Compressibility of nano inclusions in complex fluids by ultrasound velocity measurements.

We present a high precision ultrasonic velocimeter for a small volume sample (1 cm3) for a path length of 1 cm achieved. The method used is based on the time of flight measurement with an original signal processing technique: the barycenter method. With our system, we have measured the sound velocity with an accuracy of 10(-5). The detection of a difference in velocity between two liquids of about 2 cm/s is achieved. The compressibility of the reference liquid can then be deduced with an accuracy better than 0.2%. Using this custom-made system, we have studied and characterized complex fluids, systems biomimetic of biological membranes, as well as proteins included in nanometric water droplets. Under these experimental conditions, we have reached the value of protein compressibilities with an accuracy better than 10%.