Structure of capillary suspensions and their versatile applications in the creation of smart materials
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Prospective Article
Structure of capillary suspensions and their versatile applications in the creation of smart materials Katharina Hauf, Karlsruhe Institute for Technology, Institute for Mechanical Process Engineering and Mechanics, Karlsruhe, Germany Erin Koos, Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200f, Leuven 3001, Belgium; Karlsruhe Institute for Technology, Institute for Mechanical Process Engineering and Mechanics, Karlsruhe, Germany Address all correspondence to Erin Koos at [email protected] (Received 23 January 2018; accepted 21 February 2018)
Abstract In this paper, we reviewed recent research in the field of capillary suspensions and highlight a variety of applications in the field of smart materials. Capillary suspensions are liquid–liquid–solid ternary systems where only one liquid is present in a few percent and induces a strong, capillary-induced particle network. These suspensions have a large potential for exploitation, particularly in the production of porous materials since the paste itself and the properties of the final material can be adapted. We also discussed the rheological properties of the suspension and network structure to highlight the various ways these systems can be tuned.
Introduction The field of capillary suspensions is fairly young but has already gained worldwide attention.[1–7] This rapidly growing field has shown significant promise in easily creating adaptable pastes to use in a large variety of applications, e.g., heat-stable and low-calorie chocolate spreads,[8,9] battery slurries,[10,11] highly conductive pastes for solar cells,[12,13] stable slurries for biofuels,[14] and strong highly porous ceramics.[15–17] These different applications illustrate the manifold applicability of capillary suspensions due to the easy way capillary suspensions can be adapted to each desired product. It is important to understand the physical phenomenon of capillary forces to understand how the suspensions can be tuned appropriately to create tailor-made materials. Different particulate networks can be realized when creating capillary suspensions. These differences primarily originate from the wettability of the secondary fluid against the particles, quantified using the three-phase contact angle θ. In the first investigation by Koos and Willenbacher,[1] two states were defined. For θ < 90°, the secondary fluid preferentially wets the particles and the system is arranged in the pendular state. The particles are directly connected to each other through pendular bridges, a sample-spanning network is formed. The particulate system is in the capillary state when the contact angle θ > 90°. Here, clusters of particles surrounding secondary fluid droplets are formed.[18] A sample-spanning network is formed in this system through the connections between these polytetrahedral or higher order clusters. These different network structures were first imaged by Bossler and Koos,[19] and are shown in Fig. 1 together with a
schematic illustration of the two states. They used a system co
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