Oils and Gels and Creams, Oh My!
Ever wonder if some folks do find watching paint dry interesting? Welcome to the world of rheology!
What is Rheology?
The Oxford Dictionary defines rheology as, “the branch of physics that deals with the deformation and flow of matter…” This includes all phases of materials – anything from liquids and powders that behave like fluids to gels and solids. Have you ever wanted to describe how grain moves through a silo? Or how a metal structure bends in the wind? Or—yes—how recently applied paint will dry? Rheology can help! In this article, we will explore some methods for describing the rheological behavior of viscoelastic liquids.
The Roots – General Viscosity
One of the oldest and most common studies in rheology is dynamic viscosity. Very specialized methods and equipment have been developed to measure the viscosity of certain materials, as viscosity is geometry and temperature dependent. Knowing how a measurement is taken is important to get consistent results that best represent how a material is acted upon in a system.
A sample is placed either in a cup (cup-and-bob/concentric cylinder) or on a plate (cone-and-plate or plate-plate configuration) and a spindle is lowered until completely in contact with the sample. The spindle (bob, cone, or plate geometries) is then spun.
So, what is being measured? Imagine a solid structure, divided into many finite, lateral slices or layers. Now imagine the top layer is pulled in one direction at a certain speed. As the top layer moves, it drags the layer below with it, and so on with each subsequent layer. Each layer will move at a different speed relative to how far away it is from the movement of the top layer. This velocity gradient is defined as the shear rate (s-1). Conversely, the force necessary to reach and maintain a specific shear rate is known as the shear stress (dynes/cm2). Viscosity is described as the shear stress divided by the shear rate and is reported in units of milli-Pascal-seconds (mPa-s, SI) or centipoise (cP, CGS).
What analyses are available at PTL?
Rotational Tests – Dynamic Viscosity
This is the most basic of rheological measurements available at PTL. It is important to know that this analysis is best utilized for free-flowing liquids—liquids that do not exhibit viscoelastic properties—as only the viscous information is collected. An analysis can either be done at constant rotational speed (shear rate) at a range of temperatures (-20°C to 180°C) or over a range of rotational speeds at a constant temperature.
What about non-free-flowing materials, like gels and pastes? We call these viscoelastic liquids. They exhibit properties of both solids and liquids. In these cases, dynamic viscosity cannot tell us all the information we might need to know about how viscoelastic materials flow. Instead of a steady rotation of the spindle geometry, the spindle is rotated clockwise and counter-clockwise in a sinusoid with either constant amplitude or frequency. The phase angle—lag measured in sample response—can inform us about both the viscous (liquid) behavior and the elastic (solid) behavior of a material.
In this analysis, the amplitude is ramped while frequency and temperature are held constant. In doing so, we may define how much stress/deformation is necessary to break the material’s structure, also known as the linear viscoelastic limit. Knowing this limit is essential when performing the following analyses.
In this analysis, frequency is ramped while amplitude and temperature are held constant. This can be used to describe time-dependent behavior. Low frequencies can determine the materials’ viscosity at rest or simulate applied stressors with long-term recovery. High frequencies simulate applied stressors with short-term recovery. This technique is often used with cross-linked polymers or for long-term dispersion stability.
Three Interval Thixotropy Test
Thixotropy is the property of a material of becoming less viscous when subjected to applied stress, also known as shear-thinning. A cream or lotion at rest will retain its elastic structure, appearing as a solid-like dollop on a surface. Then, shear stress is applied—the lotion is spread onto the skin—and the lotion will exhibit viscous behavior and behave more like a liquid. At some point, the lotion may return to a more solid-like, elastic structure, or it may never recover from the applied stress.
A useful bit of information to know about a shear-thinning viscoelastic liquid is how quickly it recovers after applied stress and how well it recovers. In a three-interval thixotropy test, an initial oscillatory measurement is taken at low shear, simulating the material at rest. Then, high shear conditions are applied to the material, simulating the application process of a coating, squeezing a cream from a bottle, mixing, and so on. In this step, the structure is broken. Then, low shear conditions are resumed, simulating the material returning to rest. We can then determine a viscoelastic liquid’s ability to return to its initial structural conditions.
A three-interval thixotropy test can help to determine if freshly applied paint will dry with a smooth finish. If the recovery time is too short, the paint will return to a more solid structure too quickly and dry with brush streaks. If the recovery time is too long, the paint will retain its viscous structure and begin dripping before drying.
Rheology can help us answer questions like:
- Will a sunscreen apply smoothly without white patches?
- How spreadable is a cream cheese on a pastry at a specific temperature?
- How much force is needed to squeeze ketchup out of the bottle?
So, the next time someone complains about watching paint dry, tell them there is a whole branch of science dedicated to figuring out how exactly that paint will dry.
By Arielle Lopez, Particle Characterization Chemist III.