Concentrated hydrochloric acid

Untreated Al foil
(AL000612, Goodfellow) with a thickness of 200 μm (similar to
the thickness of common soda can walls) was cut into rectangular samples,
which was performed with chemical and fs-laser treatments, as shown
in Figure 1b. The foils
were first chemically etched for 30 min under ultrasound in a mixed
solution of concentrated hydrochloric acid (37.2%, Thermo Fisher Scientific
Inc., USA) and 5% cuprous chloride (98% min, Alfa Aesar Inc., USA)
with a mixing ratio of 0.25 v/v % to obtain a surface with microroughness.
The ratio of the HCl/CuCl₂ mixture solution volume to the
surface area of the sample was 1.25 mL cm^–2. After
etching, the samples were cleaned with ethanol under ultrasound. To
yield a superhydrophobic surface, the etched samples were treated
in 0.01 mol L^–1 stearic acid (98%, Alfa Aesar Inc.,
USA) ethanol solution for 24 h, followed by rinsing with ethanol and
drying at ambient temperature. Finally, the as-designed superhydrophilic
patterns were fs-laser-ablated on the superhydrophobic surface by
programmed scanning with a SCANcube III 10 galvanometer scanner (SCANLAB
GmbH, Germany) with a scanning speed of 5 mm s^–1. The laser was generated by an Astrella-USP-1K ultrafast Ti/sapphire
amplifier system (Coherent Inc., USA) with the wavelength centered
at 800 nm, pulse width of 35 fs, repetition rate of 1 kHz, beam quality M² < 1.25, and an average power of 1.25 W.

Glass substrates with dimensions of 1.2 cm × 1.2 cm (Thermo Fisher Scientific) were washed by immersing them in a mixture of methanol (Thermo Fisher Scientific) and concentrated hydrochloric acid (Thermo Fisher Scientific) (1:1 v/v) for at least 30 min followed by washing them with DI water thoroughly. The substrates were oxidized with concentrated H₂SO₄ (l) (Thermo Fisher Scientific) for at least 30 min to activate the hydroxyl groups and then washed with DI water thoroughly. Next, the Glass substrates were washed with methanol and immersed in a mixture of 3-mercapto-1-propanesulfonic acid (MPS) and methanol (1:1000) for 30 min. Finally, the Glass substrates were sequentially washed with methanol and DI water and then dried under nitrogen. The substrates could be stored in a desiccator for a short period.