HandsOn 16 - The Hele-Shaw Experiment with Carrageenan

I. Introduction

II. Preparing Carrageenan

III. Observational Analysis

I. Introduction

The purpose of this experiment is to observe the viscous fingers that form when fluids of different viscosity interact with each other, and when there is a chemical reaction due to differences in the pH of the two fluids. Specifically we will be working with carrageenan of varying concentrations and pH. Carrageenan is a polymer which in aqueous solution gels at high pH (basic conditions), but does not gel at low pH (acidic conditions). Carrageenan is added to ice cream, whipping cream, and some other foods because of its gelling properties.

The viscosity of aqueous carrageenan solutions is strongly dependent on both concentration and pH. In the previous experiments in the Hele-Shaw cell you worked with glycerol. The viscosity of glycerol is a simple function of its concentration. By contrast, the viscosity of carrageenan does not depend simply on its concentration, because it has a complicated inner structure.

pH is an important factor in carrageenan solutions. Long-chain molecules such as carrageenan can interact through electrostatic forces. The pH of the solution strongly influences this interaction. The pH reflects the ions present in the solution that mediate this interaction.

The experiments with carrageenan allow you to plan and carry out your own research project. You will repeat steps 1 and 2 of the Hele-Shaw experiment above, using carrageenan instead of glycerol and varying the pH of both the carrageenan and the injected water. There are three different variables:

Don't forget to clean and dry the Hele-Shaw cell thoroughly between experiments with different solutions.

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