LOCAL MEASUREMENTS OF VISCOELASTIC PROPERTIES OF FLUIDS USING NEAR-FIELD ULTRASONICS

It is well known that the damping effect observed in non-contact vibra ting AFM is due mainly to the viscoelasticity of the fluid clenched be tween the tip and the object. Similar effects can be reproduced in nea r-field ultrasonic microscopy and can be used to monitor the tip-sampl e distance or to characterize the properties of the fluid. Two techniq ues are compared. The AFM technique uses a tip coupled to a piezoelect ric transducer whereas the second technique uses a solid horn driven d irectly by a transducer. The sizes of the vibrating tips vary from mil limetric to submicrometric scale. The approach of these two new techni ques has been validated and leads to cross results, the first one in t erms of equivalent acoustic impedances, the second in terms of spring- dashspot constants. Both concepts can be useful for characterization.