Rheology and Viscosity
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Transcript of Rheology and Viscosity
7/26/2019 Rheology and Viscosity
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Adhitya Pitara Sanjaya, S.T.P., M.Sc.
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Have we found “rheology ” in daily life?
We eat breakfast, perhaps using a range of
spreads for toast, or perhaps we eat yoghurt
with pieces of fruit suspended in it.
We all have squeezed toothpaste tubes,
kneaded bread dough or tried to rub skin
lotion on our leg.
Rheology is simply one
way of describing those
sensations!!
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What is…err…Rheology?
In particular, rheology scienceaddresses fluid and structuralproperties of raw materials,intermediate products, ingredients,and final products of the food andpharmaceutical industries.
Consumers use subjective tests to
ascertain the perceived texture of
a product. Rheology is usefully used
to objectify these perceptions
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How do you like the sauce to flow? Liquid food productsshould be formulated to display desired rheological behaviour,
e.g., easy to pour from the bottle, but flow in controlled
manner and recover the viscosity upon pouring on the plate
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What happens when the
products are notcorrectly formulated
“rheologically”?
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But…Do I Really Need to Learn
Rheology?
“Rheological measurements can provide a rapid determination of product
quality and may serve as a tool for quality control”
For food technologists, knowledge of
rheology is important for a better
understanding of how process variables
influence specific textural characteristics,
such as pourability and mouthfeel.
Rheological measurements can aid in the
understanding of how the viscosity and
elasticity of foods are influenced by changes
in composition, processing, and storageparameters
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Definition of Rheology
“Rheology” comes from Greek rheos, meaning „to
flow
The Greek philosopher Heraclitus described rheology as
panta rei —
everything flows (if you wait long enough!) Rheology aims at measuring those properties of materials
that control their deformation and flow behaviour when
subjected to external forces
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Definition of Rheology
The subject of rheology is concerned with the study ofdeformation and flow of matter.
When subjected to external forces, solids (or truly elastic
materials) will deform, whereas liquids (or truly viscous
materials) will flow.
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FLOW OF MATERIAL
Newton’s Law of Viscosity Viscous Fluids
Newtonian Fluids
If a fluid's internal flow resistance is independent of the
external force – i.e. the shear rate - acting upon the fluid, it isideally viscous.
Typical Newtonian liquids are water or salad oil
Non-Newtonian Fluids
If a substance is not ideally viscous, its viscosity changes withthe shear rate.
For example yoghurt and shower gel show shear-thinning
behavior, while starch solutions show shear-thickening behavior.
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= rate of shear
= perbedaan kecepatan antara 2 bidang cairan yang
dipisahkan oleh jarak yang sangat kecil.
= shearing stress
=gaya per satuan luas yang diperlukan untuk menyebabkanaliran.
dr
dv
G
A
F F
'
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SISTEM NEWTON
Mengikuti hukum aliran Newton Makin besar viskositas suatu cairan, makin besar gaya per
satuan luas yang diperlukan untuk menghasilkan suatu rate
of shear tertentu.
Di mana : = koefisien viskositas ~ viskositas
G
F
dr
dv.
A
'F
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Viscosity
Viscosity is defined as the resistance of a fluid to flow. Theunit of dynamic viscosity is (Pa · s) in the SI system and
poise (g/cm · s) in the CGS system.
Satuan viskositas : poise = dyne detik cm-2
Cps (centipoise) = 0,01 poise
Fluiditas, , didefinisikan kebalikan dari viskositas
22
det
det
'
cm
dyne
cmcm
cmdyne
Adv
dr F
1
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Types of Viscosity (Dynamic viscosity)
Dynamic viscosity is known by the Greek symbolη, eta.
It is sometimes also referred to as shear viscosity. Dynamic
viscosity is obtained by reformulating Newton's Law.
The SI unit is pascal-second [Pa.s] or millipascal-second [mPa.s]:
1 Pa.s = 1000 mPa.s
Other commonly used units are poise [P] or centipoise [cP]:
1 P = 100 cP
Relation between units: 1 cP = 1 mPa.s
Dynamic viscosity is rather used in life science and research
applications.
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Types of Viscosity (Kinematic Viscosity)
a substance's flow behavior under the influence of Earth's gravity. It is
dynamic viscosity divided by density ρ, rho, which is defined as mass per
volume.
Kinematic viscosity is widely established due to historical reasons: Gravityas the driving force does not require any elaborate technical equipment. It
is simply available everywhere on earth.
Kinematic viscosity is mostly used to specify petrochemical fluids such as
fuels or lube oils.
Other commonly used units are stokes [St] or centistokes [cSt] 1 St = 100 cSt
1 cSt = 1 mm2/s
1.0 m2/s = 10000 Stokes = 1000000 Centistokes
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Types of Viscosity (Relative Viscosity)
Relative viscosity is an importantparameter when testing polymers in
solutions.
The relative viscosity is calculated by
dividing the viscosity of the polymersolution η by the viscosity of the pure
solvent ηs.
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Contoh soal :
Dengan viskometer Ostwald, didapat viskositas aseton
0,313 cp pada 250C. Kerapatan aseton (250C) = 0,788
g/cm3. Berapa viskositas kinematis aseton?
Jika diketahui viskositas air (250C) = 0,8904 cp. Berapa
viskositas aseton relatif terhadap air pada 250C?
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Factors Affecting Viscosity
Viskositas gas meningkat dengan naiknya temperatur Viskositas cairan menurun jika temperatur dinaikkan
Fluiditas kebalikan dari viskositas
Fluiditas cairan meningkat jika temperatur dinaikkan
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SISTEM NON NEWTON
Aliran plastis Aliran Pseudoplastis
Aliran Dilatan
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1. Aliran Plastis
Bingham Plastic Fluids
Kurva plastis tidak melewatititik (0,0), tetapi memotongsumbu shearing stress, dikenaldengan harga yield (yield value).
Toothpaste is a typicalexample of Bingham plasticfluid. It does not flow unlessthe tube is squeezed.
In food systems, mayonnaise,tomato paste, and ketchupare examples of this type offluid.
R a t e
o f s h e a r
Shearing stress
f
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Harga stress dibawah yield value, zat bertindak sebagaibahan elastis (seperti zat padat
Kemiringan rheogram disebut mobility ≈ fluiditas pada
sistem Newton.
Kebalikannya adalah viskositas plastis = U
G f F U f= yield value dalamdyne cm-2
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Contoh soal :
Suatu bahan plastis diketahui mempunyai yield value 5200dyne cm-2. Pada shearing stress di atas yield value, F
ditemukan meningkat secara linear dengan meningkatnya
G. Jika rate of shear 150 dt-1 pada saat F = 8000 dyne
cm-2, hitung viskositas plastis sampel tsb !
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2. Aliran Pseudopastis
Contoh : Na alginat, metilselulosa, CMC Na
Viskositas berkurangdengan meningkatnya rate
of shear (cairan menjadiencer)
Disebut shear thinningsystem ( sistem geser encer )
Contoh kecap atau saus
tomat yang untuk mengeluarkannya daribotol harus mengocoknyakuat-kuat.
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G'F N
'logFlog NGlog
• Eksponen N meningkat pada saat aliran
meningkat menjadi non-Newton
• N=1, alirannya adalah Aliran Newton
Persamaan Regresi Linear
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3. Aliran Dilatan
Suspensi tertentu (persentase zat padat terdispersi tinggi)
peningkatan daya hambat untuk mengalir dengan meningkatnya
rate of shear.
Volume meningkat dengan terjadinya shear disebut dilatan
Sistem disebut geser kental (shear-thickening) system
Suspensi dengan kandungan padatan >40-50%
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3. Aliran Dilatan
Istilah dilatan dikaitkan dengan meningkatnya volume Viskositasmeningkatdenganbertambahnyarateofshear
Mekanisme:
Pada keadaan diam partikel- artikel tersusun rapat dengan
volume antar partikel kecil Pada saat shearing stress meningkat bulk dari sistem
memuai meningkatkan volume kosong hambatan
aliran menigkat (tidak dibasahi) terbentuk pasta kaku.
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Pemilihan Viskometer
Semua viskometer dapat digunakan untuk menentukanviskositas sistem Newton dan hanya viskometer yang
mempunyai kontrol shear stress yang bervariasi dapat
digunakan untuk bahan-bahan Non Newton.
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Macam-macam viskometer
Visk. Kapiler Visk. Bola jatuh
Visk. Cup & bob
Visk. Kerucut dan lempeng
Sistem Newton
Sistem Newton
dan
Non Newton
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1. Viskometer kapiler
Disebut sebagai viskometer ostwald
Dasar : Hukum Poiseuille
Vl8
Ptr 4
(a) Viskometer Ostwald, (b) Ubbelohde, (c)
Ostwald-Cannon-Fenske
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Karena P tergantung pada kerapatan cairan (), maka :
P t K t K
22
11
2
1
t
t
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Contoh soal
Jika waktu yang dibutuhkan aseton untuk mengalir antarakedua tanda pada viskometer Ostwald adalah 45 detik,
untuk air adalah 100 detik (250C).
Diketahui kerapatan aseton 0,788 gram cm-3, kerapatan
air 0,997 gram cm-3 dan viskositas air 0,8904 cps. Berapa viskositas aseton ?
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2. Viskometer Bola Jatuh
Disebut viskometer Hoeppler
Prinsip :
Suatu bola gelas/besi jatuh ke bawah dalam suatu tabung
gelas yang hampir vertikal, mengandung cairan uji pada
temperatur konstan. Laju jatuhnya bola dengan dan tertentu adalah kebalikan fungsi viskositas sampel
tersebut.
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3. Viskometer ‘Cup’ and ‘Bob’
Prinsip :
Sampel di’shear’ dalam ruangantara dinding luar, ‘bob’ (rotor)dan dinding dalam mangkuk(‘cup’).
Viskometer Couette, mis : visk.Mac Michael
Mangkuk yang berputar
Viskometer Searle, mis : visk.
Rotovisco, visk. StormerRotor yang berputar
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Viskometer stormer
v
wK
v
Dimana :
Kv : Konstanta alatW : berat beban
V : rpm
v
wwK U f
v
Untuk aliran plastis
Dimana :
Wf : intersep yieldvalue dalam gram
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Yield valuef f wK f
)R
R
(log303,2
1x
60
2xK K
b
c
vf
Dimana :
Rc : jari-jari mangkok
Rb : jari-jari rotor
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Contoh soal : Suatu sampel gel dianalisis dengan viskometer Stormer
yang dimodifikasi. Berat w sebesar 450 gram menghasilkankecepatan rotor v 350 rpm. Suatu seri kecepatandiperoleh dengan menggunakan berat pengendali lainnya,diperoleh suatu rheogram aliran plastis. Intersep yieldvalue wf diperoleh dengan mengekstrapolasi kurvatersebut terhadap sumbu shearing stress di mana v = 0,wf = 225 gram. Konstanta alat Kv = 52,0 dan Kf = 20,0.Berapakah vikositas plastis dan yield value sampeltersebut?
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4. Viskometer Kerucut dan Lempeng
Contoh : viskometer Ferranti –
Shirley
Prinsip :
Kerucut dikemudikan motor
dengan kecepatan yang berubah-ubah, sampel dishear di antara
lempeng yang diam dan kerucut
yang berputar. Rate of shear : rpm
(dengan dial pemilih). Shearing stress: puntiran (dibaca pada skala
penunjuk).