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We develop a method for converting the unfathomed ''invisible'' behavior of the traditional batch hydrothermal process to a visual image. By utilizing a novel gelation system, the ''black box'' of hydrothermal synthesis is opened. The poloidal annular structure of the gel reveals the existence of annular natural convection in the batch hydrothermal solution. Temperature gradient is the dominating driving force. Larger reactors would strengthen the flow behavior, resulting in a non-negligible effect on the hydrothermal products, especially gels.

Solar steam generation is an emerging research field to approach the challenge of water shortages all around the world. Abundant light absorber materials have been developed to improve the broadband absorption of sunlight in the past few years, but the thermal insulating layers that often make a significant improvement in the photothermal conversion performance of the device have rarely been researched. A material that has both excellent heat insulation property and high light absorption is also uncommon. Bioinspired by the hollow hair of polar bears, here we demonstrate difunctional carbon-nanotube-based aerogels (DCTAs), which not only can be used as the thermal insulating layer that helps to resist heat loss from the surface to bulk water but also serve as the absorber layer. The hollow carbon nanofiber network structure can effectively resist thermal transport so that the carbon-nanotube-based aerogel shows great thermal insulating properties. Besides, the aerogel can self-float on the surface of water because of its lightweight and hydrophobic nature. By further compositing with Au nanoparticles, the broadband absorption of aerogel is up to 98% in the wavelength range of 250−2000 nm. This rational design endows the DCTAs with a good solar thermal conversion performance.