3.5. Solar heating performance and outdoor tests
The photothermal properties of transparent or double-sided
heterochromatic nano wrinkled fiber films were evaluated by thermal
imaging. PLLA and APLLA without CS/MWCNTs showed almost no temperature
rise (Fig. 5a-d). As shown Fig. 5e, at the initial CS/MWCNTs
concentration, the temperature of the dark side of the film continued to
increase with the increase of the sunlight radiation time, and the
higher the concentration, the higher the temperature. However, when the
concentration of CS/MWCNTs was reduced to 1/4 of the original
concentration, the photothermal response of the film was elevated, and
it could be almost instantaneously boosted to high temperatures, but
with large temperature fluctuations. Comparing the dark and light sides
of CS/MWCNTs/APLLA (Fig. 5f), it can be found that the light side has a
more rapid photothermal response and no significant enhancement with
time, but is unstable; the dark side has a slow increase in temperature
with time and better photothermal stability. Comparing the photothermal
performance of CS/MWCNTs/APLLA dark-colored surface under different
sunlight radiations (Fig. 5g), it can be found that the temperature of
the photothermal film is higher with the same time as the radiation
enhancement, and the higher the radiation intensity, the more rapid the
response. However, the temperature distinction is not obvious for
sunlight densities of 1000 kWm-2 and 1250
kWm-2.
We also evaluate the photothermal performance of the membranes on
low-temperature substrates, as shown in Fig. 4h. CS/MWCNTs/APLLA light
side (top left), dark side (top right), dark side 1/2 concentration
(bottom left), and dark side 1/4 concentration (bottom right) were
placed on cryogenic glass (-20 ˚C), and the temperature changes were
recorded under 1 sunlight radiation (1000 kWm-2). The
Max represents the maximum temperature of the 4 photothermal films, and
Sp1 represents the collimation temperature. It can be seen that the
photothermal membrane on the cryogenic substrate can also heat up
rapidly and the maximum temperature reaches nearly 55 ˚C within 4
minutes, with the darker side of the membrane with a high concentration
of CS/MWCNTs/APLLA showing the best low-temperature photothermal effect.
We also evaluate the photothermal uniformity of the films. As shown in
the Fig. 4i-k, comparing the thermal distribution of the material before
and after 5 minutes of 1 sunlight irradiation, it can be seen that the
thermal distribution of CS/MWCNTs/APLLA photothermal film is very
homogeneous and shows high in the middle and low at both ends [26].
The evaluation of wetting the photothermal film is also important since
wearable materials are often faced with situations such as sweat-soaking
[27]. As shown in Fig. S20 and Supplementary Video 1, the heat
distribution of the photothermal film before and after the drop of pure
water is recorded. Even under wetting conditions, the film can rise to
37 ˚C within 4 minutes and promote liquid evaporation for self-drying
recovery. Fig. S21 demonstrates the variation of the maximum temperature
(Max) of the area and the collimation temperature (Sp1) under the
wetting condition, and the photothermal film exhibits a fast temperature
recovery. The thermal management capability of the film is also an
important criterion. The membrane was heated in an oven at 60 ˚C for 5h,
after which it was taken out and the photothermal distribution was
recorded as shown in the Fig. S22 and 23. The photothermal film has
excellent thermal dispersion and can be cooled from 60 ˚C to room
temperature in 10 s. The distribution of the temperature along the
collinear horizontal line indicate that the film has excellent
uniformity [28]. Furthermore, we test the thermal performance of the
film in different orientations (parallel, 40 degrees and perpendicular
to the light). The orientation perpendicular to the light gives the best
performance (Fig. S24a-c). The film also shows good thermal behavior
when folded (Fig. S24d).
We conduct outdoor evaluations of CS/MWCNTs/APLLA membranes. The outdoor
test equipment was shown in Fig. S25 and the detailed outdoor test
parameters were shown in Table S3 and the test locations are shown in
Fig. 6a. At night without sunlight irradiation, the temperature of the
photo-thermal film decreases with the air temperature and is slightly
higher than the air temperature by 2 ˚C (Fig. 6b). At 7-8 am, the
photothermal membrane can reach nearly 50 ˚C with less than 0.5 sunlight
irradiation. The sharp fluctuation of the membrane temperature in two
places between 7:0-7:10 am is caused by obstacles blocking the membrane,
while the slow decrease and increase of the temperature between
7:10-7:20 am is caused by the decrease of sunlight radiation due to the
blocking of sunlight by buildings (Fig. 6c). At midday, when the
sunlight was 800 kWm-2, the temperature of the solar
thermal film was even close to 75 ˚C and remained consistently high. One
of the short temperature drops was affected by clouds covering the sun
(Fig. 6d). At sundown, both air temperature and solar irradiation
gradually decrease, the temperature of the film first rises and then
falls, and the temperature of the film can only be maintained in the
range of 38-40 ˚C when the solar irradiation falls to 100
kWm-2 and below (Fig. 6e). As the weather changes from
sunny to cloudy, solar radiation drops dramatically and the
effectiveness of the solar thermal membranes decreases. In rainy
weather, the surface of the membrane gets wet with rainwater, thus
affecting the photothermal performance (Fig. 5f-h). By comparing the
photothermal performance of the membrane under different weather
conditions, it shows the universality of the double-sided
heterochromatic photothermal membrane under varies environments
[29].