Electronic for Chemists_Goniometer

This ebook, 'Goniometer', part of the 'Electronic for Chemists', provides a practical guide to designing a functional scientific instrument for light scattering analysis. Combining theoretical depth with engineering practicality, it offers a comprehensive framework for measuring the size and structure of particles in colloidal systems.

It begins with a detailed examination of electromagnetic wave scattering and distinguishes between the three main regimes: Rayleigh (D/λ ≪ 1), Mie (D/λ ≈ 1) and Fraunhofer (D/λ ≫ 1). These models establish the relationships between scattering intensity, observation angle, particle diameter and refractive index. Particular emphasis is placed on Mie theory, which enables the precise characterisation of particles in the micrometre range through angular intensity distributions.

Building on this foundation, the goniometer is a measurement methodology based on angular scanning. It records light intensity as a function of scattering angle, enabling structural information to be extracted from diffraction patterns or continuous scattering curves. The analysis incorporates polarisation effects through the Clark–Jones formalism, enabling differentiation between the parallel and perpendicular components of scattered light.

The book's core contribution lies in the complete design of a compact goniometer. This system incorporates a 650 nm laser source, dual photodiodes operating in a balanced detection configuration to minimise noise and a NEMA 17 stepper motor that provides controlled angular resolution via microstepping. The mechanical structure is optimised through 3D printing, with a 5.3 cm arm length that balances precision (resolution of approximately 3%) and stability. An Arduino-based system handles signal acquisition, automating angular scanning, data averaging, and real-time display.

The performance of the instrument, which is built using 3D-printed components, is validated experimentally. Calibration using CD and DVD diffraction patterns shows a high level of agreement between the theoretical and measured angles. Further studies include analysing periodic biological structures, such as feather microstructures, and characterising soybean emulsions. Here, particle diameters of around 20 µm are inferred from scattering profiles.

Beyond the technical implementation, the book illustrates how advanced measurement systems can be developed at a low cost less than 40€ by leveraging accessible components, open-source tools and interdisciplinary knowledge.