open channel capillary,” J. Fluid Mech. 322(-1), 109–129 (1996). [CrossRef]
  18. J. A. Mann, L. Romero, R. R. Rye, and F. G. Yost, “Flow of simple liquids down narrow ssV grooves,” Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics 52(4), 3967–3972 (1995). [CrossRef] [PubMed]
  19. R. R. Rye, J. A. Mann, and F. G. Yost, “The flow of liquids in surface grooves,” Langmuir 12(2), 555–565 (1996). [CrossRef]
  20. L. Courbin, E. Denieul, E. Dressaire, M. Roper, A. Ajdari, and H. A. Stone, “Imbibition by polygonal spreading on microdecorated surfaces,” Nat. Mater. 6(9), 661–664 (2007). [CrossRef] [PubMed]
  21. S. Gerdes, A. M. Cazabat, and G. Strom, “The spreading of silicone oil droplets on a surface with parallel V-shaped grooves,” Langmuir 13(26), 7258–7264 (1997). [CrossRef]
  22. A. D. Dussaud, P. M. Adler, and A. Lips, “Liquid Transport in the Networked Microchannels of the Skin Surface,” Langmuir 19(18), 7341–7345 (2003). [CrossRef]

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