Short answer
Yes – hydrophobic carbon powders (graphite, carbon black, graphene platelets, soot, etc.) can be used instead of hydrophobic silica to surround and isolate tiny droplets of water. The objects you obtain are, however, better described as “liquid marbles” than as the commercial product called “dry-water”, and several of their properties differ from the silica material.
- How the material is formed
• Both dry water and liquid marbles are Pickering systems: a gas–in-liquid (foam) or liquid-in-gas dispersion stabilised by solid particles that attach irreversibly to the water/air interface.
• Two conditions have to be met
– The particles must be sufficiently hydrophobic (water/air contact angle ≥ 90°).
– The particles must be small enough (typically < 100 nm for a free-flowing powder) so that many of them can jam together on the interface and stop neighbouring droplets from coalescing.
• In commercial dry-water a high-shear homogeniser (several thousand rpm) disperses water in air while simultaneously feeding in fumed silica that has been silanised to make it highly hydrophobic (contact angle ≈ 150°).
• Exactly the same procedure can be followed with hydrophobised carbon blacks (average diameter 20-80 nm) and stable “carbon dry waters” are described in the literature (e.g. Binks & Murakami, Chem. Commun. 2010, 46, 8383).
- What happened in your experiment
• Pencil “graphite” is not pure graphite; it is ~60 % microcrystalline carbon, ~30 % clay, ~10 % waxes or stearates that lubricate the lead.
• The wax component is strongly hydrophobic, so when you stirred the powder gently into water some of the coarser carbon–wax particles coated a single macroscopic drop. The result was a millimetre-sized water drop that could be blown around – the classic demonstration of a liquid marble.
• Because the particles were tens of micrometres in size and because you did not use high-shear mixing, only one or a few large drops formed; you did not create the huge population (10^9–10^10 m-3) of micrometre droplets required for a free-flowing powder.
- Can graphite be used to make real “dry water”?
Yes, provided that
• you start with a carbon powder of sub-micrometre size (commercial hydrophobic carbon black, graphite nanoplatelets, acetylene black, etc.), and
• you apply vigorous shear while introducing the powder into the water/air mixture.
With the correct powder and equipment you obtain a black, free-flowing powder that is more than 90 wt % water, indistinguishable in hand-feel from hydrophobic silica dry-water.
- How does carbon-based dry-water differ from the silica material?
- Colour – jet-black instead of white.
- Electrical and thermal conductivity – the carbon shell makes each droplet conductive; silica dry-water is an insulator.
- Density – similar particle density (≈2.2 g cm-3) but the carbon powder is usually slightly heavier, so the bulk powder packs a little more densely.
- Hydrophobic robustness – untreated graphite gives a water/air contact angle only just above 90°, whereas silanised silica is super-hydrophobic (>150°). Carbon dry-water therefore collapses sooner if shaken with a little solvent or subjected to long storage. Fluorination or polymer coating of the carbon markedly improves stability.
- Chemical reactivity – silica is chemically inert; carbon can be oxidised, can absorb light and convert it to heat, and can adsorb gases. These features have been exploited for
• photo-thermal evaporation of the internal water,
• catalytic reactions in which reagents are brought into contact with the carbon surface,
• gas uptake and release (e.g. CO₂, H₂) that is greater than with silica dry-water.
- Terminology: “dry water” vs. “liquid marble”
In the scientific literature a free-flowing powder of water droplets (<100 µm) is called dry-water, whereas a single millimetre-scale drop wrapped in powder is called a liquid marble. Your observation belongs to the second category; to obtain the first you would need finer, more hydrophobic carbon and high-shear mixing.
Key references
- B. P. Binks & R. Murakami, “Phase inversion of particle-stabilised materials from foams to dry water”, Nat. Mater. 2006, 5, 865.
- B. P. Binks & R. Murakami, “Dry water: Particles stabilising water-in-air foams”, Chem. Commun. 2010, 46, 8383-8385.
- P. Aussillous & D. Quéré, “Liquid marbles”, Nature 2001, 411, 924.
