If you put a drop of water on a surface and then tilt the surface slightly, the drop will deform a bit, but not slide. With a steeper tilt, it will slide. This is the kind of thing that scientists find fascinating.
The way things slide is very important at the nanoscale, where even small frictional forces can be important in the workings of machinery. Recently, I read an article on how liquid droplets slide on surfaces. The researchers did a rather cool experiment, made some interesting observations, and... (edited: the news writeup did not exactly follow their paper.)
To test how a droplet slides under a variety of sideways and downwards forces, they put the droplet in a centrifuge. To watch it, they added a wireless camera to the centrifuge rotor. Simple, clever, elegant.
They found that as the downward force increased, it took more sideways force to make the drop slide. This is (at least superficially) similar to the way solid objects slide. But they also found that, if the drop is hanging upside down from the surface, it also takes more sideways force to make it slide.
The researcher speculates that hanging upside down helps the droplet molecules align relative to the surface. But I started to think: There is nothing magical about the force of gravity - no reason why a downward force of 1 G should provide minimum sliding force for the liquid droplet. But that's what the experiment seems to imply. (Visualize a curve of gravity force vs. sliding force. Does it make sense for the curve to be a U shape? Maybe. Does it make sense for the bottom of the U to be right at 1 G? No.) (Edited: The paper contains a figure that clearly shows a minimum at 0 G.)
Keep in mind that the droplet is first deposited on the surface, then centrifuged to make it move. This means that the downward force (apparent gravity) changes between the time the drop is deposited and the time that it slides. (I strongly suspect that, for the upside-down experiment, they deposited the drop on the surface first and then flipped the surface over.)
As the force changes, the droplet will deform. This deformation may either increase or decrease the force required to make the droplet slide. (Or leave it unchanged, but that seems unlikely; non-zero values are easy to change.)
In general, liquid molecules find it very easy to move past each
other. One might expect a droplet on a surface to slide with zero
static friction. The fact that it doesn't implies that some of the
molecules are pinned to the surface somehow - which means that there is
strain in the system. (Molecules don't like to be pinned; it reduces
their entropy.) That strain is part of the mechanism that keeps the droplet from moving. If the strain were zero, the droplet would not stick in place. So shouldn't we expect that, if the droplet is deformed in place, the strain will be increased? And if the strain is increased, shouldn't we expect that the droplet will be more firmly held in place? (Of course, there's a limit to this, a point past which the droplet breaks free and slides.)
Here's another line of argument. If deforming the droplet decreases the force required to make it slide, then the droplet's initial configuration might be unstable; a sufficiently different configuration might require zero force to slide, so that if the droplet reached that different minimal-force configuration, it would then be able to adjust itself to further minimize the sliding force. So it seems likely that deforming the droplet in either direction will increase the sliding force.I don't know if I'm right about this, of course; I'm going to email the researchers and see what they think of these arguments, and I'll post follow-ups. But I wanted to post this "raw" to give my readers a look "behind the scenes" of how I think about nanoscale phenomena.
(Edited: The paper also shows that the sliding force increases if the droplet is allowed to sit for a while. It increases almost linearly for a while, then rather abruptly levels off. This appears to support the researcher's belief that molecular alignment is taking place which increases the sliding force and which is affected by drop orientation. On the other hand, there might be other explanations, such as trapped gas molecules between the drop and the surface that are absorbed into the drop over time.)