Capillary Condensation in Atomic Scale Friction: How Water Acts like a Glue

We present atomic-scale friction force measurements that strongly suggest that the capillary condensation of water between a tungsten tip and a graphite surface leads to the formation of ice at room temperature. This phenomenon increases the friction force, introduces a short-term memory in the form of an elastic response against shearing, and allows us to “write” a temporary line of ice on a hydrophobic surface. Rearrangements of the condensate are shown to take place on a surprisingly slow time scale of seconds.

Figure 1

Lateral force loops ($\mathrm{\text{blue}}=\mathrm{\text{forward}}$, $\mathrm{\text{red}}=\mathrm{\text{reverse motion}}$) obtained with a tungsten tip on HOPG at two values of the RH and for two scan ranges, at a normal force of 3.7 nN. At low humidity (a),(b) the tip moves in traditional “stick-slip” fashion over the surface. At high humidity the entire loop is tilted for short scan ranges (c), while for larger ranges (d) peaks develop in the lateral force, just before the end of the forward and reverse parts of the loop.

Figure 2

Variation of the lateral force peak with the scan range ($\mathrm{RH}=23\%$). Width and height of the (negative) peak in the forward (i.e., lower) part of the force loop are both proportional to the scan range (see inset). Each loop was measured sufficiently long after setting a new scan range to avoid transient effects (Fig. 3).

Figure 3

Response of the lateral force peak to a stepwise increase in scan range ($\mathrm{RH}=23\%$). The insets show (a) the last force loop at a scan range of 25 nm, (b) the first and (c) the 102nd loop at 50 nm range ($t=0$ and 29.6 s, respectively, after the change in scan range). The force peaks shown here all exceeded the 12 nN maximum detectable force of our sensor. The main panel shows the slow relaxation of the width of the lateral force peak.

Figure 4

Scenario proposed to explain our observations. (a) Under dry conditions the tip is in direct contact with the carbon atoms of the graphite substrate and it performs stick-slip motion. (b) When the humidity is noticeable capillary condensation of water takes place between the tip and the substrate. The condensate is solid and exerts elastic forces on the tip during small scans with a range up to the diameter of the condensate, which makes the entire lateral force loop tilt. (c) When the scan range is increased beyond the original size of the capillary neck, the condensate is forced to grow to a more elongated shape; at the end of the scan range, the condensate is again exerting an elastic force on the tip, which is reflected in the lateral force peaks.