## 2 Materials and Methods
The temperature control system was not enabled because vibrations from the circulating bath significantly increased the jitter of the drop positions, but the gas stream was still used because it reduced the effect of air drafts in the room on the droplet positions. Thus, the temperature of the whole setup stabilized passively to the temperature of the experimental station and was between 26.1 and 26.25 \({}^{\circ}\)C. The temperature was measured with two K-type thermocouples, one placed in contact with the nozzle body, and the other  
Figure 1: **(a)** Experimental design. X-ray laser pulses focused to ~1 \(\upmu\)m passed through the center of ~30 \(\upmu\)m or ~60 \(\upmu\)m water microdrops, and launched shock waves. The drops were imaged optically after the X-ray free-electron laser (XFEL) pulse. The photo shows the shock wave in a 30.5-\(\upmu\)m diameter drop, 2.29 ns after the arrival of a 41.5 \(\upmu\)J XFEL pulse. To characterize the shock waves, the surface velocity of the drop was measured using multiple droplet images over a range of imaging delays. **(b)** Experimental setup. In addition to the main imaging system with a 50\(\times\) objective, a lower resolution system was used for droplet alignment. The pulse energy was measured after the droplets for each pulse. in free space, ~1 mm away from the nozzle orifice; the thermocouples reported temperatures within 0.1 \({}^{\circ}\)C of each other. The experimental setup and the data collection procedure are similar to the ones used to investigate cavitation and spallation induced in water by X-ray laser pulses [17], and we refer the readers to the previous work for general information on the experiment. Here, we will focus on the upgrades made to that setup and to the data collection and analysis. _Modifications made to the setup to improve the quality of data._ The experiments were conducted at atmospheric pressure instead of in a vacuum environment. For the purpose of increasing the accuracy of measurements, operation at atmospheric pressure had multiple advantages: (i) The droplets can be aligned better to the beam since vacuum pump vibrations are eliminated; (ii) the droplet temperature is nearly constant as evaporation, which cools the drops, is much slower at atmospheric pressure than in vacuum; and (iii) it allows better and faster setup and tuning of the optical imaging and of the drop injection, and reduces the duration of the adjustments needed during the beamtime. We note that operation at atmospheric pressure reduces the XFEL intensity through absorption in air, and that shocks from the drops will be partially transmitted into surrounding gas. Nevertheless, these processes did not significantly affect our measurements because the transmission of ~10 keV X-rays through 0.215 m of air, 88%, was sufficiently large to perform the measurements. Also, the shocks transmitted into the nitrogen gas stream can be neglected, because the ratio of acoustic impedances of liquid water and gaseous nitrogen is ~3000, therefore the intensity of the shock transmitted into the gas is approximately three orders of magnitude smaller than intensity of the incoming shock [34]. Since the magnitude of shocks depends directly on the XFEL pulse energy, the experiment was designed to accurately measure the pulse energy at the sample, and multiple measurements of the pulse energy were made for each shot. The primary pulse energy measurements were relative, provided by the two photodiodes (Hamamatsu S3590-09) measuring scattering from the Kapton foil placed after the droplets, as shown in Figure 1b. To calibrate the photodiode readings in absolute energy values, a scaling factor was calculated using an absolute energy detector [35] located upstream of the KB focusing mirrors. The absolute detector was apertured to respond only to photons that could be focused by the KB mirrors, and thus provided a measurement proportional to the pulse energy at the sample.