Influence of particle shape on the rheological behavior of three-phase non-brownian suspensions

Robust Soil Water Potential Sensor to Optimize Irrigation in Agriculture

Extreme weather phenomena are on the rise due to ongoing climate change. Therefore, the need for irrigation in agriculture will increase, although it is already the largest consumer of water, a valuable resource. Soil moisture sensors can help to use water efficiently and economically. For this reason, we have recently presented a novel soil moisture sensor with a high sensitivity and broad measuring range. This device does not measure the moisture in the soil but the water available to plants, i.e., the soil water potential (SWP). The sensor consists of two highly porous (>69%) ceramic discs with a broad pore size distribution (0.5 to 200 μm) and a new circuit board system using a transmission line within a time-domain transmission (TDT) circuit. This detects the change in the dielectric response of the ceramic discs with changing water uptake. To prove the concept, a large number of field tests were carried out and comparisons were made with commercial soil water potential sensors. The experiments confirm that the sensor signal is correlated to the soil water potential irrespective of soil composition and is thus suitable for the optimization of irrigation systems.

Giant Functional Properties in Porous Electroceramics through Additive Manufacturing of Capillary Suspensions

Dedicated hierarchical structuring of functional ceramics can be used to shift the limits of functionality. This work presents the manufacturing of highly open porous, hierarchically structured barium titanate ceramics with 3-3 connectivity via direct ink writing of capillary suspension-type inks. The pore size of the printed struts (∼1 μm) is combined with a printed mesostructure (∼100 μm). The self-organized particle network, driven by strong capillary forces in the ternary solid/fluid/fluid ink, results in a high strut porosity, and the distinct flow properties of the ink allow for printing high strut size to pore size ratios, resulting in total porosities >60%. These unique and highly porous additive manufactured log-pile structures with closed bottom and top layers enable tailored dielectric and electromechanical coupling, resulting in an energy harvesting figure of merit FOM₃₃ more than four times higher than any documented data for barium titanate. This clearly demonstrates that combining additive manufacturing of capillary suspensions in combination with appropriate sintering allows for creation of complex architected 3D structures with unprecedented properties. This opens up opportunities in a broad variety of applications, including electromechanical energy harvesting, electrode materials for batteries or fuel cells, thermoelectrics, or bone tissue engineering with piezoelectrically stimulated cell growth.

Nonequilibrium Structure Diagram of Pendular Suspensions under Large-Amplitude Oscillatory Shear

For pendular suspensions with particles in contact with immiscible secondary liquid bridges, the shear field significantly influences particle aggregates and networks. In this work, we study the structure of the pendular network and how the structure changes under large-amplitude-oscillatory shear. Using rheology and optical microscopy, we found unique network destruction followed by reconstruction with increasing strain. Two processes show different shear-field dependencies, strain-rate dependency for destruction and strain dependency for reconstruction. A nonequilibrium state diagram is constructed to show the phase behavior, where the critical particle concentration of sol-gel transition is dependent on the shear history and may depend on shear strain nonmonotonically. Two different mechanisms, shear-induced network breakdown at low strain and shear-induced agglomeration at high strain, are suggested to describe the nonmonotonic critical concentration under the upward strain sweep quantitatively.

Ultrastretchable Conductive Elastomers with a Low Percolation Threshold for Printed Soft Electronics

Stretchable conductors are required for next-generation soft electronics. Achieving both high electrical conductivity and high stretchability in conductors composed of elastomers and conductive fillers, however, is challenging. Here, a generic, versatile strategy is reported for producing ultrastretchable conductors exhibiting both superior electrical conductivity (>10³ S/cm) and stretchability (>1600%). This is achieved by adding small amounts of immiscible secondary fluid into silver (Ag)-filled inks. Capillary forces in these ternary systems induce the self-assembly of conductive particle networks at a low percolation threshold (6-7 vol %), cutting silver consumption by more than 2/3 compared to conventional conductive elastomers. Ag-filled polydimethylsiloxane exhibits superior cyclic durability sustaining 100% tensile strain for 1000 cycles with only a minor loss of conductivity. Ag-filled thermoplastic polyurethane displays unprecedented reversibility with nonretarded switching from conductive to nonconductive states during repeated stretching up to 200% strain. Patterned strain sensors and conductive wirings were 3D-printed to demonstrate the technical feasibility.

A New Solid-like State for Liquid/Liquid/Particle Mixtures with Bicontinuous Morphology of Concentrated Emulsion and Concentrated Suspension

Research in exploring the microstructures of the ternary liquid/liquid/particle mixture is still a challenging task due to the complex interface properties and compositions of each phase. In this work, we report a new kind of solid-like state for ternary mixtures after the addition of a surfactant, which has the bicontinuous morphology of two phases, that is, the concentrated emulsion and the concentrated noncolloidal suspension. The bicontinuous morphology was justified by optical microscopy and the unique two-step yielding behavior under large oscillatory shear flow, which has the yielding character of a noncolloidal suspension at smaller strain and that of a concentrated emulsion at larger strain. A phase diagram is constructed from the rheological measurements and morphological observations. The boundaries of the new solid-like state can be well predicted from three basic requirements on the glass forming or jamming conditions in the aqueous noncolloidal suspension phase, the aqueous emulsion phase, and the whole ternary mixture.

Structure of capillary suspensions and their versatile applications in the creation of smart materials

In this article, we review 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 one liquid is only 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 discuss the rheological properties of the suspension and network structure to highlight the various ways these systems can be tuned.

Fractal approaches to characterize the structure of capillary suspensions using rheology and confocal microscopy

The rheological properties of a particle suspension can be substantially altered by adding a small amount of a secondary fluid that is immiscible with the bulk phase. The drastic change in the strength of these capillary suspensions arises due to the capillary forces, induced by the added liquid, leading to a percolating particle network. Using rheological scaling models, fractal dimensions are deduced from the yield stress and from oscillatory strain amplitude sweep data as function of the solid volume fraction. Exponents obtained using aluminum-oxide-based capillary suspensions, with a preferentially wetting secondary fluid, indicate an increase in the particle gel's fractal dimension with increasing particle size. This may be explained by a corresponding relative reduction in the capillary force compared to other forces. Confocal images using a glass model system show the microstructure to consist of compact particle flocs interconnected by a sparse backbone. Thus, using the rheological models two different fractal dimensionalities are distinguished - a lower network backbone dimension (D = 1.86-2.05) and an intrafloc dimension (D = 2.57-2.74). The latter is higher due to the higher local solid volume fraction inside of the flocs compared to the sparse backbone. Both of these dimensions are compared with values obtained by analysis of spatial particle positions from 3D confocal microscopy images, where dimensions between 2.43 and 2.63 are computed, lying between the two dimension ranges obtained from rheology. The fractal dimensions determined via this method corroborate the increase in structural compactness with increasing particle size.

Influence of mixing conditions on the rheological properties and structure of capillary suspensions

The rheological properties of a suspension can be dramatically altered by adding a small amount of a secondary fluid that is immiscible with the bulk liquid. These capillary suspensions exist either in the pendular state where the secondary fluid preferentially wets the particles or the capillary state where the bulk fluid is preferentially wetting. The yield stress, as well as storage and loss moduli, depends on the size and distribution of secondary phase droplets created during sample preparation. Enhanced droplet breakup leads to stronger sample structures. In capillary state systems, this can be achieved by increasing the mixing speed and time of turbulent mixing using a dissolver stirrer. In the pendular state, increased mixing speed also leads to better droplet breakup, but spherical agglomeration is favored at longer times decreasing the yield stress. Additional mixing with a ball mill is shown to be beneficial to sample strength. The influence of viscosity variance between the bulk and second fluid on the droplet breakup is excluded by performing experiments with viscosity-matched fluids. These experiments show that the capillary state competes with the formation of Pickering emulsion droplets and is often more difficult to achieve than the pendular state.