DATE: Wednesday, December 6, 2017
TIME: 12:00 – 1:00pm
Topic: Structure of capillary suspensions and their applications
KU Leuven, Department of Chemical Engineering, 3001 Heverlee (Belgium)
Ternary liquid-liquid-solid systems exhibit a wide variety of different morphologies depending on the ratio of the three components. With small amounts of secondary fluid, the characteristic mechanical strength of such capillary suspensions arises due to the capillary force inducing a percolating network of particles bridged by small individual droplets of secondary fluid . Spatial information on the structure of such particle networks can be obtained using 3D confocal microscopy on an index-matched model system and directly correlated to changes in the mechanical strength  or mixing conditions.
We investigate microstructural properties and transitions as the particle size, contact angle, and volume ratio of secondary fluid to particles, φsec/φsolid, is changed. A transition from a granular system without added liquid, through a sparser capillary suspension network at intermediate volume ratios, to a network of dense aggregates at larger ratios is observed. For intermediate volume ratios, a change from binary bridges to more complex funicular and clustered arrangements is observed when the volume ratio is increased or when the contact angle is modified. Computational analysis of 3D confocal images provides structural parameters like the coordination number, which lies around 4 when the particle network is fully established but, due to particle agglomeration, increases when more secondary liquid is added. The fractal dimension of the network increases with the particle size. This transition may be explained by the corresponding reduction in the capillary force with increasing particle size.
These capillary suspensions can be used in several different application pathways including in the formation of porous materials, printed electronics with high conductivity, and crack-free films. The particle network can be preserved either through sintering or direct polymerization of the bridges to create materials with a high open porosity (up to 80%) with a narrow, micrometer-sized pore distribution . We can use capillary suspensions to produce stable suspensions using conductive particles that have a twofold increase in conductivity over existing formulations using polymeric stabilization . Finally, the capillary force limits the direction of particle motion during film drying and the capillary force of the bridges 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 .
- E. Koos, N. Willenbacher, Science, 2011, 331, 897.
- F. Bossler, E. Koos, Langmuir, 2016, 32(6), 1489.
- J. Dittmann, E. Koos, N. Willenbacher, J. Am. Ceram. Soc., 2013, 96(2), 391.
- M. Schneider, E. Koos, N. Willenbacher, Sci. Rep., 2016, 6, 31367.
- M. Schneider et al., ACS Appl. Mater. Interfaces, 2017, 9(12), 11095.