Microfluidic liquid sheets as large-area targets for high repetition XFELs

Abstract

The high intensity of X-ray free electron lasers (XFELs) can damage solution-phase samples on every scale, ranging from the molecular or electronic structure of a sample to the macroscopic structure of a liquid microjet. By using a large surface area liquid sheet microjet as a sample target instead of a standard cylindrical microjet, the incident X-ray spot size can be increased such that the incident intensity falls below the damage threshold. This capability is becoming particularly important for high repetition rate XFELs, where destroying a target with each pulse would require prohibitively large volumes of sample. We present here a study of microfluidic liquid sheet dimensions as a function of liquid flow rate. Sheet lengths, widths and thickness gradients are shown for three styles of nozzles fabricated from isotropically etched glass. In-vacuum operation and sample recirculation using these nozzles is demonstrated. The effects of intense XFEL pulses on the structure of a liquid sheet are also briefly examined.

3D-printed sheet jet for stable megahertz liquid sample delivery at X-ray free-electron lasers

“…On arrival of the pulse, an elliptical vacancy formed originating at the focal point which rapidly expanded and propagated along the direction of liquid flow until eventually exiting the lower rim of the flat section. This closely resembles the behavior observed with thicker impinging sheet jets at a lower repetition rate XFEL source (Hoffman et al, 2022b). A second bubble formed on arrival of the subsequent X-ray pulse and followed a trajectory similar to the initial one without perturbing the X-ray focal spot.…”

“…Their use at XFEL sources has been far less prevalent, likely due to the high liquid and gas loads that complicate vacuum operation. Recently, Hoffman and coworkers reported on liquid sheet jet injection with a hard X-ray XFEL source where they observed sheet jet explosion on exposure to a nanofocus X-ray beam at 120 Hz (Hoffman et al, 2022b). Their approach utilized impinging jet nozzles which produced sheet thickness on the order of several micrometres and flow rates of millilitres per minute.…”

Exposed interfaces and fast mixing in XFEL-friendly liquid sheets

“…The liquid heterostructures share all of the benefits of the liquid sheet jets in that they are free-standing, self-refreshing, flat, large-area, and vacuum stable [2], with the one notable downside that the liquids are essentially "consumed" through mixing (although immiscible fluids can typically be reseparated). These features make them well suited for XFEL techniques and should enable new experiments at these facilities.…”

“…The outer channels are supplied by the same liquid pump while the inner channel is fed by a separate pump. If liquid is supplied to only the outer channels, then a micron-thick liquid sheet is formed analogous to the sheets produced by colliding round jets in air with dimensions depending on the nozzle geometry and flow rate (typically several mL/min) [2]. A liquid sheet can also be produced by flowing liquid through only the center channel (100s of μL/min) and supplying pressurized gas (~100 sccm) through the outer channels [3].…”

“…The sheets produced by these nozzles at typical flow rates are several mm long and have liquid velocities of order 10 m/s, resulting in accessible mixing times on the order of 100s of μs. The length of the sheet grows quadratically in the outer fluid flow rate while the velocity only grows linearly [2], so the greatest accessible mixing time is always obtained by maximizing the outer flow rate.…”