Electro-osmosis: Moving Water With Electricity

If you ever find yourself with a glass of water, two drinking straws of different diameters, and a little free time, you might notice that if you drink out of the smaller drinking straw, you don't get as much water as you do with the larger one. In other words, if you want to drink water at the same rate through each straw, you need to exert more effort when drinking out of the smaller straw. In general, the smaller the straw, the more effort (i.e., pressure) it takes to drive fluid at a given rate through it. This principle makes good intuitive sense, and is predicted by the mathematical equations that govern fluid flows. Perhaps a little interesting, but nothing special. Why does it matter?

It matters if you work in microfluidics, which, true to its name, involves fluids flowing through extremely small passageways. These passageways are usually hundreds of times smaller than a drinking straw - usually about as big around as a human hair. So, from what you now know about the exciting world of drinking straw physics, you would suppose that we'd need a lot more pressure to make the water flow through a microchannel than a straw, if we wanted to push water at the same flow rate through the straw and the microchannel. And you would be right. The pressure demands for driving fluid through microchannels are significant (see note 1).

If you want to drink water out of a drinking straw, there is pretty much only one way you're going to get the water to go against gravity and creep upward through the straw toward your lips: exert pressure by creating suction with your mouth. How else could you do it, right? But if you want to drive water through a tiny microchannel, it turns out there is an alternative to using pressure that you don’t have when drinking out of a straw. On small size scales, there is a completely different way to make water flow.

What if I told you that if you put two metal electrodes on opposite ends of a microchannel filled with water and applied some electricity, the water would instantaneously begin to flow from one electrode to the other? This is indeed what happens. It is possible to make water flow from one place to another in a microchannel using two pieces of metal and some electricity. That's all you need. No pumps or any other moving parts are necessary. This technique is very common and people in microfluidics use it every day.

What is going on here?

More than H2O molecules

Let’s take a closer look at that glass of water. A much closer look. If you magnified a glass of water a few million times, you would discover a lot more than just H2O molecules swimming around. Among other things, in a typical glass of water there are also a vast amount of protons (the same protons that make up the nuclei of atoms) "swimming" freely about as well. How many protons? A 1-liter bottle of water contains about 60 million billion (6 times 10 to the 16th power) free protons (although there are many, many more H2O molecules than that - see note 2). Recall also that protons have a positive electric charge. This will be important in a moment.

Now that we know there are charged particles swimming around your glass of water along with the H2O molecules, let's look at what goes on at the interface between the glass and the water.

When glass is exposed to air, the silanol (SiOH) groups stay composed and mind their own business.  However, whenever water is brought into contact with the glass, the SiOH molecules deprotonate (give up their protons), rendering the glass surface negatively charged.  Protons from the bulk water solution flock to the surface to "shield" the charged surface, so that the region between the 2 red lines is electrically neutral.  This layer of protons, along with the charges on the glass surface, constitute the electrical double layer.

Glass is primarily composed of silicon dioxide (SiO2), but the outer surface of the glass, the part that "sees" the water, is made of a chemical compound called silanol. The formula for silanol is SiOH: one silicon atom, one oxygen atom, and one hydrogen atom bound together. A single grouping of these atoms is called a silanol group. The reason I bring this up is that whenever SiOH groups are brought into contact with a fluid that has a pH higher than about 3, they cannot hold on to their hydrogens anymore. Or, more specifically, they cannot hold on to the protons from the hydrogens (remember that a hydrogen atom consists of one proton and one electron), and the protons escape from the glass surface into the water, like a top hat blown off of an old man's head in the wind. This loss of protons does happen for a reason, but it is not really that important to the present discussion. The important thing is that this process happens whenever water and glass are brought into contact, and it is called deprotonation.

Positive Band-Aids

Deprotonation has some interesting consequences in a drinking glass-water system. Once the glass loses protons, its surface becomes negatively charged (see note 3). Nature would rather this didn't happen. It's a little like an open wound exposed to open air. Nature would like to apply a positively charged “band-aid” to the surface so that it becomes electrically neutral, which is the preferred configuration.

Now, remember the quadrillions of positively charged protons swimming around in the water? Recall that protons are positively charged, and as discussed above they are readily available. So it only makes sense that the protons act as the "band-aid" to the charged surface. And this is precisely what happens: some (not all) of the freely floating protons gather very (very) near the glass surface, "shielding" the negatively charged glass (see figure above). So the negatively charged "wound" is now covered with the positively charged "band-aid," and this band-aid is known as the Electric Double Layer (EDL). If all this sounds time-consuming, it's not. The whole process takes less than a millionth of a second. Something to think about the next time you pour yourself a glass of water.

Now consider that microfluidic channels are often made from glass. So whenever we fill a microfluidic channel with water, the glass deprotonates, becomes charged, and attracts protons to its surface. By the way, the electrical double layer is extremely thin. If a microchannel the size of a human hair were magnified to the size of an average-size classroom, the EDL would be as thick as the paint on the walls.

Putting it all together

Now for the reason why I've been rambling about protons in glasses of water all this time. Now let’s say you fill a glass microcapillary (basically a tiny hollow glass tube, about as big around as a human hair) with water. As always, there will be layers of protons shielding the glass walls. Now, take your metal electrodes and place them on either end of the channel and apply a voltage between them, i.e., make one electrode negatively charged with respect to the other. Remember that, as always, opposites attract. The protons in the EDL sense a positive and negative electrode in their midst. The protons, being protons, would much rather head towards the negative electrode, and that's what they do. The transport of protons (and indeed, any ions in an aqueous solution) in response to electricity is called electromigration.

Now, you might think that since protons are so tiny, they don’t exert any influence on the surrounding water when they move. Not so. When a proton moves in a fluid through electromigration, it “drags” the surrounding fluid along with it. Now, since protons are indeed incredibly small, one proton drags a minuscule amount of fluid. But remember that there are billions upon billions of these protons coating the walls of the microchannel. There are enough that you can think of the protons as a “sheath” that “coats” the microchannel edges. This proton sheath is substantial enough to drag the rest of the water in the microchannel along with it. The "micro-paint-thin" layer of protons drags the rest of the water in the "micro-room." Again, this process is extremely quick to get started, fast enough so that when you flip the switch, the flow has effectively reached full speed instantaneously.

Schematic of electro-osmotic flow.  The arrows indicate the direction of the proton migration, and ultimately of the fluid flow.

Thus, it is possible to move water through a microchannel by the simple application of electricity. This type of flow is generally known as ­electro-osmotic flow, or EOF, and it is used every day in microfluidics research and industry. It provides an alternative to using pressure to drive flow. It is a key component of capillary electrophoresis, an extremely useful technique used in analytical chemistry to separate different compounds out of a single sample. It is the basis for electro-osmotic pumps, which use EOF to “pump” water, generating sufficient pressures to do useful work using no moving parts.  At MIT, we discover new uses for EOF almost daily, and the technique will likely continue to be useful for many years to come.

Notes
1) Despite this, pressure-driven flow is still used in microfluidics when the required flow rate is not too high.

2) It is very important to note, though, that even though there are charged particles swimming around in your glass of water, there are (roughly) just enough that they cancel each other out. In a standard glass of water, there are about as many hydroxide (OH-) ions as there are protons (also known as H+ ions), so that you can think of any given droplet of water as being electrically neutral. There are about 60 million billion hydroxide ions in your 1-liter bottle of water as well. They are completely harmless and are present in every glass of water you drink.

3) Incidentally, surfaces other than glass can and do become electrically charged when brought into contact with water as well. It turns out there are many different mechanisms for surfaces to become charged in the presence of liquids.