Mechanisms of charge transfer between colliding ice particl - PowerPoint Presentation

Slide1

Mechanisms of charge transfer between colliding ice particles in thunderstormsBaker and Dash 1994

Karly Reimel

ATS 780

March 9, 2017Slide2

Baker et al. 1987Looked at charge transfer to a rimed rod moving through a cloud in which liquid water, temperature, and collision characteristics were varied over ranges believed to be present within thunderclouds

Results:

Significant charge transfer occurred during rebounding collisions between ice crystals and soft hail when there were supercooled drops present

Significant charge transfer occurred only if both particles were growing from vapor

The charge transferred to the riming surface tended to be positive at high liquid water contents and high temperatures, or at low liquid water contents and low temperatures

Conclusions:

Sign of charge transfer depends on the relative vapor growth rates of colliding particles

Particles growing fastest through vapor deposition appear to acquire positive charge during rebounding collisions

Problems with laboratory experiments

Experiments were carried out in room air under conditions that did not allow for observation of fracturing or collisions among particles

Assumptions about temperature and supersaturation gradients at the active surface are made during the time in which charge separation occurs

Experimental conditions and simulations have varied greatly and have resulted in many different proposed mechanisms for charge transferSlide3

Theory of ChargingWhat are the charge carriers?Disassociated hydronium and hydroxyl ions in the QLL

Charge distribution is not uniform

What drives the flow of charge during collisions?

Charging of an ice particle in a pure vapor environment occurs when it loses or gains QLL fluid carrying net charge

Charge separation results from mass transfer during collision

The direction of mass flow, and thus the polarity of the charge flow is determined by differences in chemical potentials between the colliding particles

Why is charge transfer correlated with environmental thermodynamic conditions?

The chemical potential of the QLL in a given location depends on the local surface curvature and local thermodynamic conditions

Mass flow direction and direction of charge flow depend on local conditionsSlide4

Gibbs free energy of the system:

:

chemical potential

N

:

number of moleculesA : area

Limitations:

effective coefficient of the dry solid-vapor interface

(a)

(b)

To satisfy (a) and (b):

Where:

f(h)

 0 when h  0 f(h)  1 when h 

Assumptions:For very thick QLL layers, the QLL has properties of bulk water

Gibbs free energy of the system for all h:Slide5

At equilibrium:

G is a minimum with respect to transformations at a constant temperature and pressure

Allows the transfer of a few molecules from one part of the system to another

Equilibrium properties of the QLL with respect to flat interfaces:

Assumptions:

Results:

 Slide6

At equilibrium:

Now setting the change in G associated with the transfer of a few molecules from the QLL to vapor equal to zero:

Approximate the chemical potentials:

Put it all together:

Conclusions

:

When a flat interface is in equilibrium, the vapor pressure over the system is that predicted by the Clausius Clapeyron equation at the flat ice-vapor interface, and is less than that over a flat bulk liquid water surfaceAs temperature increases and the equilibrium value of h increases, the equilibrium vapor pressure reaches that predicted by the Clausius Clapeyron equation for a flat liquid water surface– only reaching this value at the triple pointSlide7

Effect of surface curvature:

Assume ice particle is a sphere of radius R

Assume h/R << 1

Using the same algebra used for the equilibrium properties of the QLL over a flat surface:

For the liquid-vapor equilibrium:

The equilibrium value of the layer thickness to the first order:

Conclusions:As the radius of curvature decreases, the equilibrium vapor pressure increases, the equilibrium QLL thickness increases, and increases  Slide8

Mass TransferConsider two particles approaching one another with different chemical potentials, due to a combination of temperature and curvature effectsDiffering chemical potentials lead to differing vapor pressures and QLL thicknesses

As particles come in close proximity to each other, there is a net flow of vapor that helps reduce the difference in chemical potentials, flowing from the thicker to thinner film

Once the particles contact each other, there is a rapid equilibrating of any remaining difference in the chemical potentials

As particles move away from each other, the bridging liquid between the particles breaks and leave each particle with a fraction of the combined thickness

Mass transfer tends to be from the particle with locally warmer and/or higher curvature surfaces

Volume and thus charge transferred depend on the impact velocity between the particles, but to a weaker extent than found in laboratory studiesSlide9

Charge TransferIf the transferred liquid has a net electrical charge, mass transfer will result in charge transfer

The amount of charge flow depends on the net charge density within the flowing liquid

Past Studies:

Particles were exposed to room air in the simulations, introducing impurities that could lead to preferential absorption of ions of a specific sign into the ice

Currently it is not possible to unambiguously determine the extent of the charge segregation inside of the QLL

Must estimate the charge density required to explain charge simulation experiments

Magnitude of - the charge transferred per collision- is on the order of tens of fC Slide10

Let the surface charge density at the liquid-ice interface be:

: magnitude of electron charge

molecular area ( 5 x 10

20

m

2

)  Assuming the surface charge is positive, this results in |e|/a of about 10 C/m2The negative charge will vary throughout the layer with a characteristics relaxation distance:  If , then we can approximate the volume density of the negative charge in the QLL:

Assuming and

Results:a 1% difference in QLL thicknesses and a charge density of 1 electronic charge per 10 surface molecules (

=0.1) results in 10

-15 C <

< 10

-12 CThese results are of the order of magnitude of charge transfers found in laboratory studies

This surface charge induces a distributed charge of opposite sign with volume density

where: Slide11

Conclusions:Charge transfer during collisions is associated with mass transfer between ice particlesOur current understanding of surface melting leads to flow of surface material from:

Warm to cold

H

igh surface curvature to low surface curvature

High vapor growth to lower growth or evaporation

Observed trends in the dependence of charge transfer on environmental conditions support the charging mechanism described in this study if the fluid is charged negatively