Supporting R&D to Manufacturing.  Here to help!

Applying the appropriate science.....to answer the questions.....to drive the projects.....to meet the business needs

Knowing your sample viscosity may be important; however, understanding its rheological properties for more detailed insight into your materials may be critical and value added.

Rheology Testing Services (RTS) offers an array of analyses to support early R&D through manufacturing across many industries and materials.  These assays efficiently probe material response to various types of forces to model processes and applications to which your material may be exposed.  Material that is moved or even "pushed" without being moved, however minimal the force - even gravity, is exposed to the effects of stress, shear, strain and temperature.  Output from these assays, described in more detail below provide insight to support R&D, process optimization, manufacturing, packaging, performance, efficacy, and tactile properties.  Other applications include regulatory considerations to demonstrate structural equivalence (Q3)*, in-process control, batch consistency and stability.  


Not sure which rheology assays and parameters are appropriate?   No problem!

RTS will closely partner with you to efficiently and effectively address your technical needs.  Here to help!

rheometer plates pulling sample apart to measure adhesion, cohesion, stickiness and tackiness

*  "Draft Guideline on Quality and Equivalence of Topical Products" European Medicines Agency (18Oct2018) 


*  "Generic Development of Topical Dermatologic Products: Formulation Development, Process Develoment, and Testing of Topical Dermatological Products"

     AAPS J. 2013 Jan; 15(1): 41-52 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535108/)

* "Testing Topicals: Analytical Strategies for the In-Vitro Demostration of Bioequivalence" Pharm Tech Sept 2018



Wide range of applications of rheology for many materials and industries

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Primary Assays Overview and Applications

RTS offers an array of assays with summaries of some the most basic described further below.  Not sure what assays would be most appropriate to address your technical need?  No problem - RTS is here to provide guidance and answer your questions.

RTS is very flexible to perform an array of other analyses over a wide range of parameters to meet your needs with rapid results turn around.  Links to additional information can be found on the homepage at the end of this website.


A report for each assay set contains the following:

  • general conclusions and potential impacts for consideration

  • experimental details

  • overlay plots, often in both log and semi-log format as appropriate to highlight trends

  • summary tables to highlight trends

  • embedded data files in Excel format for more detailed analyses and comparison with other data

  • results for assay standards to demonstrate proper equipment function

  • follow-up discussions to review results, potential impacts and considerations



More detailed descriptions for each test are provided further below. 




•Yield Stress and Yield Viscosity


•Temperature Sweep

•Creep Test​

•Tribology (lubricity, friction) 


•Amplitude Sweep

•Single Frequency over time

•Frequency Sweep 

•Temperature Sweep 



•Pull Away 

•Surface Tension


Rheology Testing Services specializes in "one-off" special assays that can be taylored and implemented to meet your particular rheologcial needs.  In addition, several studies can be coupled in various interesting and informative ways. For example, a follow-up frequency sweep could be performed following a yield stress, thixotropy study, or temperature sweep to further probe how the sample rheologically changed due to exposure to shear, stress, and temperature.



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Much More Than Just Viscosity

A rheometer is a versatile R&D and problem solving tool that provides much more capability and insight than a traditional viscometer, especially regarding regulatory considerations for structural equivalence, (Q3, refs 1-3), stability, and batch consistency.  RTS offers an array of rheological analyses from basic comparative screening to comprehensive characterization using a manufacturer installed and qualified state of the art Kinexus Pro rheometer.


RTS performs a broad range of analyses on solutions, suspensions, gels, creams, lotions, ointments, and oils along with other mixtures and semisolids to support the food, cosmetic, pharmaceutical, biopharmaceutical, biomaterials, engineering and other industries.  A rheometer is well-suited to model processes and applications, and also evaluate physical stability under highly defined and controlled conditions. Outputs from these tests may provide better insight to support product development, optimization, manufacturing, delivery, handling, performance, efficacy, and sensory attributes. Evaluations may include, but not be limited to the primary set of analyses described further below.

1. "Draft Guideline on Quality and Equivalence of Topical Products" European Medicines Agency (18Oct2018) 


2. "Generic Development of Topical Dermatologic Products: Formulation Development, Process Development, and            Testing of Topical Dermatological Products" AAPS J. 2013 Jan; 15(1): 41-52                                                                        (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3535108/)


3.  "Testing Topicals: Analytical Strategies for the In-Vitro Demostration of Bioequivalence" Pharm Tech Sept 2018


Kinexus rheometer
measuring adhesion, cohesion, stickiness and tackiness


Applications in rheology of food to evaluate texture and feel.

"Do the Appropriate Science, To Answer the Questions, To Drive the Project, To Meet the Business Needs"

This is Rheology Testing Services focus. Owner & founder Mark Patrick Ph.D (LinkedIn) has served a scientist, technical manager, consultant, and mentor with over 30 years in pharmaceutical development including 24 years at GlaxoSmithKline (GSK)).


While at Stiefel, a dermatology product development subsidiary of GSK, Mark became the rheometer lead user and trainer. It quickly became apparent to Mark that there is a general lack of understanding and appreciation for the potential of rheology to address practical formulation development, product manufacturing, handling, dispensing, performance, efficacy, and sensory issues. When the opportunity arose, Mark founded Rheology Testing Services laboratory located in the Research Triangle Park (RTP), NC - USA area.


Mark welcomes the opportunity to discuss an array of technical support approaches that may provide insight to better understand and leverage various practical rheological applications for your processes and products.  He also offers a free, no obligation presentation on "Rheology Principles and Applications" (link to slides on homepage).



A rheometer provides much more insight into your product properties

A viscometer is well suited as a QC tool - simple to use and relatively inexpensive to purchase, maintain, and operate. A viscometer typically provides a viscosity result under a single set of defined conditions that may provide insight into a relatively small rheological properties window.


In contrast, a rheometer is a powerful R&D and problem solving tool that performs an array of rotational, oscillational, and vertical geometry movements to probe a broad range of rheological responses to applied forces and conditions (stress, strain, shear, temperature, amplitude, frequency, tribology (friction), and vertical compression/pull-away and surface tension). These properties include, but are not limited to stringiness, stickiness, adhesion, cohesion, pumpability, suspension stability, texture, thixotropy, structural characterization, spreadability, and physical and thermal stability.


The following sections describe in more detail only some of the primary rheology measurements and their applications that RTS offers to help you better understand your product properties.  RTS provides rapid turn-around, are very flexible and often perform investigative one-off analyses to meet your particular rheological needs.



Basic, but important!

Viscosity can be determined by several approaches across a wide range of conditions.  These may include assaying across an increasing followed by decreasing shear rate (or shear stress) with either a continuous non-equilibrium ramp or step-wise during which the sample is more stabilized at each step.  These assays can be performed with temperature ranging from 5 to 180C.  This sensitive and often discriminating assay can detect important rheological properties and subtle differences that may not be observed with a traditional viscometer. 

Figure 1 compares the effect of stepwise increasing shear rate on liquids and solids showing a viscosity decrease due to shear thinning.


Figure 2 highlights an important oversight when using a viscometer instead of a rheometer to compare viscosity vs shear rate.  In this classic example using a rheometer, mayonnaise (black curve) is more viscous than honey (gold curve) at lower shear rates (<14/sec) and then becomes less viscous than honey at >14/sec due to shear thinning.  At the cross-over shear rate at 14/sec; however, they have the same viscosity.

Figure 3 is an example of correlating viscosity of a polysaccharide in PBS at increasing concentrations within a relatively low and narrow viscosity range.  Visually they appeared to have same viscosity.

Figure 4 demonstrates the ability to easily discern among oil samples with increasing amounts of surfactant that easily shear thin at a relatively low shear rate.



Go with the flow

Yield Stress and Yield Viscosity (non-equilibrium "flow curve") quantifies the breaking point of the macrostructure with an increasing stress ramp to model properties such as spreadability, pumpability and syringability. It can be a useful tool to screen numerous formulation properties ranging from mass transfer for manufacturing and product delivery to feel on skin (soft, smooth, thick, thin).


Figure 1 shows typical shear rates for common processes. Yield stress measurements are often used to compare the ease (or lack) of dispensing, spreading, and feel during application.


Figure 2 shows the process to produce a non-equilibrium ramp flow curve. With increasing stress, viscosity often increases due to elastic "push-back" until the macromolecular structure "breaks" and flows with resultant viscosity decrease.

Figure 3 shows ketchup Brand 2 having a higher yield stress and yield viscosity that should require more energy input (hitting or shaking container) to flow.  It is important; however, to also consider the zero-shear (approximate at rest) and terminal viscosities (highest achievable shear) depending on product requirements.


Figure 4 highlights the potential impact of manufacturing process changes on rheological properties. One product may have better customer appeal, efficacy or performance than the other.



Breaking and Rebuilding...Maybe

Thixotropy investigations are very helpful to provide insight into the effects of product manufacturing processes, handling, and dispensing. In this analysis, the sample is subjected to various shear rates to model situations that typically cause thinning. The extent of rebuilding, or lack thereof, is then determined. Depending on the product purpose, the desired rebuild time and extent after shear thinning will range between immediate and never.  This test can also be used to model manufacturing processes such as pumping materials through pipes and tubes.  In particular for topical products, understanding thixotropic properties are important during application onto and penetration through skin that may affect efficacy.

Figure 1 shows two creams after being subjected to a brief increased shear have different rebuild curves that may effect product properties and performance.

Figure 2 is a simple example of designed thixotropy - toothpaste. The product stays in the tube until the squeeze force (yield stress) is achieved and product flows, yet quickly rebuilds to remain on the toothbrush.  Ketchup and paint are also thixotropic materials.



Probing sample structure and properties

In contrast to yield stress and thixotropy tests that apply a rotational force in one direction, amplitude sweep is an oscillation test (ex. clothes washer agitator) that uses an increasing amplitude (energy input) to probe sample properties such as rheological stability.  The amplitude sweep is very important to determine the LVER (Linear Viscoelastic Region) that is a critical input parameter for subsequent frequency based assays noted below.


Figure 1 shows amplitude as % strain increasing until the Linear ViscoElastic Region (LVER) limit is reached, typically defined as a 5% G' (elastic modulus) decrease beyond which the sample structure is increasingly destroyed. The value of the upper limit of the LVER tends to correlate with physical stability. The higher the LVER, the more potential for improved stability. The LVER value (% strain) is also a critical input for other oscillation tests to ensure a rheologically intact sample is being investigated - otherwise erroneous results may be obtained.


Figure 2 is a typical amplitude sweep plot showing LVER, G' (elastic modulus = solid nature), G" (viscous modulus = liquid nature), and phase angle response as amplitude increases. Formulation and processing modifications may result in product changes identified by a rheometer, but not necessarily identified using a viscometer.



Obtaining a rheological fingerprint 

Frequency sweep generates a "rheological fingerprint”.  It is used to probe viscoelastic properties such as stiffness (complex modulus, G*), solid nature (elastic or storage modulus (G')), liquid nature (viscous or loss modulus (G")), phase angle, complex viscosity, and tan delta (G"/G') across a frequency range.  This assay is useful to probe behavior of polymer and biomolecule behavior and arrangement in solution.


Figures 1 and 2 show the classic rheological response of Silly Putty to a range of frequencies. Specifically, Figure 1 shows the frequency sweep output of Silly Putty placed between oscillating plates. At low frequency it is a visoelastic liquid (G" dominant); whereas at higher frequencies it is a viscoelastic solid (G' dominant).  Figure 2 further illustrates this concept.  At rest (slow event=very low frequency), Silly Putty slowly flows as a viscoelastic liquid, yet when rolled into a ball and dropped on a surface (fast event=high frequency) it bounces as a viscoelastic solid.

Figure 3 illustrates the concept of complex modulus (G*) that quantifies the stiffness of a sample across a range of frequencies.



Effect of Temperature Cycling During Processing and Product

Temperature sweep probes rheological changes either across a temperature ramp or at a single temperature. Does your product reversibly or irreversibly change after storage in a hot or cold car? Is your product sensitive to temperature changes during manufacturing that impact product efficacy and quality? An up/down temperature ramp is well designed to efficiently probe this important property.  This assay can be performed under constant shear rate (1/sec) or shear stress (Pa) in the rotation mode and under constant strain or frequency in the oscillation mode.

Figure 1 demonstrates how temperature sweep quantifies the difference in spreadability of 2 butter products under constant frequency. The more spreadable butter (red line) has a lower G' (elastic modulus = solid nature) between refrigerator and room temperatures.


Figure 2 shows the effect of multiple heating and cooling cycles resulting in an irreversible rheological change.  A more rheologically stable form of the material is identified and confirmed with subsequent heating and cooling cycles.

Water Ripple


Quantitate stickiness, adhesion/cohesion

A pull-away test quantifies the force (stickiness, adhesion, cohesion) required to vertically separate the sample between the rheometer plates.  These values have been shown to correlate well with human sensory panel results that is an important product performance for consumer satisfaction and compliance.. 


The following results are typically reported:

   -peak pull-away force (Newtons) for tack

   -area under the curve (N-sec) for adhesion/cohesion strength

   -time (sec) for 90% of force reduction 90% for failure

Figure 1 shows the typical output of a single pull-away analysis with the red curve showing the pull-away force vs time.  The blue curve shows the increasing gap between plates as the sample is being pulled apart.


Figure 2 shows an overlay of samples demonstrating a range of pull-away forces that correlate with stickiness.



Quantitate force exerted on surface interfaces

A DuNouy ring is slowly raised through the sample to quantify the liquid-liquid or liquid-air interface tension due to the attraction force (Newtons) exerted from asymmetric intermolecular interactions that can appreciably differ from the more balanced interactions within the sample.

More information can be found in the references listed in Helpful Links page.



Tribology is the study of friction, wear and lubrication.  This assay is performed with a specifically designed attachment to quantify resistance of materials over a broad range of angular frequency (radians/sec) under a defined downward force (up to 40N) and temperature (5 to 180C).  An example output are Stribeck curves (below) that compare the coefficient of friction (CoF) vs increasing spindle speed for 2 regular and 2 high mileage brand name motor oils at 15, 25, and 125C under 1 Newton downward force.

Tribology attachment to measure friction and lubricity
Plot of CoF (coefficient of friction) vs angular frequency



Chapel Hill, NC, USA


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Wide range of applications of rheology for many materials and industries