  Two-dimensional square ice in graphene nanocapillaries at room temperature is
a fascinating phenomenon and has been confirmed experimentally. Instead of
temperature for bulk ice, the high van der Waals pressure becomes an
all-important factor to induce the formation of square ice and needs to be
studied further. By all-atom molecular dynamics simulations of water confined
between two parallel graphene sheets, which are changed in size (the length and
the width of the graphene sheets) over a wide range, we find that the critical
crystallization pressure for the formation of square ice in nanocapillary
strongly depends on the size of the graphene sheet. The critical
crystallization pressure slowly decreases as the graphene size increases,
converging to approximately macroscopic crystallization pressure. The
unfreezable threshold for graphene size is obtained by estimating the actual
pressure and it is difficult to form the square ice spontaneously in practice
when the graphene sheet is smaller than the threshold. Moreover, the critical
crystallization pressure fluctuates when the graphene size is minuscule, and
the range of oscillation narrows as the sheet size increases, converging to the
macroscopic behavior of a single critical icing pressure for large sheets. The
graphene size also affects the stability and crystallization time of square
ice.
