Stress relaxation measurement of viscoelastic materials using a polymer-based microfluidic device

• A microfluidic device measures stress relaxation of heterogeneous materials.
• The device is capable of detecting continuous distributed normal loads.
• Measured load-deflection relations of agars yield their Young's relaxation modulus.
• Maxwell models with two relaxation times are further obtained for the agars.
• Measured load-deflection relations of PDMS capture its heterogeneous elasticity.

This paper presents the stress relaxation measurement of viscoelastic materials using a polymer-based microfluidic device. Comprised of a single polymer rectangular microstructure and a set of electrolyte-enabled distributed resistive transducers, this device is capable of detecting continuous distributed dynamic loads. In a measurement, a rigid cylinder probe is utilized to apply a precisely controlled step displacement input to a material specimen on the device, and consequently the spatially varying stress relaxation behavior of the specimen is translated to time-dependent continuous distributed loads acting on the device, where such loads give rise to time-dependent continuous deflection of the polymer microstructure and register as discrete resistance changes at the locations of the transducers. To account for device-to-device fabrication variations, performance characterization is first conducted on a device as a control experiment, and then a specimen is placed on a characterized device for measurement. Several homogeneous and heterogeneous specimens are measured, and the associated data analysis is conducted for extracting the spatially varying Young's relaxation modulus, which is expressed as the Maxwell model with two relaxation times. The results demonstrate the feasibility of using a single polymer-based microfluidic device to map out the spatially varying stress relaxation behavior of viscoelastic materials.