# Non-isothermal effects on water potential measurement in a simple geometry

## TL;DR

This study examines how temperature gradients influence water potential measurements in vapor phases, using a microtensiometer to quantify effects relevant for environmental and industrial applications.

## Contribution

It provides experimental quantification of temperature effects on water potential and discusses the roles of thermodiffusion and convection in these measurements.

## Key findings

- Water potential varies with temperature difference at -7.9 MPa/K.
- Results agree with first-order theoretical predictions based on saturation pressure.
- Thermodiffusion and natural convection may explain measurement discrepancies.

## Abstract

In this paper, we investigate quantitatively the coupling between gradients of temperature and of chemical or water potential under steady state conditions in the vapor phase. This coupling is important for the measurement and modeling of the dynamics of water in unsaturated environments like soils and plants. We focus on a simple non-equilibrium scenario in which a gradient of temperature exists across an air-filled gap that separates two aqueous phases with no net transfer of water. This scenario is relevant for measurements of the water potential in environmental and industrial contexts. We use a new tool, a microtensiometer, to perform these measurements. We observed variations of water potential with difference of temperature across the air gap of $-\SI{7.9(3)}{\mega\pascal\per\kelvin}$, in agreement with previous measurements. Our result is close to a first order theoretical prediction, highlighting that most of the effect comes from the variation of saturation pressure with temperature. We then show that thermodiffusion (Soret effect) coupled to natural convection could occur in our experiment and discuss how these effects could explain the small discrepancy observed between measurements and first order theoretical prediction.