Chemical reactions and biological processes are often governed by the
structure and transport dynamics of the interface between two liquid phases.
Despite their importance, our microscopic understanding of liquid-liquid
interfaces has been severely hindered by difficulty in accessing the interface
through the bulk liquid. Here we demonstrate a method for generating large-area
liquid-liquid interfaces within free-flowing liquid sheets, which we call
liquid heterostructures. These sheets can be made thin enough to transmit
photons from across the spectrum, which also minimizes the amount of bulk
liquid relative to the interface and makes them ideal targets for a wide range
of spectroscopies and scattering experiments. The sheets are produced with a
microfluidic nozzle that impinges two converging jets of one liquid onto two
sides of a third jet of another liquid. The hydrodynamic forces provided by the
colliding jets both produce a multilayered laminar liquid sheet with the
central jet is flattened in the middle. Infrared microscopy, white light
reflectivity, and imaging ellipsometry measurements demonstrate that the buried
layer has a tunable thickness and displays well-defined liquid-liquid
interfaces, and that the inner layer can be thinner than 100 nm.Comment: 30 pages, 8 figures, 1 table. Supplement: 19 pages, 8 figure