ps_5 - (A) Assuming the lateral shrinkage is small compared...

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Problem Set 5 E-344 Due: Thursday, 22 June, 2006 Summer 2006 at beginning of lecture Callister 7.1 dislocations Callister 7.5 slip systems Callister 7.23 Hall-Petch Callister 8.5 fracture Core 6.33 Core 6.31 Court case Core 6.25 rolling Callister 7.36 driving force for recrystallization and grain growth Calister 5.7 nitrogen diffusion. 5.xx Suppose a soft contact lens is made from a hydrogel of crosslinked poly(hydroxy ethyl methacrylate) [HEMA]. At room temperature when fully hydrated this lens consists of 40% water by weight. Its hydrated dimensions are 14 mm diameter and 0.100 mm thickness. To dry the lens, it is placed in a vacuum chamber which holds the partial pressure of water in its atmosphere at 0.005 atm.
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Unformatted text preview: (A) Assuming the lateral shrinkage is small compared to the change in thickness when the lens dries, calculated the thickness of the dry lens. (B) Assuming that the drying of the contact lens is limited by evaporation of water from the lens surface to the surrounding vacuum where the water vapor concentration is essentially zero, sketch a series of composition profiles that describe the water content of the lens as a function of position across the lens thickness. (C) How would these composition profiles change if the evaporation were limited by water diffusion within the lens rather than by evaporation at the surface? Use sketches as appropriate to illustrate your thinking. Scan multiple choice review questions and practice on many of them....
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