Using Microrheology to Measure Rheology Faster and Contact Free

Interview conducted by Mychealla RiceMar 8 2019

Please tell us about LS Instruments and the work you do?

At LS Instruments (LSI) we develop analytical solutions for both academia and industry. Our core technologies and expertise are all related to light scattering techniques.

Important applications are the characterization of nanosized structures or rheological measurements. These are required in a wide range of research fields, some of the most important being colloidal, pharmaceutical or food-science. Naturally, we have customers in the chemicals, pharmaceutical, cosmetic and food-industry.

Specifcally protein characterization is currently one of the fastest growing applications areas for which we provide some unique solutions. It is an exciting time to be working in this area. There are currently many fields of light scattering in which we are achieving significant improvement. In general, we can say that novel light scattering techniques, such as microrheology for example, are far from reaching their full potential for research-driven industries.

Why has the microrheology technology not reached its full potential yet?

Microrheology allows rapid, contact-free measurement of the elastic and viscous properties (G´and G´´) on small sample volumes in an unmatched frequency range. The technique itself is however still rather new and naturally, there is still some skepticism towards it.

Furthermore, there are several different ways microrheology can be implemented. Some implementations are less suited and yield poor results. This has caused some confusion among users. In many cases, an existing technique was adapted only with software modifications to enable microrheological measurements as an add-on. But the original hardware is not designed for microrheology and thus typically has many limitations.

The most common implementation is currently Dynamic Light Scattering (DLS Microrheology). While DLS is predominantly used for particle sizing it can also perform microrheology, but with some limitations. For this reason, LSI has also developed dedicated microrheology solutions that are not add-ons to existing techniques, but stand-alone solutions optimized for microrheology. Diffusing Wave Spectroscopy (DWS) is such a technique, it is perfectly suited and predominantly used for microrheology.

Diffusing Wave Spectroscopy (DWS) is a technique that is perfectly suited for microrheology.

What industry will benefit from microrheology? How?

Most industries that require rheological measurements can benefit from microrheology as a complementary technique to classical mechanical rheology. Today we focus on those applications for which microrheology makes the biggest difference. Whenever small sample volume, contact-free measurements, high frequencies or sensitivity are a topic, then the advantages can be significant. The industries that are most interested in our solutions are biotech, pharma or life science, but also the food industry.

Tell us how Microrheology can help applications in the food industry?

Rheology has always been an important technique for the food industry. It finds application for sensory and stability studies in product development, but also for process optimization. For all these topics microrheology and especially DWS Microrheology offers significant advantages over traditional rheology. The sensitivity and reproducibility are higher, thus allowing detection of the smallest differences between production batches.

With microrheology, many production processes can be studied in a realistic manner already in a laboratory environment on a small scale. Unlike mechanical rheometers, the sample is not sheadred during the measurement. Shearing the sample is problematic because it influences any structural evolution that the sample is undergoing and might lead to poor reproducibility.

Furthermore, mechanical rheometers typically take many minutes for one measurement after which the sample is contaminated and must be discharged. Microrheology allows time and temperature dependent studies on one single sample without any contamination.

What role does microrheology play in determining shelf-life?

Cells used for DWS Microrheology can be sealed because it is a contact-free optical technique. Even complex series of measurements can be repeated on the same sample over a basically unlimited duration at any desired temperature. Which of course makes the technique so useful for shelf-life or stability studies.

Measuring gel point is important for dairy products, starch gelatinization and other foods. How does microrheology help determine this?

Whenever one wants to study rheology of a food sample during any process that does not involve strong mechanical deformation of the sample (e.g. industrial emulsification), then it is better to measure in the linear regime which is always the case for a contact-free method.  This becomes even more important in the case of a highly strain sensitive sample or process such as jellification. Again, it is the contact-free nature of microrheology combined with the short measurement duration that produces a unique solution.

Microrheology can help determine the gel point in dairy products through contact free measurement.

Tell us about the differences between microrheology and mechanical rheology when it comes to sample preparation? What sets it apart from other sample preparation techniques?

The sample preparation for a mechanical rheometer is usually straight forward. It typically only requires the availability of sufficient sample volume. The trickiest part is related to the loading of the sample into the sample chamber in a systematic manner to obtain reproducible results.

Differences between individual users can be significant. In the case of microrheology, the challenge in sample preparation depends on the specific technique used and the sample. In the best case, the sample already contains particles or droplets that scatter light, and these then function as tracer particles.

This is the case for emulsions such as basically all dairy or cosmetic products. For all transparent water-based samples, the addition of tracer particle is required, but simple. Commercially available Latex particles are a good choice since they are easily dispersed.

One of the biggest challenges for the food and pharma industry is the constant increasing requirements set by regulatory institutions, such as the FDA. How will the analytical tools of LSI help the industry on this topic?

Two aspects that microrheology offers are especially important when it comes to regulatory questions. The higher reproducibility combined with the faster measurement time is naturally an advantage since it reduces the required duration of testing significantly. The ability to store and remeasure sealed samples again and again to fulfill documentation requirements is the other.

DLS Microrheology and Multi-Speckle (MS) DWS cover smaller frequency and moduli ranges than DWS Microrheology (Echo-DWS), which gives access to up 1 MHz (1M rad/s)”. The DWS RheoLab by LSI is the only product that offers this technology.

Can you tell us about any other industries and applications that will benefit from microrheology?

Industrial applications that involve fluids with low viscosity that are moved at high velocities are an important application field. Ink-jet printing is a good example. The application-relevant properties of these fluids are given by the G´ and G´´ at a frequency well beyond 100 Hz, which are inaccessible to mechanical rheometers.

What is in store for the future of microrheology?

We believe that in 10 years most laboratories that use a mechanical rheometer today will also be employing microrheological tools of some kind. For academia, it will open new research approaches in rheology, especially the incredible frequency range of up to 10 MHz gives insight to areas that we could not access previously.

Where can our readers go to find out more

Our webpage has a technology section that covers microrheology, but also other light scattering techniques such as Dynamic or Static Light Scattering (DLS and SLS) in detail. Even more detail on microrheology is offered in the book with the same title by Eric M. Furst and Todd M. Squires.

About Dr. Andreas Charles Voelker

Dr. Andreas Charles Voelker studied at the Humboldt University of Berlin, the RWTH Aachen and the University of Fribourg in Switzerland. He has been author or co-author of numerous publications related to light scattering technologies. Since 2010 he is CEO of LS Instruments AG. New ideas and challenges in science never fail to arouse his curiosity.