Physics projects for Y3 and Y4 students

Project description

Bubble raft model of mechanical stress in granular crystalline materials

(supervisor: Rudi Winter)

Nature of project: experimental, theory

Available to students on full-time physics degree schemes or joint students.

Project description and methodology

The mechanical strength of solid materials depends on how the structure responds to external and internal stresses. Engineers use a number of standardised testing routines to quantify the mechanical properties of materials. Two of these are indentation and bending tests. These tests are designed to describe macroscopic bulk properties rather than understand microscopic structural relaxation mechanisms. Because in-situ indentation experiments with atomic resolution electron microscopy are extremely difficult to conduct, a simple two-dimensional physical model is used to study structural relaxation: soap bubbles are arranged in a strictly periodic fashion resembling a two-dimensional hexagonal single crystal (each bubble representing one atom).

Experimentally, the project involves designing an indentation experiment and, optionally, a bending experiment, producing bubble rafts, and carrying out mechanical tests on these models dependent on the force and depth of the mechanical impact.

This project explores some of the concepts discussed in the specialist Condensed Matter modules (ph335 and ph338) experimentally by translating them from the atomic scale into a readily observable macroscopic setting. However, this is not to say that this project is restricted to students taking these modules - there is help available to familiarise yourself with the concepts needed!

A successful project will develop beyond the above in one/some of the following directions:
A quantitative analysis of the data in terms of stress, strain, defect creation or mobility, interpretation in terms of stress relaxation mechanisms, and some reflection on the suitability and limitations of the bubble raft model is required for a successful project.

Following a successful implementation and analysis of an indentation experiment, a useful extension would be to build a bending experiment and analyse the data obtained with it.

A potential development of the project could consider nanocrystalline materials, where mechanical strength depends more on the interfaces between the grains than the atomic arrangement inside the crystalline grains. Therefore, single-crystalline and 'nanocrystalline' bubble rafts will be compared with respect to their response to indentation and bending tests.

As another development, it may be possible to model diatomic crystals (such as rock salt, NaCl) by using two different detergents to produce bubbles of two different sizes.

When considering where to take your project, please bear in mind the time available. It is preferable to do fewer things well than to try many and not get conclusive results on any of them. However, sometimes it is useful to have a couple of strands of investigation in parallel to work on in case delays occur.

Additional scope or challenge if taken as a Year-4 project: For a Y4 project, the student would be expected to complete a quantitative investigation of stress effects in either nanocrystals or diatomic crystals as described above.

Please speak to Rudi Winter if you consider doing this project.

Initial literature for students:

  1. http://www.doitpoms.ac.uk/tlplib/dislocations/index.php (excellent introduction on mechanical properties and bubble rafts)
  2. KJ van Vliet, S Suresh; Phil Mag A82 (2002) 1993 (Simulations of cyclic normal indentation of crystal surfaces using the bubble
  3. JC Anderson et al.; Materials Science, Cheltenham: StanleyThornes 19984 (good materials text book)
  4. http://users.aber.ac.uk/ruw/teach/334/defects.php (section of my lecture notes for ph335 on defects)

Novelty, degree of difficulty and amount of assistance required

This experiment appears as though it over-simplifies the problem, but the results obtained are quite accurate and consistent with experimental observation in real crystals. Apart from the bending test experiment, the whole programme consists of reproducing well-documented work. The bending test stage of the project is a little research project in a field not previously studied. While experimentally straightforward, this project is rather challenging on the theoretical side. This project is scalable; the half project skips the bending test.

Project milestones and deliverables (including timescale)

milestoneto be completed by
experimental setup and indenter designend of November
indentation tests on single-crystalline bubble raftsend of February
preparatory work on one of the options, e.g. bending apparatusmid-March
experimental work on the chosen option, e.g. bending tests on bubble raftsEaster

Students taking this project will have to submit a full risk assessment form