TRIBOELECTRIC PHENOMENA IN PARTICULATE MATERIALS

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TRIBOELECTRIC PHENOMENA IN PARTICULATE
MATERIALS - Role of Particle Size, Surface
Properties, and Vapor -
Scott C. Brown1 Team Yakov Rabinovich1, Jennifer
Curtis2, Jan Marijnissen3 1Particle Engineering
Research Center, 2Department of Chemical
Engineering University of Florida 3University
TU-Delft, The Netherlands
Center for Particulate Surfactant Systems
(CPaSS) IAB Meeting Gainesville, FL August 20,
2009
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Industrial Relevance
Triboelectric charging is a persistent challenge
to powder processing industries.


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SAFETY HAZARDS
SEGREGATION
AEROSOLIZATION
(e.g., dust explosions)
Mehrotra, PRL 99, 058001 (2007)
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Industrial Relevance (Contd.)
Tribocharging behavior subsequent interactions
for dielectric particulate materials are poorly
understood.

• Charge generation (e.g., tribocharging appears to
  occur even between apparently identical
  materials)
• Image Forces (e.g., existing theories always
  predict strong adhesion at close separation
  distances, regardless of charge then how do
  charged dielectric powders sometimes
  self-levitate from particle beds? theories and
  experiments are lacking)
• Discharge Behavior (e.g., influence of contact
  duration on charge transfer between dielectric
  materials)
• Vapor interactions (e.g., humidity inconsistently
  mitigates electrostatic charge)

Inability to predict and fully control
triboelectric phenomena in powder processes
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Goals of the Project

• To acquire fundamental knowledge of the role of
  particle properties and vapor content on contact
  and triboelectrification phenomena

Influence of

• Particle Size
• Surface Properties (i.e., chemistry, roughness)
• Vapor (i.e., of differing chemistries)

on

• Charging
• Image Forces
• Discharge Behavior

• To identify simple techniques for mitigating
  tribocharging.

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Research Methods/ Techniques

• Use Atomic Force Microscopy (AFM) to understand
  tribocharging phenomena at the single particle
  and subparticle level.

Advantages of AFM Methods

• Precise control of frictional engagements
• Capacity to image charge distributions on
  particles/surfaces.
• Ability to quantify triboelectric charging, image
  force interactions and discharge properties in a
  single experiment.
• Ability to precisely control local environment
  (e.g., humidity, temperature, other vapor content)

- Conductive tips for charge mapping and
monitoring charge diffusion and dissipation.
- Colloidal probes for controlled tribocharging
experiments and interaction force measurements.
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Research Methods/ Techniques - Tribocharging

• Frictional engagement followed by normal force
  measurement to determine
• developed charge
• image force contribution
• contact mediated charge dissipation

Surface imaged post-rastering with an electrified
tip via electrostatic force microscopy (EFM) or
Kelvin Probe Microscopy (KPM)

• charge mapping
• charge diffusion
• charge dissipation

Applied bias to cantilever (static for EFM,
Sinusoidal for KPM) Monitored in non-contact mode
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Results - Tribocharging
System Silica Particle Polystyrene Surface
Rastered 1um at 50 nN load for 10 cycles
Prior to rastering
Charges developed by frictional engagement can be
dissipated by normal collisions.
30 min
1 min
(40 humidity)
Water vapor increases charge diffusion and
dissipation
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Research Methods/ Results Electro/capillarity

• Vapor mediated charge dissipation appears to be
  more rapid when the formation of an liquid
  annulus (capillary bridge) occurs.
• charge mitigation occurs over seconds to a few
  minutes

• Kinetics of capillary formation in
  non-electrified systems not well investigated (2
  instances of experimental data found Kohonen et
  al. 1999 Xu et al. 1998)
• Initiated fundamental research on the impact of
  surface contact time with capillary force
  development
• Developed an equation relating annulus radius to
  capillary force
• Applied a Langmuir diffusion model validated for
  large cm sized objects (Kohonen et al. 1999 Butt
  et al. 2009)

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Capillary Formation Dynamics Deviations
T temperature ? humidity RMSfit RMS Surface
Roughness
? surface tension req Eq. Meniscus
radius k Boltzman constant T Contact angle

Theory
Experiment
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System

• 5 - 7 micron silica colloidal probes
• Opposing silica surface

• Experimental characteristic time is by five
  orders larger than predicted by theory for each
  humidity
• Similar deviation seen when analyzing the
  independent data of Xu et al. and comparable data
  obtained using silicon AFM tips.

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Summary
Preliminary experiments demonstrate

• Capacity to use colloidal probe AFM and EFM to
  study and quantify triboelectification processes
• Capillary formation leads to enhanced charge
  dissipation in vapor environments.
• Time scales of capillarity are order of magnitude
  larger than previously believed.

Future Directions
Timeline
Effect of Particle Size
Effect of Surface Chemistry
Effect of Surface Roughness
Inert
Vapor based Mitigation
Year 2
Year 1
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Outcomes/ Deliverables

• Fundamental information on the influence of
  particle size, surface chemistry, surface
  roughness, and vapor phases on triboelectric
  charging / discharging
• Materials and methods for low level vapor induced
  charge mitigation
• Empirical (potentially fundamental) models
  describing the triboelectrification behavior and
  electrostatic interactions between dielectric
  particles.

• Results are anticipated to be used for the
  development of powder flow models to predict
  segregation etc. in a future project.

Acknowledgements
Industrial members of the Center for Particulate
and Surfactant Systems for your support