Imagine a lightweight durable floating sponge for use at an ocean oil spill that attracts only oil, expands to hold nearly 200 times its weight and 800 times the volume of the stuff, moves automatically towards higher concentrations of the oil and can be squeezed clean and reused dozens of times.
The stuff of high-tech dreams, right? Well, it exists right now, at least in the labs of Chinese researchers, and I would think certain oil drilling companies and government agencies would be extremely anxious to be testing out this stuff ASAP.
This sponge – composed of randomly oriented carbon nanotubes – was discovered by a team from the Key Laboratory for Advanced Materials Processing Technology (Department of Mechanical Engineering) at Tsinghua University and the Department of Advanced Materials and Nanotechnology at Peking University. A paper on their work was published in February in Advanced Materials (note: the editors have waived the fee and are generously providing this research paper at no cost).
The gist of their research is that, unlike most CNT projects where alignment of the nanotubes is desired (and usually difficult to achieve), these sponges actually work precisely because the nanotubes are not aligned. The “sponginess” apparently results from the random orientation that allows the tubes to move but not slide along a single direction:
Shape and structural recovery of the sponges stems from the random distribution of CNTs that prevents the formation of strong van der Waals interactions even at densiﬁed state, therefore liquid re-absorption into the pores could push CNTs away and back to their original conﬁguration.
So, the Chinese group create a porous, sponge-like bulk material of self-assembled CNTs. So porous, in fact, that the CNT sponge has a density (or lack thereof) that rivals aerogels.
But, whereas some aerogels are brittle, the CNT sponge is flexible. In its original state, it also has the property of being very wettable to organic chemicals:
The sponges in densiﬁed state swell instantaneously upon contact with organic solvents. They absorb a wide range of solvents and oils with excellent selectivity, recyclability and absorption capacities up to 180 times their own weight, two orders of magnitude higher than activated carbon. A small densiﬁed pellet ﬂoating on water surface can quickly remove a spreading oil ﬁlm with an area up to 800 times that of the sponge, suggesting potential environmental applications such as water remediation and large-area spill cleanup. In comparison, the application of one of the lightest porous materials, silica aerogel, has been impeded by their structural fragility, environmental sensitivity and high production cost.
One nice thing about these sponges, apparently, is that they are rugged and can be reused repeatedly:
The sponges can sustain large-strain deformations, recover most of the material volume elastically, and resist structural fatigue under cyclic stress conditions, in both air and liquids.
[ . . . ]
The sponges show no strength degradation after compression at a set strain of 60% for 1000 cycles.
How do these CNT sponges’ absorption capacity compare to other materials?
We tested many porous materials with different pore sizes and densities including natural ﬁbrous products (e.g., cotton towel, loofah), polymeric sponges (e.g., polyurethane- or polyester-based) and pellets of activated carbon with a density of 2000 mg cm-3. In case of diesel oil, the absorption capacity of CNT sponges (Q<143) is several times that of polymeric sponges (Q<40), 35 times that of cotton and loofah (Q<4) and two orders of magnitude higher than activated carbon (Q<1).
Besides continuously floating on top of water, the performance of the CNT sponges is also excellent in other ways that almost seem to be too good to be true:
In addition, a piece of pristine sponge can continuously attract and suck most part of an oil ﬁlm when it was placed to contact the edge of the ﬁlm. Signiﬁcantly, a small particle of densiﬁed CNT sponge (with a diameter of 6 mm and a volume of 0.1 cm3) can remove a spreading diesel oil ﬁlm with an area of 227 cm2 in several minutes. We observed that the sponge was ﬂoating on water surface and moving freely throughout the oil area. Wherever it arrived, the sponge instantaneously sucked the part of oil ﬁlm in contact, resulting in a local white-color region around and behind where fresh water exposed. The sponge tends to drift to the remaining oil ﬁlm area due to its water-repelling and oil-wetting properties, leading to this unique “ﬂoating-and-cleaning” capability that is particularly useful for spill cleanup. . . The oil area that has been completely cleaned is about 800 times larger than the size of the initial densiﬁed sponge.
Adrian Miller at the Materials View blog has some great additional information about how these Chinese researchers made their discovery and other applications they have in mind. Miller’s post also contains a brief video that illustrates the ability of such a CNT sponge to return to its original shape after repeated compression cycles.