Structural characterisation

The structural features of a product define product performance and stability. For example, we can expect that if we have a formulation of big and small particles, the depletion interaction would cause aggregation of a formulation and thus it is helpful to characterize the relative sizes of components.

We can characterise individual components to understand how they interact and link this to the architecture observed in product microstructure. The following techniques aid us in characterizing individual components, interfaces and microstructure.

Individual Components

Below are a collection of the methods available for us to analyse individual components of a product.

Differential Dynamic Microscopy (DDM)

DDM is an interesting imaging microscopy technique that characterises the dynamical properties (e.g. diffusion coefficient) of colloidal suspensions over a wide range of particle concentrations. For low particle concentration, particle size can also be extracted. Size range (~10nm-10um)

Dynamic Light Scattering

Dynamic Light Scattering (DLS), in which we measure the fluctuations in intensity at a given angle, allows to probe timescales related to the Brownian motion of the particles and obtain a hydrodynamic radius (size range 5 to 500 nm).

Laser Diffraction

Our Mastersizer 3000 laser diffraction particle size analyser allow us access to larger particle sizes through the Fraunhofer approximation of Mie scattering (particle size range 100 nm to 1 mm). Commonly we use this technique to characterize emulsion droplets or aggregates.

Static Light Scattering

In Static Light Scattering (SLS), we measure the scattered intensity as a function of scattering angle. By fitting the data to an appropriate model for Mie Scattering we obtain the average particle size and variance therein (particle size range 100 to 2000 nm). SLS can also be used to determine molecular weight.

Zetapotential - Zetasizer

Using our Malvern Zetasizer Nano-Z we can determine the surface charge of particles and polymers from their mobility as they move through an electric field. Charge is important as it can cause flocculation, provide stability or dielectric interactions in a formulation.

Interfaces

Where different components meet within a formulation (e.g. particles meet a liquid or protein meets air and a liquid) the strength of interactions affect the properties of the interface . The following techniques help us to characterize interfacial properties:

Langmuir Trough

Langmuir tough allows us to measure the surface pressure of complex liquid interfaces, such as monolayers of proteins, lipids and colloidal particles at a liquid-gas or liquid-liquid interface. It enables the precise control of the monolayer thickness, homogeneous deposition of the monolayer over large areas and the possibility to make multilayer structures with varying layer composition.

Tensiometer

Our optical tensiometer can be used to measure wetting of a substrate through the contact angle of a droplet on the substrate and to measure the surface tension of an interface as a droplet of one liquid is suspended in another. We also have a Spinning Drop Tensiometer to measure very low surface tensions.

Multi-Component Microstructure

Formulations and composites are multicomponent mixtures and the following techniques help us to examine the arrangement of components deep within:

Confocal Microscope

By tagging components (e.g. oil, water, surfactants) with different fluorescent dyes we can identify their location in a product microstructure using the multi-wavelength capability of our Zeiss confocal microscope. By seeing the architecture within a product we can better understand its performance and stability.

Cryo-FIB-SEM-CT: a 'three-in-one' imaging facility for opaque soft matter.

The University of Edinburgh has been awarded EPSRC funding to purchase a cryo-SEM-FIB, housed in the School of Physics & Astronomy. FIB is a powerful technique that uses a focused beam of charged atoms (ions) to cut and section specimens very accurately inside the SEM. This not only allows the user to expose desired sections at will, but also to build up a complete 3D picture (literally) by imaging the sample section by section to a resolution of 10 nm (100 times the size of atoms). As a technique, cryo-SEM-FIB is so new that we know of only two current instruments in the UK, neither of which is dedicated to the study of soft matter.  The availability of this combined suite of instruments is transforming the ability of soft matter scientists to see inside their samples routinely. A programme of outreach and training is making this facility available to academic and industrial researchers UK-wide.  Dr Thomas Glen, an expert in SEM and TEM, is dedicated to supporting other scientists to using this facility.