Electric Fields Part 2

Electrophoresis of Pigments in Chromatography Paper

In last week's lab, you mapped out the electric fields for different configurations of charged electrodes in a tub of water. While you had the liberty to choose your configurations, we asked you to map the field given by oppositely charged parallel plates, which by now you know is given by $\vec{E} = \frac{V}{d}\hat{d}$, where $V$ is the potential difference between the plates, $d$ is the separation of the plates, and $\hat{d}$ is a unit vector perpendicular to the plates (assuming infinitely large plates). For our experiment, we also used an alternating potential (AC) instead of a constant (DC) potential, the main reason being to avoid the chemical reactions that would occur between the electrodes if we generated a steady current of ions.

This week, however, we want to see what happens when a charged particle suspended in water is subjected to a DC electric field between parallel plates. Indeed, you can imagine that the particle will experience a force that is proportional to how much charge it has as well as to the strength of the field, so that it will move parallel to the field lines. The direction of motion will be determined by the sign of its charge.

The motion of suspended charged particles in an electric field is known as electrophoresis. It has many applications in the characterization of chemical and biological systems, and it is often used to separate mixtures of chemical compounds by their charge-to-mass ratios. For instance, electrophoresis can be used to separate nucleic acids by the number of base pairs they contain, which affects the resultant charge-to-mass ratios that they possess. See gel electrophoresis for an example.

However, to properly characterize and analyze mixtures of complex molecules such as nucleic acids and proteins, a number of complications beyond the scope of this course need to be taken into consideration. Instead, in this experiment we will be focusing on the electrophoresis of a relatively simple colloidal system consisting of pigment particles (like those found in food coloring) that are suspended in water. In doing so, we hope to accomplish the learning objectives that follow.

Learning Goals

  • Develop a model of the forces acting on pigment particles suspended in water.
    • As it is often the case in science, you will need to make some assumptions about the system under consideration.
  • Apply the principles of electrodynamics and fluid dynamics to study and characterize a chemical system.
  • (Next week) Test the validity of the model through the implementation of an experimental procedure.
    • Which parameters in your model can you manipulate experimentally in order to test whether your model matches up with the experimental observations?
    • Is your data consistent with the model you develop within experimental uncertainties?

But first, a demo

Above is a footage of a colloidal system consisting of silica ($\text{SiO}_2$) microspheres of roughly 5 microns in diameter. A colloidal system is a mixture of two phases of matter, wherein one of the phases is insolubly dispersed throughout the other through suspension. In our case, the silica microspheres will be suspended in deionized (DI) water. Due to water content in the atmosphere, the silica microspheres will have $\text{SiOH}$ groups adsorbed onto their surfaces. When the spheres are immersed in water, the $\text{H}^+$ ions separate from the surface of the spheres as they dissolve due to $\text{H}_2\text{O}$'s polar nature, leaving a $\text{SiO}^-$ behind. The spheres are thus left with a net negative charge $q$ on their surfaces.

Using a power supply, we apply a potential difference of 6 V across the sample through the two parallel plates, therefore generating a uniform field. As you observe the demo, keep the following questions in mind. After seeing the demo, discuss with your lab mates and sketch out answers in your lab notebook.

  • In what direction do the particles move?
  • What is the shape of the trajectory?
  • What happens at larger voltages? Smaller ones?
  • What would you think is the role of ions in the water? Would the particles behave the same in salt water?

Modeling the system

Let us first begin by thinking about the forces that a small particle with charge $q$ suspended in water would experience in the presence of an electric field $\vec{E}$. By Newton's second law, we know that the acceleration due to the electric field will be given by

$m\vec{a_E} = \vec{F_E} = q\vec{E}.$ (1)

But as the particle moves, there will also be a viscous drag force $\vec{f}$ given by the collisions the particle will experience with the water molecules, as well as the electrodynamic interactions between its charge and ions in water. In the case that the velocity and the particle size are small, this drag force will be linearly proportional to the velocity of the particle and the viscosity of water, so that

$\vec{f} = -\alpha \eta \vec{v}.$ (2)
How small and slow is enough?

When we say that the particle is moving slowly and that it is small enough, it means that we are assuming that the system's Reynolds number is small. That is, we are assuming that the inertial forces that the particle exerts on the fluid are not too large compared to the viscous drag forces that the fluid exerts on the particle. When this is the case, the flow of the fluid is said to be laminar instead of turbulent. In this regime, we can apply Stokes' law for drag, which states that the drag force $\vec{f}$ depends linearly on the velocity $\vec{v}$. For a small sphere like those you observed in the demonstration, the geometric constant $\alpha = 6\pi r$, where $r$ is the radius of the sphere.

Here $\alpha$ is some geometric constant, $\eta$ is the viscosity of water, and $\vec{v}$ is the particle's velocity. Newton's second law thus becomes

$m\vec{a_E} = \vec{F_E} + \vec{f} = q\vec{E} - \alpha \eta \vec{v}$ (3)

If we express the acceleration as the rate of change of the velocity, so that $\vec{a_E} = \frac{\text{d}\vec{v}}{\text{d}t}$ and rearrange, the above becomes the following differential equation:

$\frac{\text{d}\vec{v}}{\text{d}t}=\frac{q}{m}\vec{E}-\frac{\alpha \eta}{m}{\vec{v}}$ (4)

Instead of solving it analytically, let's think about the limiting cases:

  • At $\mathbf{t = 0}$. Just after the constant electric field is turned on while the particle is stationary, the drag force is zero. The particle will then accelerate to some velocity $\vec{v}$ at a constant rate given by Eq. 1: $\vec{a_E} = \frac{q}{m}\vec{E}$.
  • At $\mathbf{0<t<\infty}$. As the particles acquires a velocity, it will start decelerating at a rate given by $-\frac{\alpha \eta}{m}\vec{v}$. The motion here will be governed by the time-dependent Eq. 4.
  • At $\mathbf{t \to \infty}$. In the long-time limit, the electric force and the drag force will cancel each other, so that $\vec{a_E} = 0$. At this point,$\frac{\text{d}\vec{v}}{\text{d}t}=\frac{q}{m}\vec{E}-\frac{\alpha \eta}{m}{\vec{v}}=0$. We can then solve for the terminal velocity,
$\vec{v_t} = \frac{q}{\alpha \eta} \vec{E} = \mu_e \vec{E}$ (5)
Show us the math :0 :0 :0

Let us start first with the case where there is only a frictional force $\vec{f} = -\alpha \eta \vec{v}$. Using Newton's second law we then find that $m\frac{\text{d}\vec{v}}{\text{d}t} = -\vec{f}=-\alpha \eta \vec{v}$. Orienting the system in one direction and rearranging we find

$\frac{\text{d}v}{v}=-\frac{\alpha \eta}{m} \text{ d}t$


$\ln \frac{v}{v_0} = -\frac{\alpha \eta}{m} t $

assuming initial time is $t_0 = 0$. From this it follows that

$\vec{v}(t) = \vec{v_0} e^{-t/\tau}$

where $\tau \equiv \frac{m}{\alpha \eta}$ is the time constant of the system (the $1/e$ time, that is, the time at which the particle reaches a velocity given by $v_0/e =0.3678v_0$). We can see that the velocity decreases exponentially toward zero, meaning that friction eventually slows down the particle to a stop in the absence of external forces that keep the particle moving.

What about in the presence of an electric field? In this case, Newton's law states:

$m\frac{\text{d}\vec{v}}{\text{d}t} = q\vec{E}-\alpha \eta \vec{v}$.

We see that forces balance out (that is, $\frac{\text{d}\vec{v}}{\text{d}t} = 0$) when it reaches a terminal velocity $\vec{v} \equiv \vec{v}_t = \frac{q\vec{E}}{\alpha \eta}$. We can thus reexpress the law as

$m\frac{\text{d}\vec{v}}{\text{d}t} = \alpha \eta \vec{v}_t-\alpha \eta \vec{v} = -\alpha \eta \vec{u} $.

where $\vec{u} = \vec{v}-\vec{v}_t$. Note that $\vec{v}_t$ is constant, so $\frac{\text{d}\vec{v}}{\text{d}t}=\frac{\text{d}\vec{u}}{\text{d}t}$ and the equation becomes $\frac{\text{d}\vec{u}}{\text{d}t} = -\alpha \eta \vec{u}$, which has the solution $\vec{u}(t) = \vec{u}_0 e^{-t/\tau}$ as above. Now recall that $\vec{u} = \vec{v}-\vec{v}_t$, so we have

$\vec{v}-\vec{v}_t = \vec{u}_0 e^{-t/\tau} = (\vec{v}_0-\vec{v}_t)e^{-t/\tau}$.

Assuming the particle starts from rest, $\vec{v}_0 = 0$ and we get

$\vec{v}(t) = \vec{v}_t(1-e^{-t/\tau})$

From this we clearly see that in the long time limit, the particle's velocity is given by $\vec{v}_t$.

In Eq. 5, we find that we have introduced the electrophoretic mobility, $\mu_e \equiv \frac{q}{\alpha \eta}$, so that for spheres suspended in a low Reynolds number environment this becomes $\mu_e = \frac{q}{6\pi r \eta}$.

But cows aren't spherical

For our experiment, we will not be working with spheres, but rather pigment molecules, which means that our model for spheres will likely not be accurate. However, if we assume a low Reynold's number environment, our model still predicts that the terminal velocity will be proportional to the applied electric field.

Experimental Procedure: Part 1

Lab notebook template

One member of the group should click on the link below to start your group lab notebook. (You may be asked to log into your UChicago Google account if you are not already logged in.) Make sure to share the document with everyone in the group (click the “Share” button in the top right corner of the screen) so each member has access to the notebook after you leave lab.

You will be using this notebook for today's and next week's lab. Make sure to save it and clearly mark the dates.

Separating Pigments

In this experiment, we want you to test the behavior of the particles predicted by Equation 5. More specifically, you will be performing electrophoresis of food pigment molecules in food coloring, using wet chromatography paper as the medium that provides a frictional drag force. Most food coloring dyes consist of a mixture of a handful of pigment molecules, each of which has a specific charge and molecular mass. This means that when the pigments are subjected to an electrical potential difference, they will experience an electric force that is proportional to the charge they carry. As such, you'll be able to separate the pigments based on the magnitude of their charge given that they will move at different rates. Figure 2 below shows some important parameters of these pigments.

Figure 2a: Food coloring pigments' names and relevant parameters. Taken from American Journal of Physics 83, 1003-1011 (2015) Figure 2b: Pigments we'll use in this lab.
materials.jpg meter.jpg
Figure 3a: Toothpicks, flathead, pipettes, petri dishes, and trays. Figure 3b: Volt meter. We will be using it in the DC setting.

To perform the experiment, we have provided you with the following equipment, seen in the figures above:

  • different pigments and pigment mixtures (when you're ready, ask your TA for a few drops of the pigment mixtures on a petri dish)
  • 3 cm wide strips of chromatography paper (these will be available from a roll in the front of the room - please only take what you need)
  • toothpicks and a needle
  • electrophoresis tray table with ruler
    • rectangular graphite electrodes
    • petri dishes
  • 2 g/L baking soda solution (available in front of the room; you'll only need 4 petri dishes filled half-way with this solution)
  • pipette
  • high voltage power supply
  • banana cables with alligator clips
  • flathead screwdriver

Graphite is rather brittle, so be sure not to drop it on the floor or to put much stress on it as you insert it in the slots in the tray table. If you break one, ask your TA for a replacement.

Preliminary observations

Let us first get acquainted with the experimental apparatus and samples we will be working with. To perform the experiment, we have provided you with an electrophoresis tray table, a picture of which you can see in Figure 4. It consists of a flat surface with a rectangular well at either end. In this well we can insert rectangular graphite electrodes that we can charge by connecting the metal pins that keep the electrodes in place to the positive and negative leads of a power supply.

Figure 4: Power supply. It will already be wired up. The negative lead should be connected to the ground pin (the center pin), so that we measure voltages relative to ground (zero). To measure the voltage, turn on the voltmeter (already connected in the circuit above) in the DC voltage measurement configuration.

CAUTION: Shock Hazard You will be using the power supply at voltages that can go up to 500V. Make sure not to touch the bare leads of the electrodes. Whenever you're transferring liquids or pigments onto the chromatography paper, make sure the supply is off and the voltage reading on the meter is near zero to avoid uncomfortable shocks.

You will also be working with liquids. Be careful about spills near the electronic equipment.

How do we use this power supply?

To turn on the supply, make sure that the metal switch on the upper right quadrant of the supply is down (Standby). Now turn the black switch on the left (under the voltage dial) to the “AC Power” “ON” position. This turns the machine on.

There are two knobs, one controlling the “C- Volts” and one controlling the “B+ Volts”. Between them, we can get a voltage difference of up to 500 V between the negative and positive leads. To do so, make sure the black switch between the knobs in pressed down on the right side. Finally turn on the voltage by flipping the metal switch to the “DC ON” position and measure the voltage difference on the attached voltmeter.

The electrophoresis procedure will be done on the strips of chromatography paper. Each strip will be placed on the tray so that it lies flat on the surface. Both ends of the strip will go over the well so that, by carefully inserting the graphite in the well, the strip remains pinched and stationary. To prepare this:

  • Hold the strip of paper parallel to the surface and over the well, as in the picture below.
  • Insert the graphite electrode, letting it pull the paper into the well.
  • Pull the paper softly so that its surface is flush with the surface of the tray.
  • Holding the paper fixed to the table by pressing on it, place the other electrode in its well; it should pull paper from the other end with it inside the well.
  • When both electrodes are placed in the wells and the paper is held taut and flush with the surface of the tray, tighten the screws with the bars pressing on the electrodes. These keep the electrodes in place and in electrical contact with the paper, and also serve as a point of contact with the power supply electrodes.
  • Using the pipette, wet the entire paper with the baking soda solution and place the ends of the strip in a baking soda solution in a petri dish. Make sure the region between the leads is also wet.
Figure 5a: Electrophoresis tray. Figure 5b: Push down on the graphite electrode while holding the paper taut. Figure 5c: Insert the second electrode so that the middle of the strip on the tray is flush with the surface. Figure 5d: To charge the plates, connect the outputs of the power supply to the leads using the alligator clips. Note: not all setups have long screws for you to clip on. Connect the alligator clips to the aluminum bars, which are in direct contact with the graphite bars. Figure 5e: This is how the tray looks when you are ready to make connections and start the experiment.

Aim for a uniformly moistened paper. Be careful not to puddle the solution, as this can cause the pigment to flow places. If you put too much, dab it with a bit of paper towel until you don't see any puddles.

Similarly, be careful not to make air bubbles between the paper and the tray, as this can also cause the pigment to divert its path.

Why are we using graphite?

When we apply a constant electric potential difference between two metals in solution, the ions in the water are attracted to the charged metals, causing them to react with them. This can cause unwanted electrochemical reactions between the metals, which affect the ion concentration in solution and thus how the pigment molecules behave and interact with the electric field. Consequently, we must use an inert electrode that does not react with the ions in water. Graphite (the material of which pencil lead is made) is inert in solution and its surface conductivity is enough for the potential drops across it to be minimal. Thus it can be used as an electrode for this experiment. Another inert material that is often used in electrochemical reactions and commercial electrophoresis apparatus is elemental platinum, which does not suffer from the resistivity of graphite nor its brittleness.

You will be observing what happens to pigment particles as they are subjected to an electric field using this setup. To do so, start by putting a drop of coloring consisting of a single pigment on a petri dish. Then wet the tip of a toothpick with the dye and touch the wet chromatography paper strip with the tip, so that you end up with a spot of reasonable size. You can also do this with the needle head, putting a small drop of pigment on the hole to transfer it to the paper. (After each use, clean the tip with a tissue.)

What happens if the spot is too large? What if it's too small? Make several of different sizes and determine which one you think will be effective for measuring its position over a long time.

After doing so, think about the following:

  • What do you see about the spot of colorant that may pose a challenge in making measurements?
  • How will you determine the position of the dye? How will this affect your measurement uncertainty?
  • What is the dye doing? Is it stationary? Moving? If so, in what direction?

Now connect each lead of the power supply to either crimping bar to generate a voltage potential difference across the strip and set the voltage to a high setting between 100 V to 300 V.

  • What do you see when you turn on the voltage? Are the different dyes behaving the same?
  • At what timescale is the dye moving? How will you be making time measurements?
  • In what direction is the dye moving? What does that tell you given the electric field you are applying?

Analyzing a Mixture

From the theory section above, the dye particles will reach a terminal velocity due to the frictional forces they experience in the medium. We invite you to do the calculation yourselves for a particle of nanometer length scales, which would lead you to discover that the $1/e$ time for this is in the order of femtoseconds, so that for all practical purposes the particle speed is constant. Therefore, we can determine this speed as the slope of a graph of the average position of the particles versus the time elapsed.

Ask your TA for a drop of a mixture of pigments (hint: there are three mixed together, two of which will be clearly visible - don't worry if you can't see yellow.)

Make two spots of the mixture on the paper. For the blue and red colors, measure the displacement as a function of time when a large potential gradient (above 150 V) is applied between the leads.

  • How are you determining the average position?
  • How are you ensuring that the successive measured positions of the spots are distinct from the previous measurements?
  • For how long will you need to collect? Do you see any unexpected behavior as time goes on? Why may that be?
  • Does the uncertainty in position change as time evolves? Is the uncertainty in time significant?

Is your measurement reproducible?

Using the second tray provided, repeat the experiment for the same mixture and same voltage. Compare the plots that you get. Do the same colors move at the same rate? What does that tell you about the movement of the particles and the assumptions we have made?

You can use the same power supply by connecting the second pair of banana cables on top of the ones that are already plugged in to the supply.

Submit your lab notebook

Make sure to submit your lab notebook by the end of the period. Download a copy of your notebook in PDF format and upload it to the appropriate spot on Canvas. Only one member of the group needs to submit to Canvas, but make sure everyone's name is on the document!

Make sure to clean up the materials you used and the station for the next group. Rinse the pipettes, petri dishes and trays under the sink. Dry and return them to your station.

When you're finished, don't forget to log out of both Google and Canvas, and to close all browser windows before leaving!

Post-lab assignment

Answer the questions/prompts below in a new document (not your lab notebook) and submit that as a PDF to the appropriate assignment on Canvas when you are done. You should write the answers to these questions by yourself (not in a group), though you are welcome to talk to your group mates to ask questions or to discuss.


The conclusion is your interpretation and discussion of your data.

  • What do your data tell you?
  • How do your data match the model (or models) you were comparing against, or to your expectations in general? (Sometimes this means using the $t^{\prime}$ test, but other times it means making qualitative comparisons.)
  • Were you able to estimate uncertainties well, or do you see room to make changes or improvements in the technique?
  • Do your results lead to new questions?
  • Can you think of other ways to extend or improve the experiment?

In about one or two paragraphs, draw conclusions from the data you collected today. Address both the qualitative and quantitative aspects of the experiment and feel free to use plots, tables or anything else from your notebook to support your words. Don't include throw-away statements like “Looks good” or “Agrees pretty well”; instead, try to be precise.


In addition to your conclusion, try to include in your report answers to the following questions. Feel free to discuss among your peers or with the TA for ideas, but make sure the answers are your own. Do not worry too much about a precisely correct answer. Instead, think about what you have learned about electric fields and the questions that would arise when thinking about ions moving in electric fields.

  1. For the spheres in water, what do you think is the role of ions in the water? Would the particles behave the same in salt water? What about in more acidic or alkaline water?
  2. You saw that different dyes move at different rates depending both on their geometries and the charge they have. How would you design an experiment to separate a mixture of pigments? Can this be applied to other sorts of molecules such as proteins and nucleic acids?

REMINDER: Your post-lab assignment is due 24 hours after your lab concludes. Submit a single PDF on Canvas.