Movement and propulsion of colloidal chains under an AC

electric field:

Formation and Motion Analysis of Colloidal Particle

Chains Under an AC Electric Field

Conclusion: Using a method that we developed to seal and heat a colloidal

solution, we were able to successfully form colloidal chains that were flexible, but

not durable. Additionally, we developed a mass production plate which allowed us

to produce exponentially more chains, and extract the solution. We also observed

partially correlated motion within chains when the entire chain acted under the

influence of Brownian motion.

William Trevillyan, Kelley Heatley, Dr. Ning Wu

Introduction: Research on formation and motion of colloidal particle chains

under an AC electric field is being conducted to develop micro-robotic technology

that can potentially be used in drug delivery, micro-surgery, and active sensors.

The final goal is to understand the formation of colloidal chain systems,

characterize them, and probe the fundamental mechanisms of chain propulsion

under AC electric fields.

A “Colloidal dispersion” consists of the solvent and the particles suspended in

the solvent. Colloidal particles range in size from a few nanometers to a few

micrometers. When present under a parallel electric or magnetic field, the particle

gains an induced-dipole similar to a bar magnet. Dipolar interactions between the

particles make them attract towards each other forming long chains in the

direction of the applied field. After chains have been formed, we can apply a

perpendicular AC electric field. An electro-hydrodynamic (EHD) Flow will be

induced surrounding the chain and possibly cause it to propel. This flow is caused

by the movement of cations and anions in the solution being attracted to the

oppositely charged electrodes. When there is an imbalanced EHD flow

surrounding a particle or chain, the fluid induces a force on the particle or chain

causing it to propel. When under the influence of this force, chains oscillate or

propel just like an Earth worm or a snake as seen in nature.

Future Work: We will run experiments regarding the formation of flexible,

durable chains in an electric or magnetic field while changing many variables: salt

concentration, PVP concentration, field strength, frequency, particle concentration,

and the process of cross-linking polymerizations. We will be modifying the mass

production plate procedure in hopes of reducing air bubbles in the sealed system.

Additionally, we will develop a mathematical model for the further understanding of

EHD flow affects on the chain behavior under AC electrical fields.

Figure 5: Capillary TubeTension is placed on the two

copper wires that line the walls of

the capillary tube. This is the first

of two systems for producing

colloidal chains under an AC

electric field.

References:“Inducing Propulsion of Colloidal Dimers by Breaking the Symmetry in Electrohydrodynamic Flow” F. Ma, X. Yang, H. Zhao, and

N. Wu, PRL 115, 208302 (2015) “Electric-field–induced Assembly and Propulsion of Chiral Colloidal Clusters” F. Ma, S. Wang, D.T. Wu, and N. Wu, PNAS 112,

6307-6312 (2015)

Acknowledgements:I would like to thank Dr. Ning Wu who has lent much insight, along with CSM’s

Department of Chemical and Biological Engineering and the

National Science Foundation for supporting my REU experience.

Figure 2: During FormationFigure 1: Before Formation

Figure 3: Heating UnitA heat gun heats the inside of the container. The large volume container maintains a consistent

temperature as hot air flows into the smaller container atop the microscope containing the

sealed capillary or the sealed mass production plate. Heating a solution of particles under the

influence of a field causes the physical bond between two particles to strengthen.

Figure 7: Magnetic Chain Formation SystemAn magnetic field is applied around seven different magnetic colloidal particle solutions as an air

gun cools the electromagnet.

EHD Flow Diagram:

E0e-iwt

+ + + ++ + + +

+++ +

---

-+ +++ +- - - - - -- - -

- - -- - -

--

+ + + + + ++ + +

-

--

+ + ++ + ++ + + - - - - - -- - -

100 μm ITO Glass

Slide

Chain Formation Under an AC Electric Field:Typical Experimental Condition:

Field Condition: 20Vpp (amplified 14x), 100kHz

Temperature: 70-80°C

Particle Solution: 2 wt% 4 µm Sulfonate-functionalized Polystyrene Particles

1 wt% Polyvinylpyrrolidone (Mw=1.3M) & Sodium Chloride (1mM)

Chain Formation Under a DC Magnetic Field:Typical Experimental Condition:

Field Condition: 300 Gauss

Temperature: 75°C

Particle Solution: 2.8 µm Epoxy Magnetic Particles

1 wt% Polyvinylpyrrolidone (Mw=1.3M) & Potassium Chloride (0.01M)

Figure 6: Mass Production PlateThis is the second system for producing colloidal chains

under an AC electric field. Two glass slides were sealed

together using nail polish after the colloid solution was

added. In between each of the glass slides lies a layer

of alternating electrodes. The picture on the right

shows air that was trapped beneath the electrode

leaking in to the main solution.

Characterization of Chains without Applied Field:

Bond angles are strongly correlated during chain propulsion.

Bond angles are weakly correlated.

Non-propulsive Five-bead Chain

Frame 1 Frame 31 Frame 61 Frame 91

Propulsive Four-bead Chain

Frame 8 Frame 33 Frame 54 Frame 77

Some bond angles are strongly correlated.

Brownian Motion of Six-bead Chain

Frame 1 Frame 21 Frame 41 Frame 61 Frame 81 Frame 100 Frame 121 Frame 141

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Figure 4: After

Formation