Experimental investigation of flow regime effects on metastable liquid freezing during cryovolcanic effusive eruptions

Introduction

Previous experimental studies of effusive cryovolcanism conducted in low-pressure chambers (e.g., [1–3]) have largely focused on the behaviour of liquids that remain effectively “stationary”, or where fluid motion is negligible and can be approximated as laminar. In such setups, the investigated systems typically represent quiescent water bodies, where phase transitions—boiling, freezing, and evaporation—occur without significant prior internal mixing or advective transport. However, this represents a substantial simplification of natural conditions.

On icy world surfaces, effusive cryovolcanic eruptions are expected to produce flows that move downslope under gravity (e.g., [4,5]), potentially transitioning from laminar to turbulent flow regimes [6] depending on discharge rates, slope, and fluid properties. Such flow dynamics are likely to play a critical role in controlling heat transfer, phase stability, and the spatial distribution of freezing and evaporation processes. Turbulent regimes may also be relevant to past liquid water activity on Mars (which currently has a low-pressure atmosphere; [7]), for example where flowing water entered palaeolakes from inlet valleys.

Recent theoretical work by Morrison et al. [6] suggests that flow regime exerts a first-order control on cryolava evolution. In particular, turbulent flow should enhance internal mixing and heat transport, which can delay surface freezing, sustain liquid mobility over longer distances, and should modify the balance between conductive and evaporative cooling. This contrasts with laminar flows, where limited mixing promotes rapid development of insulating crusts and more localized freezing [1,2]. As a result, the transition to turbulence may fundamentally alter both the longevity and morphology of cryovolcanic flows.

Here, we aim to address this knowledge gap by experimentally investigating how different flow regimes influence the behaviour of metastable liquids exposed to low atmospheric pressure. Specifically, we seek to understand how the transition from laminar to turbulent conditions affects boiling dynamics, evaporative cooling, and ice formation in saline and pure water systems under low pressure (>4 mbar) surface conditions.

Methods

We employ a modified experimental setup based on previous studies [2,3], consisting of a transparent 40 × 40 × 40 cm plexiglass tank placed within a low-pressure simulation chamber ‘George’ situated at the Open University (UK). The tank is filled with aqueous solutions spanning a range of salinities, including low-salinity water (0.5 wt%), intermediate salinity (5 wt%), and eutectic compositions representative of MgSO₄ and NaCl brines.

In contrast to earlier experiments, which investigated largely static systems, the newly developed setup is equipped with a pair of counter-facing rotors mounted on opposing vertical walls of the tank. These rotors allow controlled stirring of the fluid, with adjustable rotation speeds enabling systematic variation of the flow regime from weakly mixed (near-laminar) to strongly turbulent conditions.

The evolution of the fluid under decreasing pressure is monitored using a combination of visual imaging (cameras), thermal measurements (thermocouples), mass-loss monitoring using a weighing system, and pressure sensors for improved characterization of the pressure conditions surrounding the evaporating medium, allowing direct comparison of phase transitions and flow behaviour across different dynamical regimes.

Results

Initial exploratory experiments suggest that fluid motion within the experimental tank exerts a first-order control on the boiling and freezing behaviour of water under low-pressure conditions. In static experiments, where the fluid remains largely quiescent, we consistently observe an initial phase characterized by the development of large macroscopic bubbles, followed by a relative calming of the water surface and the onset of floating ice crystal formation. These ice crystals subsequently and progressively coalesce into a continuous ice cover, which is periodically fractured by escaping water vapour. In contrast, stirred conditions produce a markedly different evolution. At times when static systems have already developed a coherent ice crust, the surface of dynamically mixed systems remains entirely ice-free.

Preliminary observations further indicate that mixing of the water suppresses bubble growth and reduces bubble coalescence. As a result, the formation of large macroscopic bubbles appears to be relatively limited compared to static experiments. Thermal measurements further reveal that, despite temperatures dropping well below the nominal freezing point throughout the fluid column, ice formation is initially suppressed. The liquid thus persists in a metastable state across the entire volume. This state, however, is inherently unstable. Once a critical threshold is reached, the system undergoes a rapid, system-wide transition: crystallization is initiated nearly simultaneously throughout the fluid, resulting in near-instantaneous freezing of the entire volume within seconds.

The resulting ice structure appears to be strongly controlled by salinity. Low-salinity experiments produce a coherent and continuous ice layer, whereas higher salinity systems yield a slush-like mixture composed of suspended ice crystals within a residual liquid phase. This remaining liquid is interpreted as a salt-enriched brine, consistent with progressive solute rejection during ice formation. However, these interpretations remain tentative and will require validation through a substantially larger experimental dataset and systematic parameter-space exploration.

Conclusions

These observations are consistent with, and provide experimental support for, the framework proposed by Morrison et al. [6], which predicts that turbulent flow enhances internal mixing and delays surface freezing. The current experiments therefore provide an initial indication that such mixing can prolong metastability and promote abrupt, bulk crystallization once critical conditions are exceeded.

Further experiments are ongoing to systematically quantify the role of flow intensity, salinity, and pressure in controlling these processes. In particular, we aim to constrain the thresholds governing the transition from delayed freezing to rapid crystallization, and to assess the implications for the morphology and longevity of cryovolcanic flows on icy worlds. Future work will also focus on verifying the reproducibility and robustness of the trends identified in these initial exploratory experiments.

References

[1] Brož et al. (2025), Earth Planet. Sci. Lett. 668, 119531  [2] Patočka et al. (2026), Earth Planet. Sci. Lett. 685, 119999  [3] Fox-Powell et al. (2026), EGU abstract 26-21408 [4] Fagents (2003), J. Geophys. Res., 108(E12), 5139 [5] Lesage et al. (2020), Icarus 335, 113369. [6] Morrison et al. (2022), J. Geophys. Res.: Planets 128, E007383, [7] Moreland et al., (2025), AGU Advances, e2025AV001891.