Estimation of liquid viscosity using radiation forces from a potential well

Radiation forces provide a remote, non-contact method to evaluate the elastic or mechanical properties of materials. For instance, noninvasive ultrasound techniques can assess the health of tissue by measuring its modulus of elasticity. In this study, we employ radiation forces from a multi-elementarray to estimate the viscosity of a medium. The array generates an effective potential well with stabilizing radiation forces that levitate spheres in water. An ultrasound pulse induces oscillation of the sphere about its equilibrium position. The sphere’s motion is tracked using a laser diode and a photoresistor. By analyzing the exponential decay of the motion, we estimate the viscosity of the medium. Our measurements agree with known viscosity values. Moreover, we observe that the sphere’s oscillations exhibit characteristics of nonlinear spring constant under directional drag forces. This approach provides a non-contact method for measuring liquid properties in benchtop settings and demonstrates potential in vivo applications. [This work was supported by NIH K25-DK132416.]