Markus Reiner, Israeli physicist and engineer (d. 1976)

Markus Reiner (Hebrew: מרכוס ריינר, born on January 5, 1886, and passed away on April 25, 1976) was an exceptionally influential Israeli scientist and a truly foundational figure in the burgeoning field of rheology. His extensive work and groundbreaking insights were pivotal in establishing rheology as a distinct and rigorous scientific discipline, profoundly shaping our understanding of how materials deform and flow.

Early Life and Path to Israel

Born in Czernowitz, then part of the Austro-Hungarian Empire (modern-day Chernivtsi, Ukraine), Markus Reiner initially pursued studies in civil engineering. His academic journey led him to a distinguished career, culminating in his immigration to Palestine, then under British Mandate, in 1922. He quickly became an integral part of the developing scientific and academic community there, contributing significantly to the foundational institutions that would later become the bedrock of Israel’s world-renowned research landscape. Reiner was a long-serving and esteemed professor at the Technion – Israel Institute of Technology in Haifa, where his teachings and research inspired countless students and colleagues.

Pioneering the Science of Rheology

Rheology is the scientific discipline dedicated to the study of the deformation and flow of matter. It investigates how liquids flow and how solids deform, bridging the gap between classical fluid dynamics and solid mechanics. This field is crucial for understanding a vast array of substances, including polymers, paints, biological fluids, food products, and even geological formations, all of which exhibit complex flow behaviors.

Markus Reiner’s contributions to rheology were monumental and multifaceted:

• Co-founding the Term "Rheology": In collaboration with Professor Eugene C. Bingham, Reiner is widely credited with coining the term "rheology" in 1928. This pivotal moment marked the formal recognition of the study of flow and deformation as a dedicated scientific discipline, drawing its name from the Greek words "rheos" (to flow) and "logos" (study of).
• Introduction of the Deborah Number: Perhaps one of his most iconic and widely cited contributions is the introduction of the Deborah number (De) in 1968. This dimensionless quantity is fundamental in rheology, providing a critical metric to characterize the fluidity of materials under specific conditions. It quantifies the ratio of a material's characteristic relaxation time (how long it takes for a material to respond to a stress or deformation) to the characteristic time scale of an applied stress or observation. A high Deborah number implies the material behaves more like a solid, while a low number suggests more liquid-like behavior. Reiner famously articulated this concept with the phrase, "The mountains flow," highlighting that even seemingly rigid materials can exhibit flow given a sufficiently long time scale of observation.
• The Reiner-Rivlin Equation: Developed in collaboration with Ronald S. Rivlin, the Reiner-Rivlin equation is a cornerstone constitutive equation in continuum mechanics. It is essential for describing the mechanical behavior of certain non-Newtonian, viscoelastic fluids, extending beyond the simple linear relationships found in ideal Newtonian fluids and accounting for complex stress-strain relationships.
• Advancements in Plastic Flow and Viscoelasticity: Reiner's research significantly advanced the understanding of plastic deformation (permanent deformation of materials under stress) and viscoelasticity (materials exhibiting both viscous and elastic characteristics). His work helped to unify concepts previously treated separately in solid mechanics and fluid dynamics, providing a more comprehensive framework for material behavior.

Through his rigorous theoretical work and practical applications, Markus Reiner not only defined but also expanded the boundaries of rheology, making it an indispensable field in materials science, engineering, and various industrial applications.

Frequently Asked Questions about Markus Reiner

What is Markus Reiner best known for?

Markus Reiner is primarily known as one of the founding figures of rheology, the science of deformation and flow of matter. His most significant contributions include co-coining the term "rheology," introducing the influential Deborah number, and developing the Reiner-Rivlin equation.

What is the Deborah number and why is it important?

The Deborah number (De), introduced by Reiner, is a dimensionless quantity in rheology that compares a material's characteristic relaxation time to the time scale of an applied stress or observation. It helps to determine whether a material will behave more like a solid (high De) or a liquid (low De) under specific conditions, providing crucial insight into material fluidity.

What is rheology?

Rheology is the branch of physics and continuum mechanics that studies the flow of liquids (especially complex fluids) and the deformation of solids. It is concerned with materials that exhibit properties of both solids and liquids, such as polymers, gels, paints, and biological fluids, and how they respond to applied forces.

Where did Markus Reiner conduct his primary research and teaching?

Markus Reiner conducted a significant portion of his distinguished career as a professor at the Technion – Israel Institute of Technology in Haifa, Israel, where his pioneering work greatly influenced the scientific community.