Experimental Evidence for Ice Formation at Room Temperature

The behavior of water under extreme confinement and, in particular, the lubrication properties under such conditions are subjects of long-standing controversy. Using a dedicated, high-resolution friction force microscope, scanning a sharp tungsten tip over a graphite surface, we demonstrate that water nucleating between the tip and the surface due to capillary condensation rapidly transforms into crystalline ice at room temperature. At ultralow scan speeds and modest relative humidities, we observe that the tip exhibits stick-slip motion with a period of $0.38\pm 0.03\mathrm{nm}$, very different from the graphite lattice. We interpret this as the consequence of the repeated sequence of shear-induced fracture and healing of the crystalline condensate. This phenomenon causes a significant increase of the friction force and introduces relaxation time scales of seconds for the rearrangements after shearing.

Figure 1

Measurements of the lateral force between a tungsten tip and a graphite surface, scanning back and forth over a distance of 3 nm at three different velocities ($v$) and two relative humidities (RH): (a) at $\mathrm{RH}=1\%$ and $v=0.5\mathrm{nm}/\mathrm{s}$, stochastic thermal jumps of the tip between the neighboring wells in the tip-substrate potential lead to very low friction; (b) at $\mathrm{RH}=1\%$ and $v=5\mathrm{nm}/\mathrm{s}$, graphite lattice appears over thermal noise, (c) at $\mathrm{RH}=1\%$ and $v=30\mathrm{nm}/\mathrm{s}$, the tip performs stick-slip motion with the 0.25 nm period of the graphite lattice; (d) at $\mathrm{RH}=5\%$ and $v=0.5\mathrm{nm}/\mathrm{s}$, the tip experiences strong friction and undergoes pronounced stick-slip motion with a period of approximately 0.4 nm; (e) at $\mathrm{RH}=5\%$ and $v=5\mathrm{nm}/\mathrm{s}$, the slip events become more noisy, (f) at $\mathrm{RH}=5\%$ and $v=30\mathrm{nm}/\mathrm{s}$ velocity, the friction force is lower and the graphitic lattice reappears.

Figure 2

Velocity dependence of the friction force (average lateral force) at two different humidities. At $\mathrm{RH}=1\%$, friction follows a sublinear velocity dependence due to the thermally activated jumps of the tip in the tip-graphite interaction potential [19]. At $\mathrm{RH}=5\%$, the friction force is unexpectedly high at velocities below $10\mathrm{nm}/\mathrm{s}$.

Figure 3

Distribution of stick-slip distances observed at low velocities ($0.1\mathrm{nm}/\mathrm{s}$ to $3\mathrm{nm}/\mathrm{s}$) and modest RH values (5% to 33%). The gray curve is a Gaussian fit to the data, from which we derive an average stick-slip distance of $0.38\pm 0.03\mathrm{nm}$. (The error-bar includes the systematic error in the measurements and the calibration error of the instrument). This value lies well within the range of lattice constants of 0.27 to 0.45 nm corresponding the known structures of crystalline ice.

Figure 4

Dependence of friction on the normal force, measured at $\mathrm{RH}=25\%$ at a constant velocity of $30\mathrm{nm}/\mathrm{s}$. The gray boxes indicate the presence of force plateaus, which forms clear evidence of water (ice) being squeezed out of the contact layer by layer when the normal load is increased.

Figure 5

A typical lateral force loop at $\mathrm{RH}=33\%$ at a velocity of $30\mathrm{nm}/\mathrm{s}$. After the turning points, the lateral force at this relatively high velocity passes through an overshoot, visible as the “static peaks” in the left-to-right and right-to-left curves (lower left and upper right, respectively), each followed by a decay to a substantially lower kinetic friction force. The absence of graphite lattice features in the force loop shows that the tip is not in direct contact with the graphite substrate, suggesting the presence of intervening water. The fully elastic behavior around the turning points indicates that this water still behaves like a solid, while the absence of icelike lattice features at this high velocity implies that the ice is significantly disordered. We interpret the static peak as the signature that the condensate somewhat recovers in crystal quality and therefore in yield strength at the turning points—the residence time was 0.3 s at the left turning point and 0.03 s at the right.