Ultrasonic Interferometer for First-Sound Measurements of Confined Liquid 4He

We present a new technique for probing the properties of quantum fluids in restricted geometries. We have confined liquid 4He within microfluidic devices formed from glass wafers, in which one dimension is on the micro scale. Using an ultrasonic analog to Fabry-Perot interferometry, we have measured the first-sound of the confined liquid 4He, which can be a probe of critical behavior near the lambda point ($T_{\lambda}$). All thermodynamic properties of liquid $^4$He can be derived from first-sound and heat capacity measurements, and although quite a bit of experimental work has been done on the latter, no measurement of first-sound has been reported for a precisely confined geometry smaller than a few tens of micrometers. In this work, we report measurements of isobaric first sound in liquid $^4$He confined in cavities as small as ~ 5 ${\mu}$m. Our experimental set-up allows us to pressurize the liquid up to ~ 25 bar without causing deformation of the confined geometry, a pressure which is about 4 times larger than previously reported with similar microfluidic devices. Our preliminary results indicate that one can possibly observe finite-size effects and verify scaling laws, by using similar devices with smaller confinement.