Snow crystals coarsen over time through Ostwald ripening: large ice grains grow at the expense of small ones because small crystals have higher surface curvature, higher surface energy, and therefore slightly higher solubility and vapour pressure. Molecules migrate from small grains to large ones, the snowpack loses its fine structure, and machine-made snow drifts toward the icy, dense state skiers complain about.
Key takeaways
- Ostwald ripening is the thermodynamic process by which larger crystals grow and smaller ones disappear, driven by the higher surface energy of small, highly-curved grains (the Gibbs–Thomson effect).
- In snow it shows up as grain coarsening / recrystallization — the fresh, fine crystal network relaxes into fewer, larger, more rounded grains, which is why snow "sets up" and gets icy with age.
- Coarser snow is denser, harder, faster to melt at the surface, and less pleasant to ski and groom — the structural change that matters even when total mass is unchanged.
- Ice recrystallization inhibition (IRI) is the countermeasure: molecules that adsorb to ice surfaces slow the migration of water between grains, holding the fine structure longer.
- SL6733's ultra-high-molecular-weight anionic polyacrylamide co-polymer is designed to disrupt this coarsening — its carboxylate groups interfere with the grain-boundary migration that drives ripening, complementing the starch nucleant that seeds distributed ice formation.
What is Ostwald ripening?
Ostwald ripening is the coarsening of a population of particles in which large particles grow while small ones dissolve and vanish. The driver is surface energy: a small, sharply-curved crystal has more surface area per unit volume and a higher chemical potential than a large one, so material spontaneously moves from small crystals to large ones to lower the system's total energy.
The quantitative core is the Gibbs–Thomson relationship — the solubility (or, for ice against vapour, the equilibrium vapour pressure) at a curved surface rises as the radius of curvature falls. A tiny ice grain therefore "wants" to shed molecules, and a large flat-faced grain "wants" to accept them. Over time the number of grains falls and the average grain size rises. This is a universal process seen in metal alloys, emulsions, crystallising pharmaceuticals — and in ice.
How does Ostwald ripening affect snow?
It transforms the snow's internal architecture without necessarily changing its mass. Fresh snow — natural or machine-made — starts as a network of many small, intricately branched crystals. Ostwald ripening and related recrystallization mechanisms relax that network into fewer, larger, rounded grains bonded at their necks. The snow becomes denser and harder, and its bright, light-scattering fine structure gives way to a coarser, icier surface.
For a resort this is the difference between snow that skis and grooms well and snow that has "gone off." The change is fastest when the snow is near its melting point, because molecular mobility rises steeply as temperature approaches 0 °C — which is exactly the marginal-temperature window where machine-made snow is most vulnerable. It is also why the initial crystal quality matters so much: snow made in marginal conditions starts wetter and coarser, and then ripens from a worse starting point. The density gap between light natural snow and dense machine snow is covered in artificial vs natural snow, and the ice-formation step that sets the starting structure is in ice nucleation in snowmaking.
| Snow property | Fresh / fine-grained | Coarsened by ripening | |---|---|---| | Grain structure | Many small, branched crystals | Fewer, larger, rounded grains | | Density | Lower | Higher | | Surface feel | Soft, grippy | Hard, icy | | Melt behaviour | Slower surface melt | Faster surface melt at coarse grain boundaries | | Groomability | Tills easily | Resists, breaks into chunks |
Why does inhibiting Ostwald ripening improve snow?
Because slowing the coarsening keeps the snow in its useful, fine-grained state longer — denser, more durable, better to ski and groom, and slower to degrade at the warm edge of the season. If you can hold the fine crystal network against the thermodynamic pull toward large grains, you extend the working life of every cubic metre of snow you paid to make.
This is where chemistry earns its place. The molecular strategy that counters ripening is ice recrystallization inhibition (IRI): compounds that adsorb onto ice surfaces and slow the migration of water molecules between grains. By pinning grain boundaries and locally altering the ice–water interface, an IRI-active molecule raises the kinetic barrier to coarsening even though it cannot change the underlying thermodynamics. The mechanism, the splat-cooling assay used to measure it, and the mean-grain-size metric are laid out in the IRI visual primer, and the umbrella review of engineered ice-control compounds is William, Mangan, Ben & Acker 2023, Annual Review of Biomedical Engineering. Nature's most potent IRI agents — antifreeze glycoproteins — do exactly this at vanishingly low concentrations.
How does SL6733 disrupt snow-crystal coarsening?
Through its high-molecular-weight anionic component. SL6733 pairs a starch nucleant that seeds distributed ice formation with an ultra-high-molecular-weight anionic poly(acrylamide-co-sodium acrylate). The polymer's negatively-charged carboxylate groups interact with the growing ice surface and the surrounding water structure in a way that interferes with grain-boundary migration — the transport step that Ostwald ripening depends on.
Two properties make the design deliberate:
- Molecular weight (15–20 MDa). A very long chain spans many grain contacts and dramatically increases solution viscosity at ppm doses, physically slowing the water migration between crystals that coarsening requires.
- Anionic charge. The carboxylate density gives the polymer affinity for the ice–water interface and lets it modify local water ordering around growing grains, rather than acting as an inert thickener.
The result is a slower drift toward the coarse, icy state — snow that holds its fine structure and density longer. This is distinct from, and complementary to, the nucleation job done by the starch component; nucleation controls how the snow first forms, while ripening inhibition controls how it ages. The dosing that achieves this — 6–7.6 ppm — and why so little material is needed is explained in how snowmaking additives are dosed, and the full two-component design in what SL6733 is.
A necessary caveat: the classic IRI literature measures grain-size reduction in controlled laboratory assays, and SL6733's snow-durability benefits are modelled and being validated in pre-commercial pilots. The physics of Ostwald ripening is textbook; the field-scale magnitude of the improvement is what pilots exist to quantify.
Can grooming and snow management slow ripening too?
Yes — chemistry is one lever among several, and good snow management already fights coarsening without any additive. Grooming breaks up and redistributes grains, compacts the surface, and resets some of the structure that ripening degrades; timing grooming to the coldest part of the night, when molecular mobility is lowest, locks in a finer result. Building a deeper, denser base early gives more material to work with as the season warms.
The practical levers an operator already controls:
- Make snow cold and dry when you can. Snow made well below the wet-bulb margin starts finer and denser, so it ripens from a better position.
- Groom at the right time. Grooming in the coldest hours resets structure with the least immediate re-coarsening.
- Build depth early. A thicker base buffers the surface layers most exposed to warm-up and ripening.
- Manage exposure. North-facing and shaded pitches ripen slower simply because they stay colder.
Recrystallization-inhibiting chemistry sits alongside these, not instead of them. It addresses the part of coarsening that grooming and siting cannot reach — the molecular-scale migration between grains that proceeds continuously, even in well-managed snow, whenever the temperature is close to melting. In the marginal-temperature markets where additives are permitted, it is an additional tool for the specific problem of snow ageing too fast at the warm edge of the season. The broader "chemistry as an efficiency lever" argument is developed in the missing fifth lever.
Is Ostwald ripening the same as melting?
No — and the distinction matters for how you manage a snowpack. Melting is a phase change that removes snow mass when temperature rises above freezing. Ostwald ripening is a solid-state (and vapour-mediated) redistribution of mass between crystals that can proceed even below freezing, coarsening the snow without melting it. A snowpack can lose its quality to ripening long before it loses volume to melt.
This is why "snow durability" is not only about keeping snow cold. Two snowpacks at the same sub-zero temperature and the same total mass can be very different to ski if one has ripened into coarse grains and the other has held its fine structure. Managing coarsening — through initial snow quality, grooming, and, where permitted, recrystallization-inhibiting chemistry — is a lever distinct from simply fighting the thermometer. It is one more reason the chemistry of the snow itself, not just the machinery that makes it, belongs in the operator's toolkit.
The bottom line
Snow gets icy with age because thermodynamics rewards big crystals at the expense of small ones — Ostwald ripening, quietly coarsening the fine structure that makes fresh snow good. You cannot repeal the thermodynamics, but you can slow the kinetics: ice recrystallization inhibition holds the fine grains together, and SL6733's ultra-high-molecular-weight anionic polymer is built to do exactly that alongside its starch nucleant. Snow that ripens slower skis better, grooms better, and lasts longer per litre and per kWh spent making it.
If you want to evaluate how a recrystallization-inhibiting additive could improve the durability of your machine-made snow, request a pilot or send us a message.
SL6733 is pre-commercial: EU lab pilots are targeted for 2026/27 and commercial deployment for 2027/28; snow-durability outcomes are modelled and under pilot validation. DeepSnow is the platform brand of SnowLabs Limited (Ireland); DeepSnow Srl (Italy) is in formation.