Emulsions and Foams.ppt

7/25/2019 Emulsions and Foams.ppt

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Properties of Emulsions and Foams

FDSC400


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Goals

Properties of emulsions Type

Size Volume fraction

Destailization of emulsions Creamin!

Flocculation Coalescence

Foams


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Emulsion

" fine dispersion of one li#uid in a

second$ lar!ely immiscile li#uid% &nfoods t'e li#uids are ine(italy oil

and an a#ueous solution%


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Types of Emulsion

)il*in*+ater emulsion ,ater*in*oil emulsion

,ater

)il

m


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-ultiple Emulsions

,ater*in*oil*in*+ater emulsion )il*in*+ater*in*oil emulsion

,ater

)il

m


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Emulsion Size

. 0%/ m

0%/*%/ m

%/*1 m

21 m


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3umer Distriutions

.0%/

m

0%/*%/

m

%/*1

m

21m

3u

m

er Very fe+ lar!e

droplets contain

most of t'e oil


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Median

Po

lydis

persit

y

Large droplets

often contribute

most to

instability

(VolumeinclassTotalv

olumemeasured)

3ote lo! scale


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Volume Fraction

Total (olume of t'e dispersed p'ase

Total (olume of t'e system

Close packing, max-onodisperse

&deal 50%67

8andom 50%/

Polydisperse

-uc' !reater


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ViscosityViscosityViscosity is t'e force

re#uired to ac'ie(e unit

flo+ rate

Force 93

D

istance9m

3o slip att'e +all

-a:imum induced flo+ rate 9ms*

S'ear rate 9s*

Forceperunitarea93m*;

Slop

e(isc

osity

93m

*;s


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Emulsion Viscosity

Emulsion dropletsEmulsion dropletsdisrupt streamlinesdisrupt streamlines

and require moreand require more

effort to get theeffort to get the

same flow ratesame flow rate

/%;0+=

Viscosity of emulsion

Continuous p'ase (iscosity

Dispersed p'ase(olume fraction


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C'emical Composition

&nterfacial layer% Essential to

stailizin! t'e emulsion

)il P'ase%


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Emulsion Destailization

Creamin!

Flocculation

Coalescence

Comined met'ods


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Creamin!=uoyancy

>"rc'imedes?

Friction>Sto@es*Einstein?

dv1=

c

sgdv

A

;

=

Continuous p'ase (iscositydensity difference

! "cceleration due to !ra(ity

ddroplet diameter

( droplet terminal (elocity

(sSto@es (elocity

=6

1gdFb


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Flocculation and Coalescence

Film

ruptu

re

8e'om

o!enization

Collisionandstic@in!>reaction?

Stirorc'an!ec'emicalconditionsF


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8'eolo!y of Flocculated

Emulsions

Flocculation leads to an

increase in (iscosity ,ater is trapped +it'in

t'e floc and must flo+

+it' t'e floc

Effecti(e (olume fraction

increased

r!


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Gelled Emulsions

T'in li#uid Viscous li#uid Gelled solid


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Creamin! Sli!'t Flocculation

Flocs 'a(e lar!er

effecti(e size

Smaller Tend to cream muc'

faster


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Creamin! E:treme Flocculation

ea(ily flocculated

emulsions form a

net+or@ Solid*li@e properties

>!el?

Do not cream >may

collapse after la!

period?


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Foams

Concentrated

Dilute


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Dilute Foams

Some+'at similar to emulsions

Various modes of formation

5mm? sp'erical ules

Very fast creamin!

)st+ald ripenin!


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Concentrated Foams

Distorted non*

sp'erical !as cells

Very 'i!' (olumefraction$ often 277


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Foam Draina!e

,ater drains from

foam under !ra(ity

"s +ater lea(es$ facesof film are rou!'t

closer to!et'er


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Film 8upture

Film must t'in t'en urst

&n'iited y surfactant repulsion9interfacial

film

Self*repair y t'e Gis*-aran!oni effect

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Emulsions and Foams.ppt