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].