Supporting R&D thru 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 is important; however, understanding its rheological properties for a 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 for pharmaceutical, material science, food, cosmetic, agricultural, nutraceutical and other industries to efficiently probe material response to rotational, oscillational, and vertical forces to model processes and applications.  Material that moves in any way, however minimal the force - even gravity, is exposed to the effects of stress, shear, strain and temperature.  Output from these assays provide insight to support R&D, process optimization, manufacturing, packaging, delivery, 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.

*  "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



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Primary Assays Overview and Applications

Below are summaries for some of the primary assays offered.  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. More detailed descriptions for each test are provided further below.  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



•Viscosity determination across increasing and decreasing shear rate or shear stress can be performed either as a continuous non-equilibrium ramp or as equilibrium steps.  This sensitive and discriminating assay can highlight properties that may not be observed with a traditional viscometer.  

•Yield Stress and Yield Viscosity (non-equilibrium "flow curve") quantifies the breaking point of the macrostructure across 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).

•Thixotropy is typically a 3-step test that quantifies material rebuild extent and rate after shear thinning that often occurs during manufacturing, dispensing and application. For example, toothpaste is rheologically designed to not flow from an open tube until sufficient stress is applied to initiate flow, after which it should quickly rebuild to remain intact on the toothbrush. Paint and ketchup are also classic examples of thixotropic materials.  In particular for topical products, understanding thixotropic properties are important during application onto and penetration through skin that may affect efficacy.

•Temperature Sweep in the rotation mode measures viscosity change over increasing and decreasing temperature ramps. Is your product reversibly or irreversibly rheologically changed after storage in a hot or cold car? Is your product sensitive to temperature changes during manufacturing that affect product consistency, efficacy and quality?

•Creep Test​ quantifies the elastic recovery and viscous loss following applied and then released stress.  This is analogous to measuring the energy absorbed while squeezing a sponge followed by the amount and rate of energy dissipation when allowed to relax, along with any net change due to irreversible changes and heat loss.


•Tribology (lubricity, friction) is performed with a specifically designed attachment to quantify resistance of a material over a broad range of angular frequency (radians/sec) under a defined downward force (up to 40N) and temperature (5 to 180C). 


•Amplitude Sweep determines the LVER (Linear ViscoElastic Region) typically as % strain that tends to correlate with physical stability and is also a critical input parameter for subsequent oscillation assays noted below.

•Single Frequency, typically performed within the LVER determines if a sample is rheologically stable over time and temperature. Results of this test may also define parameters for subsequent studies.


Monitoring elastic modulus (G', solid-nature), viscous modulus (G", liquid-nature), complex modulus (G*, stiffness), complex viscosity, and phase angle (solid-liquid nature) over time quantifies sample change.  Samples containing volatiles that may rheologically change from evaporation or materials that undergo curing (caulk, glue) should undergo this assay.  Not accounting for these potential changes during various investigations may confound results of other assays.

•Frequency Sweep  across a frequency range is typically performed within the LVER to provide a rheological "fingerprint” and also determines viscoelastic properties such as elastic modulus (G'), viscous modulus (G"), phase angle, complex modulus (G*) for stiffness, and complex viscosity across a frequency range (frequency=1/time).  This assay is useful to probe behavior of polymer and biomolecule behavior and arrangement in solution.


For example, Silly Putty rolled into a ball is an excellent example of the effect of the event frequency on material properties. At rest (slow event=very low frequency) the ball slowly flows as a viscoelastic liquid (G">G'), yet when dropped on a surface (fast event=high frequency) it bounces as a viscoelastic solid (G'>G").


•Temperature Sweep in the oscillation mode determines G', G", G*, complex viscosity and phase angle within the LVER to model properties over increasing/decreasing temperature ramps. Is your product reversibly or irreversibly rheologically changed after storage in a hot or cold car? Is your product sensitive to temperature changes during manufacturing that affect product efficacy and quality?



•Pull Away quantifies material stickiness, tackiness, and cohesion/adhesion that is an important product performance and sensory consideration for consumer satisfaction and compliance.

•Surface Tension uses a DuNouy ring to quanitfy the force (Newtons) exerted on the surface film from asymmetric intramolecular interactions


In general, it should be noted that 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 rheological changes due to exposure to shear rate, stress, and temperature.



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Much More Than Just Viscosity

A rheometer is an 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.  We offer an array of rheological analyses from basic comparative screening to comprehensive characterization using a manufacturer installed and qualified state of the art Malvern (now Netzsch) 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 can quickly and quantitatively 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




"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 is a scientist, technical manager, consultant, and mentor with over 30 years in pharmaceutical development (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 various ways and under a wide range of conditions.  These include across either a static or increasing/decreasing stepwise or continuous ramp over orders of magnitude range of shear rate or shear stress 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 (Input plot - solid green line) resulting in viscosity decrease (Input plot - dashed green line).


Figure 2 highlights the potential oversight with a comparative assay using a viscometer instead of a rheometer - rank ordering viscosity vs shear rate.  In this classic example using a rheometer, mayonnaise (black curve) is more viscous than honey (gold curve) at low shear rates that becomes less viscous than honey at higher shear due to shear thinning.  At a particular cross-over shear rate, they have the same viscosity.

Figure 3 is an example of correlating viscosity of a naturally occurring macromolecule in PBS at increasing concentrations within a relatively low and narrow viscosity range.  Visually they appeared very similar.

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, also known as "flow curve" analysis is a fundamental and useful rheological property. The sample is subjected to either increasing controlled shear stress or shear rate until its macromolecular structure "breaks", resulting in flow measured as decreased viscosity (shear thinning). This assay is often applied to develop and optimize formulations, processes, and sensorial properties (feel).


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.

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. laundry agitation cycle) that uses an increasing amplitude (energy input) to probe sample properties.


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 modulus (G')), liquid nature (viscous modulus (G")), phase angle, complex viscosity, and tan delta (G"/G') across a broad range of frequencies is inversely proportional to time.


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 semisolid across a range of frequencies.  Complex viscosity is another helpful value to evaluate semisolids in the oscillation mode.



Effect of Temperature Cycling During Processing and Product

Temperature sweep quantifies rheological changes over a broad temperature at a specified rate. 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.

Figure 1 demonstrates how temperature sweep quantifies the difference in spreadability of 2 butter products. 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.


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. 


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 reference 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 a material 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 is a Stribeck curve shown below.



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