A prototype osmotic tensiometer with polymeric gel grains

Abstract

Summary An accurate field method that directly measures the entire range of relevant soil water potentials for periods of months to years is not yet available. The osmotic tensiometer (OT) has the potential to fulfil these demands but is still in an early stage of development. We have designed a novel OT that should overcome several drawbacks of former designs by the use of cross‐linked polymeric gel grains instead of an aqueous solution of linear chain polymers inside the porous ceramic cup. For this reason, the cup can have large pores without polymer leakage. This ensures that water exchanges rapidly with the soil, resulting in a negligible pressure lag with changing temperatures, which is the most important source of a systematic measurement error of an OT. A second reason for better performance might be the inherent elasticity of the grains which should inhibit formation of a polymer cake at the filter. To study several of the key variables of the new sensor, transient and stationary experiments were performed in an aqueous solution the temperature, salt type and salt concentration of which were varied. Under similar conditions the swelling of the polymer grains was studied in more detail. The OT that we developed responds rapidly to changes in water potential, but is very susceptible to the influence of salts. To study and minimize salt influence, swelling tests of unconfined grains are proposed in combination with tests of the OT itself.

Desalination by Electrodialysis Using a Stack of Patterned Ion‐Selective Hydrogels on a Microfluidic Device

“…Both hydrogels showed an identical swelling behavior also in the microchip as shown in Supporting information S3. In addition, hydrogels immersed in salt solutions are expected to swell less than in deionized water . The reason is that hydrogel swelling decreases with increasing ion concentration, as the charged side groups present in the hydrogels are increasingly screened by the ions in the salt solution …”

Use of synthesised polymers for the development of new osmotic tensiometers

“…Matric suction is a stress state variable required to determine the engineering properties of unsaturated soils in geotechnics (Fredlund and Rahardjo, 1993, Guan and Fredlund, 1997, Ridley et al, 2003, Rahardjo et al, 2019a and also affects plant growth in soil sciences (Biesheuvel et al, 2000, van der Ploeg et al, 2010, Pagay et al, 2014. Matric suction is caused by the capillarity of soil particles on pore-water as well as the physicochemical interactions between the soil particles and the pore-water.…”

“…The second method is to fill the water chamber of the tensiometer with polymer particles or solutions to increase the osmotic pressure of water in the tensiometer chamber. This type of tensiometer is usually called osmotic tensiometer (OT) and its measuring range is equal to the osmotic pressure of water inside the polymer chamber of OT (Bocking and Fredlund, 1979, Biesheuvel et al, 2000, Bakker et al, 2007, van der Ploeg et al, 2010. Within the measuring range of OT, water with positive osmotic pressure will not go into tension so that the cavitation problem can be avoided.…”

Use of osmotic tensiometers in the determination of soil-water characteristic curves

“…This increase in the osmotic water pressure is regarded as the measuring range of the OT. Cross-linked super water-absorbent polymers have been proved to be suitable for the development of OTs (Biesheuvel et al, 2000;Liu et al, 2022b). Compared with HCTs, the preparation procedure of OT is simpler and the pre-pressurization of water inside the tensiometer is not required.…”