Ferrofluids (FFs) can adapt their shape to a magnetic field. However, they cannot maintain their shape when the magnetic field is removed. Here, with a magneto-responsive and reconfigurable interfacial self-assembly (MRRIS) process, we show that FFs can be structured by a magnetic field and maintain their shape, like solids, after removing the magnetic field. The competing self-assembly of magnetic and nonmagnetic nanoparticles at the liquid interface endow FFs with both reconfigurability and structural stability. By manipulating the external magnetic field, we show that it is possible to “write” and “erase” the shape of the FFs remotely and repeatedly. To gain an in-depth understanding of the effect of MRRIS on the structure of FFs, we systematically study the shape variation of these liquids under both the static and dynamic magnetic fields. Our study provides a simple yet novel way of manipulating FFs and opens opportunities for the fabrication of all-liquid devices.

Aims. Our goals are to investigate the relative absorption strengths of ammonia and methane using low-resolution H-band (1.5−1.7 microns) spectra obtained in the laboratory and compared with observational spectra of late-T and Y dwarfs, and to estimate what can be expected from the wide-angle low-resolution near-infrared spectroscopic survey that will be provided by the upcoming Euclid space mission. Methods. Gas cells containing ammonia and methane at atmospheric pressure were custom-made in our chemical laboratory. Low-resolution near-infrared spectroscopy of these gas cells was collected in our optical laboratory. It is compared with simulated spectra using the high-resolution transmission molecular absorption database (HITRAN) for temperatures of 300 K and 500 K, and with near-infrared spectra of late-T dwarfs, Y dwarfs, Jupiter, and Saturn. We selected for this investigation the spectral region between 1.5 and 1.7 microns (H band) because it is covered by the Euclid red grism, it is particularly sensitive to the relative proportions of ammonia and methane opacity, and it is free from strong contributions of other abundant molecules, such as water vapor. Results. The laboratory spectra showed that the ammonia and methane features present in the simulations that used the HITRAN database are incomplete. Using our laboratory spectra, we propose a modified version of the NH3-H spectral ratio with expanded integration limits that increases the amplitude of variation in the index with respect to spectral type. Combinations of our laboratory spectra were used to find the best fits to the observed spectra with the relative absorption ratio of ammonia to methane as a free parameter. A relationship was found between the Teff and the ratio of ammonia to methane from spectral classes T5 to Y2 (1100 K–350 K), in fairly good qualitative agreement with theoretical predictions for high-gravity objects and temperatures from 1100 K to 500 K. The ammonia-to-methane ratios in late-T and Y dwarfs are similar to that of Jupiter, suggesting a similar chemical composition. Simulations of the spectroscopic performance of Euclid suggest that it will yield Teff values and ratios of ammonia to methane for over 103 late-T dwarfs in the entire wide survey.