7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 1/37

Adhitya Pitara Sanjaya, S.T.P., M.Sc.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 2/37

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!!

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 3/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 4/37

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 5/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 6/37

What happens when the

products are notcorrectly formulated

“rheologically”?

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 7/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 8/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 9/37

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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 10/37

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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 11/37

= 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 

  '

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 12/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 13/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 14/37

 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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 15/37

 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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 16/37

 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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 17/37

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?

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 18/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 19/37

SISTEM NON NEWTON

Aliran plastis Aliran Pseudoplastis

Aliran Dilatan

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 20/37

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 

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 21/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 22/37

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 !

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 23/37

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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 24/37

G'F N

'logFlog NGlog  

• Eksponen N meningkat pada saat aliran

meningkat menjadi non-Newton

• N=1, alirannya adalah Aliran Newton

Persamaan Regresi Linear

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 25/37

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%

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 26/37

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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 27/37

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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 28/37

Macam-macam viskometer

Visk. Kapiler Visk. Bola jatuh

Visk. Cup & bob

Visk. Kerucut dan lempeng

Sistem Newton

Sistem Newton

dan

Non Newton

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 29/37

1. Viskometer kapiler

Disebut sebagai viskometer ostwald

Dasar : Hukum Poiseuille

Vl8

Ptr 4

(a) Viskometer Ostwald, (b) Ubbelohde, (c)

Ostwald-Cannon-Fenske

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 30/37

Karena P tergantung pada kerapatan cairan (), maka :

 P t  K               t  K 

22

11

2

1

t

t

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 31/37

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 ?

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 32/37

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.

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 33/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 34/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 35/37

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

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 36/37

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?

7/26/2019 Rheology and Viscosity

http://slidepdf.com/reader/full/rheology-and-viscosity 37/37

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).