Oriented composition fluctuation domains of a two-dimensionally confined critical Ising fluid

A quasi-binary two-dimensional Ising critical system with the main components D2O and butyric acid confined by surfactant layers has been studied. The surfactant forms large planar layers and is the basis of the charge density waves with wave fronts aligned with the layers. To orient the domains in an external magnetic field, thulium ions were added to the system (replacing sodium in the surfactant with thulium and adding more TmCl3). The critical behavior of the forward scattering and the correlation length were observed to be more mean-field-like. This can be explained by the presence of the trivalent thulium ions mediating between water and butyric acid. The high-Q scattering could be distinguished in the different directions and the ideal two-dimensional critical composition fluctuation exponent ηxy = 1/4 was observed, while the other exponent ηz = -0.08 ± 0.06 was slightly negative due to a finite acceptance angle and the finite magnetic field. The orientationally averaged high-Q exponent x of this study is well-explained by ηxy = 1/4 of the two-dimensional Ising behavior and ηz = 0.