What to wear to the Oscars?

Do engineered systems play a role in Hollywood style?

Engineering is all around us, whether we recognize it or not, from the homes we sleep in, water we drink from, and roads we drive on.  Have you ever thought engineering could play a role on the Silver Screen?  Perhaps you think engineering systems don’t impact Hollywood, but here is an example of the impact of form in a romantic drama…

In the book Atonement, by Ian McEwan, as well as the movie, a love story is told between Cecilia Tallis and Robbie Turner.  The two young cross paths at an eventful dinner, but are torn apart by lies.  The author, Ian McEwan, spends pages of the book focused on Cecilia’s choice in dress for the occasion.  She finally chooses a “bias-cut”, emerald dress which accentuates Cecilia’s figure, causing heads to turn and hearts to ache.  In fact, the actual dress Kiera Knightley wore in the movie was recently voted “Best Costume of All Time” by Sky Movies and readers of In Style [1].  So what does this have to do with engineering?

atonement dress
Screenshot of Keira Knightley wearing a bias-cut emerald dress in the movie Atonement [1]
The key word is: “Bias-Cut”.  This refers to the orientation of the fabric threads.  A fabric’s grain is determined by the direction of the warp and weft threads of the fabric which changes depending on the direction in which the material is cut.  This direction affects the response of the material in tension.  Does your shirt stretch more in one direction than the other?  How does this compare to the direction of the threads?

On the left is a diagram of the thread directions in fabric.  On the right is the direction in which the fabric can be cut.

During the 1930s and 40s, when this love story takes place, most garments were cut straight across, so that the grain ran parallel to the main, or warp, threads.  All material has some self-weight, and in garments this causes vertical tension in the weave.  When cut cross-grain, the cloth deforms very little when draped, especially if the weave is tight.  For example, the pink dress (on the left) has a stiff thread structure.  The form of the dress is largely determined by the orientation of the threads that make up the material, little deformation of the material occurs when it hangs under its own weight.


Cross-grain cut dress (left) and bias cut dress (right)

On the contrary, material that is bias-cut has been cut at a 45 degree angle to the warp and weft threads.  The initial square mesh becomes a diamond shaped thread pattern when the material hangs.  Under increased downward tensile loads, these diamonds have greater deformation and produce high lateral contraction.  You can see in the gold dress (on the right), that bias cut and lateral contraction creates a closer, more form-fitting shape of the dress.

The grid on the left under vertical tensile loading deforms to the grid on the right.

However, not every material will have this dramatic draping effect. For example, a paper dress would not have hang the way Cecilia’s does in Atonement.  The amount of lateral contraction under tension is dependent on the shear modulus, a measure of the elasticity or rigidity of a material under shearing force, of the material.  A higher shear modulus means the material is more rigid and deforms less under shearing.  Materials with low shear moduli, like silk and cotton, deform easily making them desirable in dress making.  So perhaps the shear modulus of silk can partly be to blame for Keira Knightley’s glamorous Hollywood look.

We would like to thank  Keating Helfrich, Lewis Center of the Arts, Princeton University  for lending the pink and golden dresses to us, and Aatish Bhatia for creating the mesh animations.

Author: Kendall Schmidt

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