The correct answer is Vibrational granular manipulation.

- In microfluidic devices, the separation and mixing of dry granular substances pose unique challenges compared to liquids or suspensions.
- Unlike continuous fluids, dry granular materials require methods that can physically agitate or mobilize the particles to achieve effective mixing and separation.

- Vibrational granular manipulation is commonly employed because it uses controlled vibrations to induce movement and rearrangement of granular particles within micro-scale channels.
- This method leverages the mechanical energy to overcome inter-particle friction and adhesion forces, facilitating the segregation or blending of different granular components.

- Electrokinetic manipulation is primarily effective for charged particles suspended in fluids, using electric fields to drive their motion, and is less suitable for dry granular substances.
- Magnetophoretic sorting relies on magnetic properties to separate particles, but only works if the materials are magnetic or can be functionalized accordingly.
- Hydrodynamic focusing aids in aligning fluid streams but is not designed for dry particulate matter.

Therefore, the mechanical vibrations provided by vibrational granular manipulation offer a direct and practical approach to mix and separate dry granular substances within microfluidic systems.

Reference: Microfluidics and Nanofluidics Handbook, Volume 1, Chapter 5: Particle Manipulation Techniques, p. 120-135