JANUARY 2014 • Vol. 20 No. 1
New York Society of Cosmetic Chemists
www.nyscc.org
Rheology, Theology, Salvador Dali, and God
…Joseph Albanese
N
o, this is not going to be one of those children’s games, such as “One of These Things is Not Like the Other” that you might still see on Sesame Street. Instead, rather than highlighting their dissimilarities, I will try to draw a connection between the very diverse elements listed in the title of this article.
Rheology As you probably already know, rheology is the science that studies the deformation and flow of materials. Theology is a way of thinking, or a belief system—like an ideology, religion, etc. Even though the derivation of both words may be traced back to the Greek words rheos (flow, stream), theos (God, god), and ology (a branch of learning), they are ‘seemingly’ two very different branches of learning.1,2 Now, your computer, which thinks it knows everything, will certainly confuse the two. Therefore, I caution you that the first time that you type the word ‘rheology’ using a word processer do not trust spell check to get it right. Invariably it will change the word to ‘theology’. At least that’s been my experience. Like Sgt. Phil Esterhaus (played by the late, Michael Conrad) said at the end of every TV episode of Hill Street Blues, “Let’s be careful out there.” In fact, go ahead and add the word ‘rheology’ to your software program’s dictionary right now. I will wait. As you will see in the next section, the connection between the
The Persistence of Memory (Spanish: La persistencia de la memoria; Catalan: La persistència de la memòria) is a 1931 painting by artist Salvador Dalí.
two words runs so much deeper than the similarity of their spelling. The rheological realm covers a continuum that extends from purely viscous materials that deform or flow irreversibly (e.g., water, mineral oil, etc.) to purely elastic solids, that spring back unchanged (a steel ball, for example)—see Figure 1.3 Pure liquids exhibit Newtonian rheology and their viscosity remains constant regardless of the amount of stress applied. Most, if not all, materials are viscoelastic, which means that they contain varying degrees of both properties. Take, for example, Silly Putty (Dow Corning 3179 dilatant compound). Under low shear stress it exhibits viscous behavior (will flow irreversibly) and elastic behavior under high shear (it will bounce or break; thus, behaving very much like a solid)—see Figure 2.4 So, technically speaking, selling Silly-Putty as the “real solid-liquid” is truth-in-advertising.5 Figure 1. Rheology
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