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Category: Particle Manipulation

Particle Manipulation denotes the physical technologies and microfluidic mechanisms—including dielectrophoresis (DEP), acoustic radiation forces, optical tweezing, thermophoresis, and inertial Dean flow—employed to focus, concentrate, separate, and trap micro- and nanoparticles in continuous flow.

Foundational Pillars
Principles & Methods
Topic Directory
Frequently Asked Questions

Foundational Pillars (Start Here)

NANOPARTICLE CONCENTRATION

Microfluidics Nanoparticle Concentration: Techniques Revealed

Electrokinetic focusing, membrane micro-filtration, and acoustic levitation for concentrating dilute sub-micron colloid suspensions.

INERTIAL SPIRAL SEPARATION

What Does a Microfluidic Spiral Do: Separation Mechanism Explained

Inertial lift forces balancing Dean secondary vortex drag in curved channels, enabling continuous high-throughput size-based cell sorting.

DIELECTROPHORESIS (DEP)

Frequency Optimization Dielectrophoresis: Separation Enhanced

Clausius-Mossotti factor frequency sweeps, crossover frequency tuning, and positive vs negative DEP particle sorting.

Electro-Acoustic and Inertial Force Balances

Continuous particle manipulation balances opposing hydrodynamic drag and non-contact body forces. In Dielectrophoresis (DEP), non-uniform AC electric fields induce dipole moments on neutral particles: F_DEP = 2π · ε_m · r³ · Re[K(ω)] · ∇|E|², where ε_m is medium permittivity, r is particle radius, E is electric field, and K(ω) is the Clausius-Mossotti factor. When polarizability exceeds the suspending medium (Re[K] > 0), particles experience positive DEP (pDEP), pulling them toward electrode edges; when Re[K] < 0, negative DEP (nDEP) repels them.

Manipulation MechanismPrimary Driving FieldParticle Size ScalingThroughput vs Selectivity Profile
Dielectrophoresis (DEP)Non-uniform AC electric field (∇|E|²)F ∝ r³ (scales with volume)High selectivity based on dielectric membrane properties; moderate throughput (< 1 mL/hr)
Acoustic Radiation Force (ARF)Ultrasonic standing wave (∇⟨U_rad⟩)F ∝ r³ · Φ (density/compressibility contrast)High biocompatibility, contact-free; excellent throughput for cell and bead sorting
Inertial Dean VorticesHydrodynamic inertia in spiral microchannelsF_L ∝ r⁴ vs F_Dean ∝ rUltra-high volumetric throughput (> 100 mL/hr); size-based separation without external fields
Thermophoresis (Soret Effect)Microscale thermal gradient (∇T)F ∝ -S_T · ∇TSub-100 nm nanoparticle and biomolecular concentration without shear stress

In spiral microfluidics, fluid inertia at moderate Reynolds numbers (10 < Re < 100) couples with centrifugal forces, generating two counter-rotating Dean vortices across the channel cross-section. The balance between net inertial wall lift forces (F_L ∝ r⁴) and Dean drag forces (F_D ∝ r) forces particles of distinct diameters into unique equilibrium lateral streamlines, separating blood cells from cancer cells at liters-per-hour flow rates.

Topic Directory & Methodologies

Microfluidic Concentration & Inertial Sorting

Acoustic, Electrical & Paper Manipulation

Frequently Asked Questions

What is the Clausius-Mossotti factor and why does it change with frequency?

The Clausius-Mossotti factor K(ω) = (ε_p* – ε_m*) / (ε_p* + 2ε_m*) represents the complex effective polarizability of a particle relative to its suspension medium. At low frequencies, surface conductivities dictate polarization; at high frequencies, permittivities dominate. The frequency where Re[K(ω)] flips from positive to negative is the crossover frequency, enabling selective separation.

How does acoustic radiation force scale with particle diameter?

Acoustic radiation force scales directly with particle volume (F_rad ∝ r³). Consequently, doubling particle diameter increases acoustic steering force eightfold, allowing acoustofluidic chips to easily separate 15 μm circulating tumor cells from 7 μm white blood cells and 2 μm platelets.

Why can spiral microchannels sort particles without using electrical or acoustic energy?

Spiral channels exploit passive hydrodynamic forces: the curvature of the channel induces a centrifugal pressure gradient driving counter-rotating Dean vortices, while fluid velocity shear induces inertial wall lift forces. Because lift force scales with r⁴ and drag scales with r, particles equilibrate at distinct size-dependent radial positions.

What is thermophoresis in nanoparticle manipulation?

Thermophoresis (the Soret effect) describes the directed migration of suspended particles or macromolecules along a temperature gradient (∇T). Most nanoparticles migrate away from hot regions toward colder zones due to solvent-particle interfacial free energy minimization.