Static multiple light scattering

What is static multiple light scattering?

Static multiple light scattering (SMLS) is an advanced optical method designed to directly characterize concentrated liquid dispersions—such as emulsions, suspensions, and colloidal formulations—without the need for dilution or sample preparation. Unlike traditional techniques that require optically transparent samples, SMLS operates in opaque or turbid media, where light is scattered multiple times before detection, providing information on particle concentration and size over a wide range (10 nm to 1000 μm) and concentrations up to 95% v/v.

Working principle

The technique is based on emitting a light beam (typically near-infrared, 880 nm) that passes through the sample contained in a glass cell. Photons are scattered multiple times by the suspended particles or droplets. Two synchronous detectors measure the intensity of transmitted light (at 0°) for transparent samples or backscattered light (at 135°) for opaque samples. The backscattering intensity is related to the transport mean free path (l*), the distance after which the photon loses the original direction of the incident beam. The transmission is related to the photon mean free path (l), the average distance between scatterers. Both parameters depend on particle size and concentration. The instrument scans the cell vertically over time, generating intensity profiles that reveal instability phenomena such as sedimentation, creaming, flocculation, and coalescence.

Laboratory and industrial applications

SMLS is widely used in R&D laboratories and industrial quality control to assess formulation stability across sectors such as pharmaceuticals, cosmetics, food, petrochemicals, and materials. Key applications include:

• Accelerated stability studies: monitoring dispersion evolution over time to predict shelf life.
• Nanocellulose characterization: evaluating dispersibility and aggregate size in concentrated cellulose nanocrystal suspensions, as reported in a study published in Cellulose.
• Formulation development: optimizing industrial emulsions and suspensions, such as paints, pesticides, and personal care products.
• In vitro toxicology assays: simultaneous screening of stability and dosimetry of nanoparticle dispersions.

The method is standardized by ISO/TS 21357, which outlines procedures for evaluating the mean equivalent particle diameter in liquid dispersions.

Advantages and limitations

Key advantages of SMLS include non-invasiveness, no sample preparation ("as-is" analysis), high resolution in detecting size and concentration variations, and fast measurements. Additionally, data can be correlated with mathematical models to estimate particle size distributions and migration velocities in polydisperse systems. A limitation is that the technique requires proper calibration and scattering models (e.g., Mie theory) to convert intensities into mean diameters, and it may be less accurate for non-spherical or highly porous particles.

Recent trends and innovations

Current research has combined SMLS with predictive approaches, such as the Hansen method, to rationalize the dispersibility and colloidal stability of particles in different solvents. Another innovation is coupling SMLS with mathematical models of vertical motion under gravity, enabling the measurement of velocity and size distributions of particles in highly concentrated suspensions. A recent article in Advanced Powder Technology demonstrates the application of SMLS to characterize sedimentation of concentrated submicrometric particle suspensions, showing good correlation with gravitational sedimentation data.

What is the difference between SMLS and DLS (dynamic light scattering)?

Dynamic light scattering (DLS) measures temporal fluctuations of scattered intensity to determine the diffusion coefficient and hydrodynamic size of particles in diluted suspensions. SMLS, on the other hand, is a static technique that measures the average scattered intensity in concentrated samples, where light undergoes multiple scattering, enabling direct characterization of opaque systems without dilution.

Can SMLS be used for non-spherical particles?

Although Mie theory, commonly applied to convert intensities into diameters, assumes spherical particles, SMLS can provide qualitative and comparative information on stability and aggregation of non-spherical particles. For elongated or irregularly shaped particles, results should be interpreted as equivalent diameters.

What are the critical parameters for reliable SMLS measurements?

The most important parameters include dispersed phase concentration, refractive index of particles and medium, mean size, and size distribution. Additionally, proper selection of the scattering model (Mie, Rayleigh-Debye-Gans) and instrument calibration with known size standards are essential for result accuracy.