Simultaneous Stress and Weight Measurements for Particulate Films Made from Capillary Suspensions

Cracking and particle mobility are a significant problem in the drying of films from hard sphere suspensions. One pathway to avoid these problems is by using capillary suspensions [1]. In the pendular state, the preferentially wetting secondary fluid forms capillary bridges between the particles. The capillary force from such bridges induces the formation of a sample-spanning particle network [2, 3], which limits the direction of particle motion during film drying, and the capillary force also counters the capillary force within pores generated by evaporation. Crack-free films can be produced at thicknesses much greater than the critical cracking thickness for a suspension without capillary interactions, and even persists after sintering. This method is applicable to a broad range of materials and can be easily implemented using well-established industrial methods. A simultaneous weight and stress measurement of a drying film allows direct relation of dynamic changes in stress with the corresponding drying rate. The overall stress of a film can be measured using the cantilever deflection method [4] as the film dries in a temperature and humidity controlled chamber. Capillary suspensions, with a higher shear modulus, can lower the overall stress in the film while drying. Although no visible cracks occur in any of the films, a characteristic stress increase followed by relaxation occurs. Capillary suspensions change the shape of the stress profile from a slow stress rise followed by a rapid, uniform relaxation towards a more rapid rise followed by a two-stage relaxation, where the stress reduction in the second stage is significantly slower. Moreover, the peak stress occurs at a higher film loading compared to the pure suspension, shifting from around to X = 16 % to 26 %. An increase in relative humidity has little effect on the drying behavior for each sample composition.status: publishe