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 He-4 within microfluidic devices formed from glass wafers, in which one dimension is on the micrometer scale. Using an ultrasonic analog to Fabry-Perot interferometry, we have measured the first sound of the confined liquid He-4, which can be a probe of critical behavior near the lambda point (T lambda). All thermodynamic properties of liquid He-4 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 He-4 confined in cavities as small as similar to 5 mu m. Our experimental setup allows us to pressurize the liquid up to similar to 25 bar without causing deformation of the confined geometry, a pressure which is about four 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.