Exposed interfaces and fast mixing in XFEL-friendly liquid sheets

Abstract

Microfluidic liquid sheet jets have rapidly grown in popularity for extreme ultraviolet and soft X-ray spectroscopies as they are vacuum stable, constantly refreshing, and are easily able to reach sub-micron optical path lengths required for transmission measurements. We have recently demonstrated the generation of a new class of sheet jet comprised of two liquids (a "liquid heterostructure") by colliding two jets of one liquid onto opposite sides of third jet of another liquid. The resulting structure is a layered sheet jet where a thin sheet of one liquid is completely enveloped by a larger sheet of a separate liquid. If the component liquids are miscible, the thin component layers result in fast diffusive mixing on submillisecond time scales based on measurements using FTIR microscopy. If the component liquids are immiscible, the resulting structure contains well-defined, large-area liquid-liquid interfaces with a minimized bulk liquid background as determined from ellipsometry and FTIR microscopy measurements. The inner liquid layer in these structures was found to be as thin as tens of nanometers, comparable to the thinnest sheet jets that can be produced. These new heterostructures provide the same benefits as conventional sheet jets for XUV and SXR spectroscopy and could enable new mix-and-probe spectroscopic techniques or support developing methods such as XUV/SXR second harmonic generation for examining buried liquid interfaces.