Laser Makes Silicon Superwicking
A.Y. Vorobyev and Chunlei Guo
Silicon is the most widely used material for semiconductor devices,
especially in integrated circuits used in
microelectronics and computers. Currently, a bottleneck that limits computer
speed is the overheating of the computer’s
central processing unit (CPU). ;erefore,
the cooling of silicon is in the center stage
for further increasing the speed of computers. Liquid is one of the most e;ec-tive coolants. However, the cooling efficiency depends crucially on the wetting
property of the surfaces to be cooled.
Furthermore, silicon is also widely used
in microfluidics and lab-on-chip technologies. ;erefore, the possibility of
altering the surface-wetting property of
silicon and enhancing its cooling e;ect
is of paramount importance.
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. 1 In a gravity-defying way, water sprints vertically uphill along the structured silicon surface
at an unprecedented velocity.
To structure silicon surfaces, we use
an amplified Ti:sapphire laser system
that generates 65-fs pulses at a maximum repetition rate of 1 kHz with a
central wavelength of 800 nm. ;e laser
beam is horizontally polarized and focused normally onto the sample mounted vertically on a translation stage. By
scanning the silicon sample across the
laser beam, we produce a 22-mm-long
microgroove along the vertical direction.
Next, the sample is shifted horizontally
by 100 µm and another microgroove is
produced. ;is process is repeated to create an extended array of parallel microgrooves of an area of 22 × 11 mm2.
In a photo of the laser-treated silicon
sample—shown in (a) of the figure—we
notice a dramatic change of optical
property of the structured sample, where
the processed area appears pitch black. In
the SEM image of the processed silicon
( 11× 22 mm)
Si plate ( 25× 25×0.6 mm)
(a) Photo of the treated silicon sample. (b) SEM image of parallel microgrooves with superim-
posed micro- and nanostructural features. (c) and (d) Snapshots of water running uphill on a
vertically standing silicon sample with vertically oriented microgrooves.
surface (b), we can see that the treated
surface has multiple parallel microgrooves
with a period of 100 µm, corresponding to
the horizontal step between vertical scanning lines. ;e microgrooves are covered
with nano- and fine microstructures. ;e
nanostructures include nanoprotrusions
and nanocavities, while fine microstructures include microcavities and microscale
aggregates from nanoparticles that fuse
onto each other and the silicon surface.
When we stand the silicon sample
vertically with the grooves pointing perpendicular to the table and place a pipette
with a water droplet on the bottom of
the groove area, the water immediately
sprints vertically uphill against the gravi-
ty, as shown in (c) and (d) as well as in the
supplementary video. ;e water rapidly
travels vertically uphill over a distance of
20 mm in 0.8 s.
Our study shows that the spreading
dynamics on the processed surface area
follows a square root of time dependence
that is characteristic for capillary actions.
Hence, the driving force that moves
water vertically uphill on the treated
surface has a capillary nature.
;e extremely rapid self-propelling
uphill motion of water indicates that
the wicking force in our experiment
is extremely strong, and we essentially
transform a regular silicon surface to
superwicking. ;e unique wetting and
wicking properties of our femtosecond
laser-structured silicon may find applications in optofluidics, nano/microfluidics,
lab-on-chip technology, fluidic microreactors, chemical and biological sensors,
biomedicine, and heat-transfer systems for
cooling devices such as computer CPUs,
high-power light-emitting diode arrays,
and exothermic chemical microreactors. t
to view the video that accompanies
Anatoliy Y. Vorobyev and Chunlei Guo (guo@optics.
rochester.edu) are with the Institute of Optics, University of Rochester, Rochester, N. Y., U.S.A.
1. A. Y. Vorobyev and C. Guo. Opt. Express, 18, 6455-60
2. Supplementary video of water running vertically uphill on
the surface of the femtosecond laser-structured silicon.
38 | OPN Optics & Photonics News