The Man Who Measured Squish

Nicholas Tschoegl and the Science of Stuff

Exploring how Professor Nicholas W. Tschoegl's work in rheology helps us understand everything from Silly Putty to industrial polymers.

Ever wondered why Silly Putty bounces but also flows? Or why ketchup is so stubborn in the bottle until you shake it? The answers lie in the fascinating world of rheology—the science of how things flow and deform. At the heart of this field stands the quiet, meticulous work of Professor Nicholas W. Tschoegl, a scientist who gave us the tools to understand the complex personalities of materials, from the rubber in your tires to the cream cheese on your bagel.

It's Not a Liquid, It's Not a Solid... It's Viscoelastic

We're taught that matter comes in three primary states: solid, liquid, and gas. But step into the real world, and you'll find a universe of materials that defy this simple classification. These substances are viscoelastic, meaning they display both viscous (liquid-like) and elastic (solid-like) properties.

Elastic Materials

Like a rubber band, they stretch when pulled and snap back to their original shape. They store energy.

Viscous Materials

Like honey, they resist flow and deform permanently when force is applied. They dissipate energy as heat.

A viscoelastic material does both, and its behavior depends on the timescale. Pull Silly Putty slowly, and it flows like a liquid, stretching into a string. Yank it quickly, and it snaps like a brittle solid. Professor Tschoegl dedicated his career to developing the mathematical and experimental frameworks to measure and predict this dual nature.

A Deep Dive: The Stress Relaxation Experiment

One of the most revealing experiments in rheology is the stress relaxation test. It's a simple concept that yields a wealth of information about a material's internal structure. Let's use the analogy of a memory foam mattress to understand Tschoegl's work in this area.

The Methodology, Step-by-Step:

  1. The Setup: A sample of the material (e.g., a polymer) is clamped between two plates in an instrument called a rheometer.
  2. The Sudden Deformation (Strain): The bottom plate is instantly moved, stretching the material by a fixed, small amount. This is like you suddenly lying down on the memory foam—you've deformed it.
  3. The Measurement: The instrument continuously measures the force (stress) required to hold that deformation over a long period.
  4. The Observation: Initially, the force is high as the material's molecular chains are stretched and entangled. But over time, this force decays as the chains slowly slide past each other, relaxing and rearranging.

Tschoegl's genius was in analyzing the stress relaxation curve. He used models like the "KWW (Kohlrausch-Williams-Watts) stretch exponential function" to describe this decay mathematically. This wasn't just abstract math; it told scientists about the distribution of molecular relaxation times within the material.

Results and Analysis

The result is a Stress Relaxation Modulus Curve. For a purely elastic solid, the stress would remain constant. For a simple liquid, it would instantly drop to zero. For a viscoelastic material, it follows a decaying curve.

Table 1: Raw Data from a Stress Relaxation Experiment
Time (seconds) Stress (kPa)
0.01 1050
0.1 520
1 250
10 85
100 22
1000 5.5
Table 2: Analysis - Calculating the Relaxation Modulus
Time (seconds) Relaxation Modulus, G(t) (Pa)
0.01 1,050,000
0.1 520,000
1 250,000
10 85,000
100 22,000
1000 5,500
Stress Relaxation Modulus Curve

This visualization shows how the relaxation modulus decreases over time, characteristic of viscoelastic materials. A broad distribution of relaxation times means the material has a complex mix of fast-moving and slow-moving molecular segments, which is the signature of a tough, impact-resistant polymer.

Table 3: The Scientist's Toolkit - Essential Research Reagents & Materials
Item Function in Rheology
Polymer Melts & Solutions The primary test subjects. Their long, entangled chains are the classic example of viscoelasticity.
Rheometer The workhorse instrument. It applies precise deformations or stresses and measures the material's response.
Time-Temperature Superposition (TTS) Software A powerful principle championed by Tschoegl. It allows data collected at different temperatures to be shifted to create a single "master curve," predicting material behavior over a vast range of timescales.
Constitutive Equations The mathematical models (like the KWW function) that describe the relationship between stress, strain, and time for a specific material. Tschoegl was a master at comparing and validating these models.
Cross-Linked Elastomers (e.g., Rubber) Materials with permanent molecular links. They help scientists understand the purely elastic component of behavior and the limits of network deformation.

The Lasting Squish: Tschoegl's Legacy

Nicholas W. Tschoegl may not be a household name, but his impact is felt in countless industries. His foundational work, compiled in his authoritative book The Phenomenological Theory of Linear Viscoelastic Behavior, provided a clear and rigorous path through a complex field.

Automotive

Design car tires that grip the road elastically on quick turns but don't flow viscously under the car's constant weight.

Coatings

Formulate paints that flow easily off a brush (shear-thinning) but don't drip once on the wall.

Medical

Develop medical implants with soft-tissue-like mechanical properties that can withstand a lifetime of use.

Tschoegl taught us that the world is not just solid or liquid. It's a spectrum of squish, and by understanding it, we can build a better, safer, and more functional world—one carefully measured deformation at a time.