Negative Ease in knitting (both in flat rows and in rounds)

[Tomato Evony]

I am a knitter and am interested using physics to explain the negative ease we see in knitting especially in rib [ (k1, p1) rib for example). I think some could be explained by the stretch-ability of the materials, e.g. wool, acrylic, but is there a movement of the yarn in between the "live" knots (the yarn form "knots" when they knit / purl) and if so I can see the yarn move when applying force on both ends, but how can they recoil back to original position when force is remove ?

 

[berkeman]

I am a knitter and am interested using physics to explain the negative ease we see in knitting especially in rib [ (k1, p1) rib for example). I think some could be explained by the stretch-ability of the materials, e.g. wool, acrylic, but is there a movement of the yarn in between the "live" knots (the yarn form "knots" when they knit / purl) and if so I can see the yarn move when applying force on both ends, but how can they recoil back to original position when force is remove ?

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[Tom.G]

Here are some photomicrographs of 2-strand 2-ply Peruvian Highland Wool yarn. It is unused and still in a ball. The fine marks on the ruler are 1/100 inch apart.

The first image is the relaxed yarn just unwound from the ball. Note that the individual fibers (hair) are crinkled, not straight, and rather tangled.

The second image is the yarn stretched by pulling on it. Here the fibers are straighter and closer together and the yarn diameter is smaller.

The third image is the yarn again relaxed. You can see it has regained much of its diameter and can probably detect that individual strands are again not straight.
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It is the tendency of the hair to recover its shape (crinkled) that causes the finished product to at least partially recover the original shape after being stretched. Each fiber acts somewhat like a spring.

Hope this helps.

Cheers,
Tom

p.s. Additional factoid:
Just like your hair, if it doesn't want to be neatly combed, get it wet then try again. The water weakens the molecular bonds in the hair so it isn't so springy. You then style as needed and let it dry. When the water dries, the molecular bonds re-establish and it magically stays put.

That's why it is not a good idea to hang a wet wool garment to dry it. The weight of the wet wool stretches it and then the molecular bonds re-establish themselves as it dries... and your nice piece of work no longer fits!