Scientists have been studying liquids like water for hundreds of years, and we have a pretty good idea how they behave. New technology has allowed us to study liquids on smaller space and time scales than ever before: sending them through tiny channels to make a chemistry “lab on a chip,” studying the fluid inside a single cell, and measuring single protein molecules in water. And it turns out that, on those small and fast scales, ordinary liquids behave very differently than they do on the ordinary length and time scales of our everyday experience.
When you run your hand through water, it just flows out of the way. There’s some resistance to that flow, called viscosity, and it takes more work to move your hand through a more viscous liquid like honey than through a less viscous liquid like water. But the liquid doesn’t push back the way a solid would—say, if you push your hand against a wall. Yet if you’ve ever played with “oobleck” (a mixture of cornstarch and water), you know this liquid can act like a solid if you hit it or shake it fast enough. Oobleck and other non-Newtonian fluids get their unusual properties because they’re made up of relatively big pieces (for oobleck, it’s blobs of cornstarch floating around in the water) that get jammed together when you push them around quickly (Fig. 1).
But even water itself is made up of molecules, and if you push these molecules around quickly enough, they’ll jam up as well. Nanoscale objects move really quickly—about as quickly as the molecules in liquids can move out of the way. Recently, scientists have shown that even “simple” liquids made up only of small molecules behave like complex, non-Newtonian fluids at the nanoscale.
So how do we “see” liquids flowing over such small distances and times? In our lab, we use nanometer-sized metal particles to help us out (Fig. 2). We send a short laser pulse to heat up the particles, and as they heat, they expand, which sets them vibrating—sort of like ringing a bell (Fig. 3). By measuring how long the vibrations last, we can see how much energy the particles are transferring to the liquid and infer the properties of the liquid flow. (Read here to learn more about the technique.)
This work is challenging another assumption scientists make about simple liquids—namely, that when they flow around solid objects, the liquid right next to the solid surface doesn’t move. For example, when water flows through a pipe, the speed of the water in the middle of the pipe is high, but the water gets slower and slower toward the edges, and the speed at the surface of the pipe is zero (Fig. 4). It turns out that this “no-slip boundary condition” also breaks down if things are small enough: the liquids “slip” along the surface of the solid. How much they slip can be described by a “slip length,” which has been measured to be a few nanometers. We don’t notice this slip most of the time, because we deal with objects that are millions or billions of times larger than the slip length, but it starts to matter when we look at liquids flowing through and around tiny things.
Understanding how liquids behave on ultrasmall length scales is important for developing applications in the emerging field of nanofluidics. It also bridges the gap between the ordinary properties of the liquids and the properties of the individual molecules that make up the liquids. By studying what happens in this in-between region, we can hope to get an understanding of how the behavior of liquids emerges from the complicated interactions among all the molecules in the liquid.