Measurement protocol proposal for the rheological characterization of volcanic sediment suspensions

The study of the rheological behavior of sediment suspensions is one of the most important tools for understanding the fundamental physical processes that occur between the interaction of particles and the homogeneous fluid. As lahars are natural flows that occur along the slopes of volcanoes and consist of large blocks of sediment supported by a matrix of fine sediment suspended in water, their behavior can be studied from their rheological characterization. Most of the stresses are distributed mainly within the fine matrix, due to its abundance in the flow and its capacity to support the large blocks.

This work proposes an appropriate measurement protocol for the rheological characterization of fine sediment suspensions of volcanic origin (also known as lahar matrix) from a small-scale concentric cylinder geometry, which achieves the necessary physical conditions to establish a laminar and steady flow within a homogeneous suspension and without a slippage phenomenon. This protocol was carried out using an M702e Anton Paar rheometer. The proposed protocol consisted of a staircase function testing a measurement range between and  with a homogenization step between each measurement step.

Different measurement times were tested according to the maximum sediment settling time in a virtual sample column of homogeneous particle suspension. The settling time was calculated from the shape-dependent drag formula of Dioguardi et al. (2018), which includes a wide range of fluid dynamics regimes and not only perfectly spherical sediments.

The apparent viscosity curves obtained from this protocol show its dependence on shear rate, exhibiting an inverse exponential relationship with increasing shear rate. A Herschel-Bulkley type behavior is proposed as a preliminary rheological model.