TRIBOELECTRIC PHENOMENA IN PARTICULATE MATERIALS - Role of Particle Size, Surface Properties, and Vapor - Scott C. Brown 1 Team: Yakov Rabinovich 1, Jennifer Curtis 2, Jan Marijnissen 3 1 Particle Engineering Research Center, 2 Department of Chemical Engineering University of Florida 3 University TU-Delft, The Netherlands Center for Particulate & Surfactant Systems (CPaSS) IAB Meeting Gainesville, FL August 20, 2009

Industrial Relevance Triboelectric charging is a persistent challenge to powder processing industries SEGREGATIONAEROSOLIZATION SAFETY HAZARDS Mehrotra, PRL 99, (2007) (e.g., dust explosions)

Industrial Relevance (Contd.) Tribocharging behavior & subsequent interactions for dielectric particulate materials are poorly understood. Inability to predict and fully control triboelectric phenomena in powder processes 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)

Particle Size Surface Properties (i.e., chemistry, roughness) Vapor (i.e., of differing chemistries) on: Charging Image Forces Discharge Behavior Influence of: To acquire fundamental knowledge of the role of particle properties and vapor content on contact and triboelectrification phenomena; Goals of the Project To identify simple techniques for mitigating tribocharging.

Research Methods/ Techniques 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) Advantages of AFM Methods - Conductive tips for charge mapping and monitoring charge diffusion and dissipation. - Colloidal probes for controlled tribocharging experiments and interaction force measurements. Use Atomic Force Microscopy (AFM) to understand tribocharging phenomena at the single particle and subparticle level.

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) Applied bias to cantilever (static for EFM, Sinusoidal for KPM) Monitored in non-contact mode ─ charge mapping ─ charge diffusion ─ charge dissipation

Results - Tribocharging Charges developed by frictional engagement can be dissipated by normal collisions. System: Silica Particle – Polystyrene Surface Rastered 1um at 50 nN load for 10 cycles Prior to rastering Water vapor increases charge diffusion and dissipation 1 min 30 min (40 % humidity)

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)

Capillary Formation Dynamics – Deviations Theory Experiment 66% 56% 45% System: 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. γ – surface tension r eq = Eq. Meniscus radius k – Boltzman constant Θ – Contact angle T – temperature Ψ – humidity RMS fit – RMS Surface Roughness

Summary Future Directions 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. Timeline Effect of Particle Size Year 2 Year 1 Effect of Surface Roughness … Inert Vapor based Mitigation Effect of Surface Chemistry

Acknowledgements Industrial members of the Center for Particulate and Surfactant Systems for your support 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. Outcomes/ Deliverables