In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Measuring Viscoelastic Deformation with an Optical Mouse T. W. Ng Faculty of Engineering, Engineering Block EA-07-32, National University of Singapore, 9 Engineering Drive 1, Singapore 117576;
[email protected] Students generally have no problem distinguishing between viscous fluids and elastic solids. This is because many physical entities such as water, oil, aluminum, and steel fit approximately into either one of these classes. As expected, the behavior exhibited by each entity can be described by laws and theories that are exclusive to either a viscous or elastic response. The combination of both types of behavior gives rise to a third category of response known as viscoelasticity (1, 2). Such a material response is not uncommon, being typical of polymers. A very simple demonstration of viscoelasticity can be carried out by attaching a weight to a polymer film and watching it extend over time. For quantifiable data, a ruler may be stationed beside the polymer film to measure the extension. Such an approach will likely give coarse measurements that will make it difficult to ascertain important parameters such as the relaxation time. Improved measurements may be obtained via an ultrasonic motion sensor (3) attached to a universal laboratory interface (4). Such a sensor and interface can be expensive. Cheaper alternatives to this would be desirable. Until recently, the operation of computer mice had been based almost exclusively on the rolling ball principle. Popularly called the mechanical mouse, it houses a rubberized ball that rolls according to the planar movement imposed on the
A
mouse. Two rollers located within the mouse are in constant contact with the rubberized ball. One of the rollers detects for motion in the x direction whereas the other detects for motion in the y direction. Quite naturally, the mechanical mouse suffers from the problems of wear and dirt accumulation over time. For this reason, it is common to find them incapable of registering movement after several months of heavy usage. In 1999, Agilent Technologies unveiled the first optical mouse that was immune to the problems of wear and dirt accumulation. With resolutions currently reaching 0.03175 mm, optical mice are gradually replacing their mechanical predecessors as the pointing device of choice in computers. Owing to the economics of large volume production, the cost of an optical mouse is low. Currently, it is possible to acquire a reasonably good quality unit for as low as U.S. $20. One advantageous feature of the optical mouse is that it comprises both an optical sensor and an interface to the computer. Here, we report the feasibility of using an optical mouse to track the viscoelastic deformation of low-density polyethylene films that have a fixed attached load. The choice of such films will strike a chord with students as transparent grocery bags are often made from low-density polyethylene. The mechanical properties of low-density polyethylene are highly dependent on processing conditions (5, 6).
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moving reference surface C-clamp
optical mouse C-clamp
polymer specimen
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Figure 1. The heart of the experiment (A) is a stationary optical mouse that is vertically aligned and a reference surface that moves according to the extension of the polyethylene film. (B) A schematic of the setup.
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Vol. 81 No. 11 November 2004
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In the Laboratory
Materials • Two C-clamps • A loading fixture comprising (a) an upper portion—a clamp for specimen attachment, and (b) a lower portion—a clamp for specimen attachment, a cord to hang a weight, and a reference surface • A vertically mounted optical mouse: the mouse used here has a universal serial bus (USB) connection and possesses a resolution of 0.0635 mm • A screwdriver
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• Low-density polyethylene films of 20-mm width and 0.1-mm thickness from three different manufacturers; two horizontal lines are marked 100-mm apart • A 1.5-kg mass • A stopwatch
Experimental It is important to ensure that the polyethylene film is properly fixed to the loading fixture (Figure 1). To do this the screws of the clamps at the upper and lower portions of the loading fixture are first loosened. The film is slipped inbetween the clamps and adjusted so that the two marked lines correspond to the edges of the clamps. The heads of the screws on the upper and lower portion of the loading fixture should all be on one side. After careful adjustment, the screws are firmly tightened. This is an important procedure as poor tightening can result in erroneous measurements. The vertically mounted optical mouse is then secured on any table using a C-clamp. The connector of the optical mouse is plugged to the computer’s USB port. On the computer, a program that polls the x and y positions from the optical mouse is activated. Computer savvy students may be challenged to write more sophisticated programs that will have features such automatic data storage and graphical display. The upper portion of the loading fixture is affixed securely and firmly on the table using a second C-clamp and positioned so that there is a small gap between it and the optical mouse. The reference surface should be able to move freely. This can be confirmed by pulling the reference surface up and down slowly. The gap between reference surface and optical mouse should also be kept within 1 mm; otherwise the optical mouse will fail to register any movement. Once these precautions are adhered to, the weight can be hung on the fixture. The extension of the polyethylene film is recorded at fixed time intervals (typically 10 seconds) for a certain period of time (typically 5 minutes) using the stopwatch. The beauty of using a USB port mouse is that the original mouse of the computer can be operational in tandem. The original mouse can thus be used to help set the starting position for measurement. Nevertheless, care must be exercised to prevent touching the original mouse once measurement is commencing as any such movement will result in error in the measurements made. One assured way to pre-
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Elongation / mm
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Time / s Figure 2. The extension data versus time of low-density polyethylene film from three different manufacturers (A, B, and C) show a viscous creep whose speed decays over time. These trends are typical viscoelastic response expected from polyethylene. The results also demonstrate variation in extension magnitude for each film despite the similarity in dimensions.
vent this from happening is by disconnecting the original mouse during measurement. Another precaution worth mentioning is the possibility of the polyethylene film failing altogether. Students should hence be cautioned from the outset to wear proper footwear and to be watchful over this possibility during the experiment. Results With the seemingly rudimentary experimental paraphernalia and arrangement, good measurements of viscoelastic deformation were achievable. The extension data versus time of low-density polyethylene film from the three different manufacturers (A, B, and C) showing viscous creep whose speed decays over time are typical of polyethylene (Figure 2). It is also obvious that the responses were dissimilar despite the samples having similar dimensions. Students can quite easily identify that the source of difference lies in the material itself. This creates an opportunity for students to be assigned to undertake research to uncover causes for the difference. Students may also be asked the simple question of identifying which film, A, B, or C, would be more appropriate for packaging heavy objects. Literature Cited 1. Elias, H-G. An Introduction to Plastics; Weinhem: New York, 1993. 2. McCrum, N. C.; Buckley, C. P.; Bucknall, C. B. Principles of Polymer Engineering; Oxford: New York, 1997. 3. Viela, P. M.; Thompson, D. Eur. J. Phys. 1999, 20, 15–20. 4. Pasco Scientific. http://www.pasco.com (accessed Jul 2004). 5. Krishnaswamy, R. K.; Lamborn, M. J. Polymer Eng. Sci. 2000, 40, 2385–2396. 6. Patel, R. M.; Butler, T. I.; Walton, K. L.; Knight, G. W. Polymer Eng. Sci. 1994, 34, 1506–1514.
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