Question

Question 9 Diffusion is an important process at the micro and nano scale and different devices may be designed to minimize it, compensate for it, or take advantage of it. (a) Explain what is meant by the term diffusion and describe how it occurs, illustrating your answer with references to each of the following: (i) a hole in a semiconductor such as silicon (ii) a protein molecule within a living cell. (6 marks) Explain why diffusion could be a particular problem in the metal wire of a micromachined Pirani sensor and suggest a design feature to minimize its effect. (b) (3 marks) (c) Explain how the structure of a cell membrane is designed to block the diffusion of hydrophilic substances. (2 marks) Insulin is a protein involved in the processing of glucose: a lack of insulin causes diabetes. Microfluidic devices to monitor glucose concentration and deliver insulin are under development. The diffusion constant of insulin in water is approximately 6x 10-10 m2 s-1 (d) (i) Describe how a solution of insulin can be diluted to half its strength with water, using a microfluidic channel. (ii) Calculate the length of channel, of diameter 120 μm, required to obtain uniform mixing if the flow rate of the two liquids is 3 mm s-1, stating any assumptions you have made. (ii) Draw a diagram to show how a microfluidic H-filter can be used to separate large molecules from small molecules in a flowing solution and explain the principle on which it works. (9 marks)

Answer 1

Diffusion is the movement of a substance from an area of high concentration to an area of low concentration.

Diffusion happens in liquids and gases because their particles move randomly from place to place.

Diffusion is an important process for living things; it is how substances move in and out of cells.

In gases and liquids, particles move randomly from place to place. The particles collide with each other or with their container. This makes them change direction. Eventually, the particles are spread through the whole container.

Diffusion happens on its own, without stirring, shaking or wafting.