Toni atmosphere lecture 8 Flashcards
(26 cards)
How do aerosols form from a pressurised container
When fluid is released from a pressurised container, the sudden drop in pressure causes bubbles to form and burst, producing droplets. As the droplets move into the air, the volatile components evaporate very quickly, leaving behind much smaller particles mostly made of the active ingredient.
Describe evaporation and condensation processes after it has came out of the pressurised container
In delivery of aerosol, first stage is loss of volatiles, both the propellant and any solvent. This leads to a residual particle size. But the aerosol can often change in response to environment, e.g. hygroscopic growth of the aerosol in the respiratory tract leads to a growth in particle size.
Define atomisation
Atomisation is the process of breaking up a bulk liquid into tiny droplets
What does evaporation of droplets depend on
mass transfer and heat transfer
What is mass transfer
Mass transfer (how quickly molecules escape the droplet)
What is heat transfer
Heat transfer (how quickly heat flows into or out of the droplet)
What happens after atomisation
Droplets then:
Collide, coagulate,Break up further
Evaporate, Undergo chemical reactions (e.g. in combustion)
Once atomised, droplets evolve rapidly due to physical and chemical processes.
What is gas particle partitioning
Gas-particle partitioning is the process where a gas in the air moves into (or out of) an aerosol particle or droplet.
What is gas particle equilibrium
Gas-particle equilibrium happens when the rate at which gas molecules enter a particle equals the rate at which they leave.
what does the characteristic timescales for gas particle equilibrium mean
how long it takes for gases to dissolve into aerosol droplets
Give examples of a low solubility gas
ozone, oxygen carbon monoxide and nitrogen
Give examples of high solubility gases
nitric acid, ammonia, HCl
What does characteristic timescales for gas for aqueous phase diffusion mean
how long it takes for a gas, after it has entered a droplet, to spread out evenly inside
What does the timescale for aqueous phase diffusion depend on
The time depends on droplet size and how easily the gas moves through the liquid.
What is a heterogeneous reaction in terms of particle gas reactions
When the gas enters the droplet it may react with the molecules already in the droplet. This changes what’s in the droplet it ages the particle and might even change its properties
What properties might change when there is a heterogeneous
Its size
Its stickiness
How it reflects light
How long it stays in the air
What happens when a droplet evaporates
Mass transfer: Molecules of liquid leave the droplet and become vapour in the surrounding air.
Heat transfer: Because evaporation uses up energy (the droplet cools down so it needs to absorb heat from the surrounding air to continue evaporating.
What is unsteady evaporation
Unsteady evaporation is when a droplet has just formed and is rapidly changing — it shrinks quickly, cools down, and the air around it hasn’t stabilised yet. Both the evaporation rate and the heat transfer are shifting over time, so the process is chaotic and not balanced
What is steady evaporation
Steady evaporation is when a droplet has been in the air long enough for the conditions around it to stabilise. The rate of evaporation and the heat gained from the surroundings reach a balance, so the droplet shrinks at a constant rate and its temperature stays steady.
What is quasi-steady theory
Quasi-steady theory assumes that although conditions around a droplet are slowly changing over time, they are changing slowly enough that we can treat them as momentarily steady at each point
What is the experimental challenge in studying mass and heat transfer
when a droplet evaporates quickly, the surface can dry faster than the inside can keep up, causing solutes to accumulate and form a solid shell or “skin” around the droplet
What is the maxwell equation used to calculate
used to calculate how fast a droplet evaporates when conditions are in steady state
What does the Maxwell equation help us predict
How fast a droplet loses mass
How long a droplet will last
How vapour pressure, temperature, and diffusion affect evaporation
What is the diffusional gradient
Diffusional growth is the process by which a droplet or particle increases in size as vapour molecules from the surrounding air diffuse toward it and condense on its surface.
