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Femtosecond laser fabrication of SWSA surfaces
A description of the experimental setup for fabricating the SWSA surface is provided in Supplementary Fig. 1a. In particular, a sheet of Al foil of 200 µm thickness and 22 mm × 40 mm dimensions, mounted on an xy translational stage, was scanned normal to the femtosecond laser beam (Ti: Sapphire, Spittfire, Spectra Physics) operating at a wavelength of 800 nm, 780 µJ per pulse energy and 1 kHz repletion rate. In a typical experimental procedure, the laser beam was focused onto the target surface with a focal spot size of ∼100 µm using a planoconvex lens with a focal length of 250 mm and scanned in a spiral manner. The scan speed was optimized for 0.5 mm s−1 and interline spacing was 100 µm to generate an SWSA surface with maximum optical absorbance.
Optical absorbance measurements in the ultraviolet–visible–near-inrared and mid-infrared regions
The hemispherical optical reflectance of the SWSA surface was measured in the spectral range of 0.25–2.5 µm using a PerkinElmer Lambda-900 double-beam spectrophotometer coupled with a 50-mm-diameter integrating sphere. Similarly, hemispherical reflectance in the mid-infrared region (2.5–25 µm) was measured using a Thermo Fisher Scientific Nicolet 6700 FTIR spectrometer coupled with a PIKE research integrating sphere. An accessory (B0137314) was used with the PerkinElmer 900 spectrophotometer to measure specular reflectance in the spectral range of 0.25–2.5 µm for incident angles ranging from 15° to 75° to demonstrate omnidirectional or Lambertian absorptive surfaces. In a similar manner, an accessory (PIKE VeeMAX III) coupled with the Thermo Scientific Nicolet 6700 FTIR spectrometer was used to measure specular reflection in the spectral range of 2.5–25 µm for incident angles ranging from 30° to 80°. As the SWSA sample is opaque, hemispherical/specular absorbance is complimentary to measured scattering/reflectance in ultraviolet–visible–near-infrared and mid-IR range; therefore, absorbance is obtained using A = 1 − R.
Surface topography and surface morphology measurements
The surface topography and depth profile of hierarchical microstructures on the surface of the SWSA sheets was measured using a 3D scanning laser microscope (Keyence VK 9710-K) with an elevation resolution of 0.2 µm. The surface morphology of the SWSA surface was measured using a SEM/FIB Zeiss-Auriga scanning electron microscope.
Wetting dynamics measurements
Two types of water-wetting-dynamics measurements were performed. In the first measurement, the SWSA sample was mounted onto a vertical platform (Supplementary Fig. 6) and 200 µl water was placed at the lower end of the SWSA sample. The video of water-wetting dynamics was recorded with a high-speed camera at 200 frames per s (Supplementary Videos 1 and 2). The video, shown in Supplementary Videos 1 and 2, speed was slowed down by 10× with video processing software and snapshots were captured at different moments to generate Fig. 2d and Extended Data Fig. 2a. In another set of measurements, the SWSA sample was mounted on a micrometre-control translational stage with motion in the vertical direction (Supplementary Fig. 7a). The lower end of the SWSA surface was allowed to touch the water surface placed on the computerized weighing balance. When the lower end of the SWSA surface touched the water surface, a sudden decrease in the mass of water was observed. The decrease in the water mass gives the water-wetting rate on a vertically mounted SWSA surface and, ultimately, the rate of water-mass uplifting.
Calibrating the solar simulator and power meter and designing the sample plane
The solar simulator (Sanyu) with an AM1.5G airmass filter was first calibrated for 1 Sun (1,000 W m−2) using a NREL-certified PV reference solar cell (PV Measurements). An output of a thermopile power meter (FieldMax II TO, Coherent), set at a wavelength of 500 nm, corresponding to 1,000 W m−2 from the calibrated solar simulator was used as a unit of 1 optical concentration. For example, thermopile power meter results for 283 mw for 1,000 W m−2 of incident flux from the solar simulator. The head of the pyroelectric power meter was circular in shape with a diameter of 19 mm (area, 2.83 cm2). A planoconvex lens (focal length, 300 mm; diameter, 150 mm) was mounted at the output port of the solar simulator to concentrate a 10 cm × 10 cm square beam of the solar simulator into a 4 cm × 4 cm square beam in the horizontal plane. The most uniform area of 2.5 cm × 2.5 cm (the sample size that was exposed with light is 2.0 cm × 2.0 cm) at the centre of the 4 cm × 4 cm beam was used for the measurement. To vary the optical concentration in the xy plane, we varied the current in the xenon lamp and waited for 20–30 min every time the current was changed for the solar simulator to stabilize before measurement. A time of 5–10 min was given to the thermopile head to obtain stabilized readings. For a given current passing through the xenon lamp, power was measured at the 2.5 cm × 2.5 cm central region of the 4 cm × 4 cm beam with an average time of 20 s. The detector was then moved 6 mm in the x direction to measure the time-averaged power at the next spot. Using this method, we measured the power at four locations in the xy plane (Supplementary Fig. 17) to estimate the error in the optical concentration (Fig. 3e). The current in the solar simulator was varied to adjust solar irradiance from 1,000 W m−2 (283 mW at thermopile head) to 5,000 W m−2 (1,415 mW). The solar simulator was turned on for 20–30 min to obtain a stabilized output, and times of 5–10 min were given to the thermopile head to obtain stabilized readings.
Temperature measurement of the SWSA surface
The SWSA sample was mounted onto the surface of polystyrene foam in the horizontal plane. Two thermocouples (TC1 and TC2) were mounted onto the front and back surface of the SWSA sheet or unprocessed Al sheet (Supplementary Fig. 17). The surface of the SWSA samples was irradiated with light from a vertically downward beam of the solar simulator. The outputs of the thermocouple were fed to the computer through an electronic data logger (TC08, Omega Engineering) and stored in the computer for further processing. The temperatures of the front and back surfaces of the SWSA and unprocessed Al sheets were measured for different optical concentrations (Extended Data Fig. 3).
Indoor water-evaporation measurements in the vertical and horizontal planes
Water-evaporation measurement in the horizontal plane
The SWSA sample of 20 mm × 36 mm was bent into a U shape to generate a square 20 mm × 20 mm horizontal light absorber and evaporator surface (working area) and two parallel surfaces, each of 8 mm × 20 mm, (auxiliary surfaces) to transport water on the absorber surface. The U-shaped sample was mounted onto a low-cost polystyrene thermal insulating foam (which is generally used in packaging; thickness, 7 mm), and cut into a circular shape to fit into the opening of a glass or plastic water container (Extended Data Fig. 4). The diameter of the insulating foam that was used for mounting was adjusted in such a way that it floated on the water surface. The water container along with the U-shaped SWSA sample mounted onto the insulating foam was placed onto a computerized electronic weighing balance (Radwag SMB-60/AS 60/220.R2) to measure the mass of the water with time with a sampling rate of 1 data point per second. A square aperture of 20 mm × 20 mm, which was cut into a piece of black thick cardboard and which was carefully aligned with the absorber surface, was used to prevent additional light falling onto the non-sample area to avoid additional solar-thermal heating (Extended Data Fig. 4e–g). First, the water vaporization mass in the dark, with and without the SWSA surface, under the same opening was measured for 1 h as a reference for self-evaporation. Later, the loss in water mass, with and without the SWSA surface, was recorded for different optical concentrations (COpt = 1–5). Water was changed after each measurement to delete any thermal storage history in the water.
Two K-type thermocouples, of which the first was mounted onto the SWSA surface and the second was installed just below the insulating foam, were used to measure the absorber surface and water temperature using an electronic data logger (TC08, Omega Engineering). Thermal images of the absorber surface and bulk water were recorded using an infrared camera (FLIR TG167; Extended Data Fig. 4h–k).
Water-evaporation measurements in the vertical plane
The SWSA sample (area, 20 mm × 28 mm) was used for water-evaporation measurements in the vertical plane. In this case, the surface area of 20 mm × 20 mm was used as a solar-thermal vapour generator (working area), while the remaining 8 mm × 20 mm area was used as an auxiliary surface to transport water to the absorber surface. The SWSA sample, which was vertically mounted onto the surface of polystyrene foam (Supplementary Fig. 20, Extended Data Fig. 4), was floated on the water surface. The complete system was placed onto a computerized balance to measure the loss in water mass for a different angle of mounting. The SWSA sample plane was bent at different angles (0°, 30°, 45° and 60°) from vertical to measure effects of angle of light incidence (light flux) on the evaporation rate. We first measured the dark-condition water-evaporation rate for each angle for 50 min, and then irradiated the absorber surface with COpt = 1 Sun to measure the loss in water mass under solar irradiation. The dark-condition evaporation rate was subtracted from the corresponding evaporation rate under light illumination. The average evaporation rate was measured by linear fitting of five different segments of 10 min.
Outdoor water-evaporation measurements
Two SWSA samples, each with an area of 20 mm × 30 mm, were bent at a height of 10 mm to obtain a 20 mm × 20 mm working area and a 10 mm × 20 mm auxiliary area for the water transport. The first sample was bent at a right angle to generate a flat absorber, whereas the second sample was bent at 65° from vertical to generate an absorber plane to directly face the Sun at a zenith angle of 25°. Both of these SWSA samples were installed onto the surface of polystyrene foam and allowed to float on the surface of the water. To compare the rate of water evaporation from flat and tilted samples (30 June 2018), the mass of water in each container was measured at intervals of 30 min for 8 h. For the next three consecutive days (1 July 2018–3 July 2018), the water container with the Sun-faced sample was placed onto a computerized balance to measure the water loss for 10 h, 12 h and 7 h (Extended Data Fig. 9) on 1 July, 2 July and 3 July, respectively. The temperature of the absorber surface was measured using a thermocouple and the corresponding solar irradiance was measured using a Apogee 420 pyranometer.
Solar-based water sanitation and water quality testing
Preparation of contaminated water
Contaminated water samples were prepared by dissolving a known amount of impurities into double-distilled water. To simulate 500 ppm of heavy-metal-contaminated water (Cd, Cr, Pb, Ni), 20 mg salt (Cd(NO3)2, Cr2O3, PbCl2 or NiCl2) of the corresponding heavy metal was dissolved into 40 ml of double-distilled water. Similarly, standard salt solutions of 104 ppm (saline water) were simulated by dissolving 400 mg of corresponding salts (NaCl, KCl, MgSO4 and CaCl2) into 40 ml of double-distilled water. Ethylene glycol and dyes are industrial pollutants used as coolants and colouring, respectively. A 10 ml solution (11.1 g) of ethylene glycol was ultrasonically dissolved into 40 ml of distilled water to prepare 2.77 × 105 ppm aqueous solution of ethylene glycol. Similarly, 4.33 mg of R6G dye was dissolved into 40 ml of double-distilled water to obtain a 108.25 ppm dye solution. Detergent and glycerin are two common domestic pollutants. Sodium dodecyl sulfate (SDS; C12H25SO4Na) is a surfactant that is generally used in detergents, dishwashing liquids, toothpaste and all types of soaps. The 10 mM solution of SDS was prepared by dissolving 144.186 mg of SDS in 50 ml of double-distilled water, which is the equivalent of 2.88 × 103 ppm detergent solution. Urea is an agricultural pollutant and the main component of human and animal excretory product. A urea solution of 800 ppm was prepared by dissolving 32 mg of urea (NH2CONH2) into 40 ml of double-distilled water.
Solar-based water-sanitation setup
The SWSA device, which consisted of the SWSA sample mounted onto polystyrene foam, floated on the water surface with the absorber surface in the horizontal plane. The water-wet absorber surface of the device was irradiated with normally incident light from a spectrally calibrated solar simulator to generate vapour. The water vapour condensates at the walls of a transparent and cleaned container were collected as pure water. In detail, the SWSA sample bent into a U shape with a working area of 30 mm × 30 mm and two parallel water transport surfaces of 30 mm × 8 mm was mounted onto the surface of polystyrene insulating foam (similar to that shown in Supplementary Fig. 21a,c). The system could float on the surface of contaminated water in a glass container (first container). The first container, with contaminated water and the absorber, was placed into another precleaned glass container (second container) with a transparent glass lid with ∼95% optical transparency. The outer surface of the first container and the inner surface of the second container was cleaned several times with double-distilled water to avoid pre-existing contaminants in the region between two containers in which pure water was collected. The complete system (Supplementary Fig. 21c) was placed below the solar simulator with COpt = 2. Water evaporated, condensed on the top and interior walls of the second container and collected in the region between two containers. Each contaminated water sample was evaporated for 2 h to obtain about 5–7 ml of purified water.
Water testing
Pure water samples, which were obtained from the solar sanitation of simulated contaminated water such as heavy metals and light metals (salts), were tested by Culligan Water—a NELAP-accredited water testing laboratory—using inductively coupled plasma mass spectroscopy. EPA standard method 200.8 R5.4 was used to measure the concentrations of heavy metal in the purified water, whereas EPA 200.7 R4.4 was used to measure the concentrations of Ca, Mg, K and Na.
Ultraviolet–visible optical absorption spectroscopy was used to measure the concentration of dye and urea in purified water using Beer–Lambert law. The standard solutions were first prepared by dissolving a known amount of solvents followed by its dilution. For example, 13.5 ppm to 0.005 ppm solutions of R6G were prepared to draw a calibration line. The concentration of dye/urea in the purified water was measured on the basis of the absorbance value at a given wavelength (Supplementary Fig. 24).
The concentration of ethylene glycol, glycerol and detergent in purified water was measured using a physical method of contact-angle measurements36. The standard solutions of each impurity in double-distilled water were prepared to draw a line of calibration. The contact angle for each of the standard solutions was measured by putting 200 µm droplets on a superhydrophobic surface. A calibration curve (contact angle versus concentration) was drawn for each impurity to obtain a concentration of impurity in the purified water.
A sample of dirty water, collected from a nearby pond, was solar sanitized using the SWSA surface (Supplementary Fig. 25a,b for dirty water and purified water, respectively). The water samples before and after solar sanitation were tested for the presence of bacteria using the commercially available Bacteria in Water Test Kit (PRO-LAB, BA110). The water sample (1 ml) from dirty or sanitized water sample was mixed thoroughly with the bacterial growth medium provided in the kit. The bacterial growth medium with the dirty or purified water sample was transferred separately into different sterilized Petri dishes and left for 48 h for bacterial growth. The bacterial density from solar-sanitized dirty water and the control sample was calculated using the colony-counting method32. The Petri dish corresponding to dirty water sample had more than 500 colonies (Supplementary Fig. 25c), whereas no colony was visible in the other Petri dish (Supplementary Fig. 25d) in which sanitized water was used. More

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