Many living organisms track light sources and halt their movement when alignment is achieved. This phenomenon, known as phototropism, occurs, for example, when plants self-orient to face the sun throughout the day. Although many artificial smart materials exhibit non-directional, nastic behaviour in response to an external stimulus, no synthetic material can intrinsically detect and accurately track the direction of the stimulus, that is, exhibit tropistic behaviour. Here we report an artificial phototropic system based on nanostructured stimuli-responsive polymers that can aim and align to the incident light direction in the three-dimensions over a broad temperature range. Such adaptive reconfiguration is realized through a built-in feedback loop rooted in the photothermal and mechanical properties of the material. This system is termed a sunflower-like biomimetic omnidirectional tracker (SunBOT). We show that an array of SunBOTs can, in principle, be used in solar vapour generation devices, as it achieves up to a 400% solar energy-harvesting enhancement over non-tropistic materials at oblique illumination angles. The principle behind our SunBOTs is universal and can be extended to many responsive materials and a broad range of stimuli.
This work investigates
the effect of wall thickness on the thermal
conductivity of mesoporous silica materials made from different precursors.
Sol–gel- and nanoparticle-based mesoporous silica films were
synthesized by evaporation-induced self-assembly using either tetraethyl
orthosilicate or premade silica nanoparticles. Since wall thickness
and pore size are correlated, a variety of polymer templates were
used to achieve pore sizes ranging from 3–23 nm for sol–gel-based
materials and 10–70 nm for nanoparticle-based materials. We
found that the type of nanoscale precursor determines how changing
the wall thickness affects the resulting thermal conductivity. The
data indicate that the thermal conductivity of sol–gel-derived
porous silica decreased with decreasing wall thickness, while for
nanoparticle-based mesoporous silica, the wall thickness had little
effect on the thermal conductivity. This work expands our understanding
of heat transfer at the nanoscale and opens opportunities for tailoring
the thermal conductivity of nanostructured materials by means other
than porosity and composition.
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