What makes kitchen plastic wrap cling so maddeningly to itself, and so conveniently to the sides of bowls and plates?

 

　　Plastic wrap clings for two reasons: it’s elastic and it’s sticky. That probably seems pretty obvious, but let’s explore the chemistry beneath the elasticity and stickiness of plastic wrap.

 

　　Plastic wrap is elastic because of its molecular structure. A molecule of plastic is incredibly long and coiled up like a spring. When you stretch out a piece of plastic wrap, you uncoil these long molecules.

 

　　Like a spring, the stretched molecules tend to spring back, perhaps tucking under the rim of the bowl you just covered! Over time, certain types of plastic wrap lose some of their elasticity, which could explain why that dish you wrapped tightly may loosen up after a day or two.

 

　　Now that you know the secret to plastic wrap’s elasticity, let’s explore why it’s sticky.

 

　　Most thin plastic films easily pick up a static electric charge by friction. In other words, plastic wrap is static electricity waiting to happen. As you pull the plastic wrap from the roll, or as it brushes against your sleeve, the friction could cause electrons to be displaced onto one area of the plastic, giving it a negative charge.

 

　　The surface that lost the electrons would then have a positive charge. Since opposites attract, the plastic wrap and any oppositely charged surface, whether that’s the plate of food, your sleeve, or even another part of the plastic wrap itself, will cling to each other by electrical attraction.

 

 

 

 

　　保鲜膜的秘密

 

　　厨房保鲜膜为何能自我粘附，并且能轻易地贴合于碗盘的边缘？

 

　　保鲜膜可粘附有两大原因：它具有弹性和粘性。这看起来似乎颇为明显，但还是让我们探究深藏于保鲜膜弹性及粘性下的化学原理。

 

　　保鲜膜的弹性取决于它本身的分子结构。塑料的一个分子极长且像弹簧般蜷曲。当你将一张保鲜膜伸展开时，同时也将长长的分子拉伸开来。

 

　　保鲜膜被伸展后会像弹簧般回缩，它可能刚好紧贴在你刚覆盖的碗边。有些品种的保鲜膜会随时间而逐渐失去弹性。这也就能解释为何之前紧贴在碗边的保鲜膜会在一两天后松开。

 

　　既然有关保鲜膜弹性的秘密已揭开，那现在就来探究下为何保鲜膜会具有粘性。

 

　　大多数保鲜薄膜可通过摩擦轻易获得一个静电荷。换言之，保鲜膜随时会产生静电。当你将保鲜膜从卷筒中拉出，或是保鲜膜轻擦过衣袖时，摩擦使得电子转移到保鲜膜的某一区域，进而使该区域产生负电荷。

 

　　而失去电子的表面会产生正电荷。由于异性相吸，不论是装有食物的盘子、衣袖甚至保鲜膜本身的其它部分，保鲜膜都会由于电荷间的相互吸引而与之紧贴在一起。