Key concepts
Physics
Fluids
Flow
Process

Introduction
Do you ever wonder why we perform tasks in a particular way? Very often we learn by copying someone and never stop to question if there is a more efficient or easier way to do something. This activity is about the tricky task of pouring from a full container. Could there be an easier way to do it?

Background
Almost everything around us is either a solid (such as a wooden block), a liquid (like water you drink) or a gas (as in the air you breathe). These are all made up of tiny particles but the strength by which they connect to one another differs. In solids these particles hold on to one another tightly. For liquids and gases these particles can flow over or alongside one another. That is why liquids and gases are also called fluids: because they can flow. This flow can be smooth, chaotic or anything in between. A smooth flow is called laminar and its opposite is called a turbulent.

When you pour fluid out of a container you remove particles from that container leaving space behind. One of two things can happen as a result: The container can collapse and become smaller or another fluid can rush in and fill the emptiness. This happens because the tiny particles that make up a fluid press against everything around them. As long as the particles inside the container press about as hard as the particles on the outside, the container will hold its form. If fluid flows out, there are suddenly many fewer particles pressing on the inside of the container while the pressure on the outside remains constant. As a result, the container might collapse.

In other circumstances another fluid will rush in, fill the space and help press against the inner walls so the inside and outside pressures equalize again. This activity explores the different ways a fluid can flow in while you pour water—and how that can affect the flow.

Materials

  • Empty rectangular soup or milk box (such as shelf-stable soy milk) with a spout on top
  • Water
  • Marker
  • Glass or cup to pour into
  • Paper towels or rag (to dry up spills)
  • A workspace that can tolerate water spills
  • Paper and pen (optional)
     

Preparation

  • The box has six sides: one on the top with the spout, one on the bottom and four side panels that connect the top and the bottom. Write the letter A on the side panel farthest from the spout. Go around the box, lettering the next side panel B, then C and, finally, D.
     

Procedure

  • Fill the box with water. The water level should be about one half inch from the top.
  • Pour some of the box’s water rather quickly into a glass in the way you usually do. Pay attention. Which side panel do you grip while you pour? Is the flow fast or slow? Does the water flow fluently? Do you think there are other ways to pour? If so, would those work better, as well or worse?
  • If you poured quite slowly, repeat the previous steps but pour faster and see how this changes how the water flows.
  • Empty the glass and add more water to the box so it is filled to the same level as before.
  • Pour some water rather quickly into the glass, but this time hold the box by a different side panel. Is this easier or harder compared with the way you usually do it? How does the water flow when you do it this way?
  • Repeat two more times, each time holding the box by a different side panel. Rank the ways you poured water from most to least preferred and from a laminar (or fluent) water flow to a turbulent (or chaotic) one. If you have paper and a pen, write down your ranking.
  • Repeat the whole activity two more times. Pay attention to how far you tilt the box before water pours out and whether or not the spout is fully covered with water. Do you get the same ranking each time?
  • Can you explain why the water flows differently depending on which side you hold the box to pour?
  • Extra: Do you think your findings are still true if the box is half full or nearly empty? Why? Try it and see if your prediction was correct!
  • Extra: Test whether your findings are the same if you pour really slowly.
  • Extra: Repeat the activity after making a hole in the top panel of the box. Keep using the spout to pour. How does this change your rankings? Did it make pouring easier? Why?
  • Extra: Test out different water bottles. Do some show turbulent flow when water is poured quickly? Some water bottles have a small hole near the spout in the lid—why would this be? Why does liquid not flow out from this small hole?

Observations and results
Did the water flow the smoothest when you held the box on side panel C? Did it flow most turbulently when your hand covered part of side panel A? Maybe you even developed a new preference. Preferences are personal choices and can differ from person to person. Laws of physics, by contrast, determine how the water flows.

You might think the box was empty when you poured all the water out, but it was not. Something else filled the box—something you cannot see: air. If liquid is poured out, another fluid such as air will take its place or the container will collapse. Unless you take very special measures, the empty space created by pouring out water does not stay empty for long! In your case the container probably held its form and air rushed in.

Sometimes air can flow in and water can pour out simultaneously. The two flow alongside each other, resulting in fluent or laminar flows. You probably saw this when you poured while gripping side panel C. Holding the box by this side allows you to tilt the box quite far before the water level reaches the spout. This makes it easier to create a nice stream of water without filling the spout opening entirely, leaving space for air to flow in while water pours out.

When you poured while holding side panel A, a much smaller tilt of the box made the water pour out but it almost instantly covered the spout completely, leaving no room for air to flow in. As water pours out and there is nothing to replace it, a pressure imbalance builds up. If this imbalance is great enough, the water flow can stop for a moment to suck in air, which results in water glugging out. Each glug is accompanied by a break for air to flow. This is an example of a chaotic or turbulent water flow, which is much harder to control.

Cleanup
Dry off your workspace, clean the glass and recycle your box if you can.

More to explore
Separation Science: Suck up an Egg Yolk, from Scientific American
Wild Winds: Turbulent Flow around Structures, from Scientific American
Spilling Science: Can Solid Candies Flow Like Liquids?, from Scientific American
Science Activities for All Ages!, from Science Buddies