This paper demonstrates the dynamic stressing of viscous liquids in microfluidic channels. An infrared laser pulse is focused within the testing liquid in a microfluidic channel and a spherical shock wave near an air-liquid interface is created. The shock is reflected as a tension wave by the free surface due to the acoustic impedance mismatch. The displacement of the free surface within hundred nanoseconds is captured by a developed double-exposure optical system. The tensile strength can be estimated by extrapolating a series of shock pressure approximation at several different distances between the bubble and the free surface to the measurement accuracy. This study has a great potential in the optical breakdown of biomaterials.