Fu u e o elec o echnics: e o luids 9
FUTURE OF ELECTROTECHNICS: FERROFLUIDS
D. Maye
Uni e si y o Wes Bohemia, Facul y o Elec ical Enginee ing, Uni e zi ni 26, 306 14 Plzen. maye @.zcu.cz
Summa y: Magne ic liquids enabled de elopmen o new de ices and echnologies ha a e a use ul al e na i e o he
exis ing ones. Many o hese applica ions a e s ill in p og ess and do no ep esen any b eak- h ough disco e ies ye .
Ne e heless one may expec ha owing o hei ema kable quali ies magne ic luids will become in he u u e a pa o
o iginal p ojec s. The esea ch o magne ic liquids has a s ongly mul idisciplina y cha ac e . I is hus desi able o
echnicians o di e en specializa ions o o he specialis s (such as physicians, biologis s, pha macis s e c.) o be acquain ed
wi h he quali ies and exis ing applica ions o hese pe spec i e ma e ials.
1. INTRODUCTION
In he 19
h
cen u y some physicis s (M. Fa aday, T.
J. Seebeck and o he s) e i ied hei p esump ions o
he quali ies o he magne ic ield using liquids whe e
ine me al dus was dissol ed. A disad an age o his
en i onmen was i s ins abili y - unde he in luence
o g a i y he dus ended o se le down. A e mo e
han 100 yea s physicis s eopened he issue and
ound ou ha hese liquids can be s able i he dus
pa icles a e e y ine. Wi h ex emely ine
e omagne ic pa icles hei sedimen a ion occu s
a e a longe ime. In hese liquids he magne ic
iscosi y phenomenon was disco- e ed i means ha
when hey a e exposed o he magne ic ield hei
iscosi y inc eases. These liquids we e labeled as
magne ic heological. Soon p oduc ion echnology
was de eloped, hei physical - chemical quali ies
we e examined and hei applica ions we e sea ched
o in echnical, medical and biochemical p ac ice.
The 1
s
pa en s o using e o luids we e gained by
Jacob Rabinow [13] in 1940.
In he 50s and 60s o he 20
h
cen u y esea ch in
e o luids was emba goed. Since 1970s he
knowledge was disclosed and many pape s in jou nals
and books ha e been published. Many in e es ing and
use ul applica ions ha e been ealized and pa en ed.
Complex ma hema ic-physical heo ies ha e been
desc ibed ha p o ide in o ma ion abou s uc u e
and beha io o e o luids in s a iona y and dynamic
s a e. The solu ion o magne ic ields (elec o-
magne ic, he mal powe e c.) is di icul in sys ems
con aining e o luids as hey a e in a e y non-linea
and aniso opic en i onmen . Al hough many
ma e ials we e published on e o luids, he
de elopmen and mainly usage o hese p ospec i e
ma e ials is no ully exploi ed ye . Con empo a y
de ailed knowledge abou he e o luids is deal wi h
in wo ks [1], [2] [11] [12] [15].
2. PHYSICAL-CHEMICAL PRINCIPLE OF
FERROFLUIDS
Fe o luids a e pe manen ly s able colloid suspensi-
ons o e omagne ic pa icles in ca ie liquid. Sus-
pension s abili y he e means he quali y ha he sus-
pension emains pe manen ly homogenous, i is he
e omagne ic pa icles do no sepa a e om he
ca ie liquid and do no se le a he bo om o
he con aine (do no sedimen ), nei he c ea es
mu ual agg ega ions. To each s abili y e o luids
mus con ain e omagne ic pa icles o he size 5 o
15 nm (1 nm = 10
-9
m), so called nanopa icles.
Nanopa icles a e usually o med by one Weiss
domain. As a esul o spon aneous magne iza ion i
has a magne ic momen and ep esen s an
elemen a y magne ic dipole. Elemen a y dipoles
in luence each o he . To p e en hei agg ega ion
hey a e co e ed wi h s abilize , i.e. a polyme ous
(mac omolecula ) coa ing, so called de e gen
o med by he chains o pola molecules (e.g. a y
acid), long 1 o 2 nm. E e y chain is a one end
bound wi h a nanopa icle and a he o he end
loosely a ac ed by he molecules o he ca ie
medium, Fig. 1. De e gen is hus a su ace ac i e
ma e ial ha p e en s di ec con ac be ween
nanopa icles, causes epulsi e o ces be ween hem
and so p e en s hei agg ega ion.
chains
o molecules
o de e gen
ca ie
medium
magne ic
nanopa icle
Fig. 1. Fe omagne ic nanopa icle wi h de e gen
coa ing in ca ie liquid.
The mos common ma e ials o e omagne ic
nanopa icles a e magne i e (Fe
3
O
4
), maghemi e
(Fe
2
O
2
), cobal (Co), i on (Fe) o i on ni ide Fe
x
N).
The ca ie liquid can be wa e , a ious oils, usually
syn ac ic on hyd oca bon base, glycol and hei
compounds. Typical magne ic liquid con ains (in
olume): 5% e omagne s, 10 % de e gen and
85 % ca ie liquid. I s ela i e pe meabili y µ
≈ 5
10 Ad ances in Elec ical and Elec onic Enginee ing
d ops wi h empe a u e and a Cu ie empe a u e ge s
he alue µ
= 1, sa u a ion magne iza ion is abou
1,3 T and wo king empe a u e is om –125 o
200
o
C. Wi h highe empe a u e and empe a u e
changes chemical de e io a ion o de e gen chains
occu on he su ace o nanopa icles, which leads o
des abiliza ion o e o luid. Fe o luids du abili y is
e.g. om 8 o 10 yea s. High quali y e o luids wi h
long du abili y a e mo e expensi e.
Nanopa icles mo e in ca ie liquid by he mal
(B own) mo ion. I he liquid is no in he magne ic
ield, magne ic momen s o nanopa icles a e andom-
ly o ien ed and he liquid is non-magne ic, Fig. 2. I
he liquid is in he magne ic ield, he nanopa icles
a e pola ized, i is hey u n in he di ec ion o he
magne ic ield and make chains lying in he di ec ions
o he lines o o ce. This p ocess leads o conside a-
ble changes o physical chemical quali ies o e o-
luids. As o hei mechanic-elas ic quali ies i is
mainly iscosi y – he magne ic iscosi y phenome-
non. S able e o luid emains liquid e en in a s ong
magne ic ield, i means i s pa icles do no sedimen
and do no agg ega e in a s ong magne ic ield. Un-
less exposed o he magne ic ield, i is iso opic, bu
in he magne ic ield i becomes s ongly aniso opic.
The dependence o he magne ic induc ance on he
in ensi y o he magne ic ield has in e o luids a
cou se simila o ha o solid e omagne ics: wi h
g owing H inc eases B and asymp oma ically app oa-
ches he s a e o sa u a ion. In he linea magne ic
ield as a esul o losses (hys e esis and eddy cu -
en s), du ing emagne iza ion o nanopa icles hese
a e hea ed and he ca ie liquid is hea ed as well
which leads o dec easing o i s iscosi y.
Fig. 2. Fe o luid wi hou he in luence o ou e magne ic
ield: magne ic momen
o nanopa icles has andom dis ibu ion.
In some applica ions e o luids a e used wi h
mic opa icles i is pa icles o he size om 5 o
15 µm. Fo hese liquids he name is used – magne o-
heoloigical liquids. Mic opa icles a e mul i-domain
ones, (non single-domain ones like nanopa icles) and
a e no magne ically pola ized, do no ha e a magne-
ic momen . Magne o- heological liquids con ain a
conside ably la ge olume o e omagne ics, up o
70 % (o weigh ). They a e no usually s able, i
means hei mic opa icles sedimen and agg ega e,
he de elopmen o s able magne o- heological
liquids is one o he aims o he p esen esea ch.
They a e used in si ua ions when ex emely s ong
magne ic iscosi y phenomenon is equi ed, in
magne ic ield hey lose hei liquidi y and become
solid.
Fe o luids do no exis in na u e; hey a e de e-
loped syn he ically. Olde p oduc ion echnologies
we e based on long e m magne ic c ushing o mag-
ne i e o e i e pa icles in ball mills in de e - gen
solu ion. The p ocess o g inding las ed om 500 o
1000 hou s and a e inishing he cen i ugal sepa-
a ion o bigge pa icles ollowed. A p esen as e
and mo e e ec i e ways a e used, based on a ious
chemical p ocesses leading o p ecipi a ion o nano-
pa icles om solu ions o e ous sal s. The
ob ained p oduc mus be pu i ied, i is bigge pa i-
cles a e emo ed o by cen i uga ion o by sedi-
men a ion caused by g a i y o non-homogenous
magne ic ield.
The p oduce s o e a wide ange o e o luids o
magne o- heological liquids ha di e in hei
composi ion, physical chemical quali ies and p ice
and hey ecommend wha applica ions hey a e
sui able. Among impo an p oduce s o hese
liquids and equipmen using hem a e e.g. Ame ican
company Lo d Co po a ion Inc. and B i ish
company Liquids Resea ch L d. [17].
3. FERROHYDRODYNAMICS THEORY
In his pa we in oduced a sho synopsis o
ma hema ical desc ip ion o sys em wi h he e o-
luid, in he o m o bounda y alue coupled p o-
blem, based on magne ically/mechanical ields.
Magne ic ield in he domain Ω is desc ibed by he
equa ion
1
o o ( ) =
µ
− ∈Ω
A J, A
, (1)
oge he wi h bounda y condi ion o A, whe e by
ec o is de ined he poin in he ield,
µ
is he
pe meabili y,
γ
is he conduc i i y and J is he
cu en densi y. The magne ic ield s eng h is
1
o (di = 0)
µ
H = A A (2)
Fo 2D o a ional symme ic sys em ( , z) wi h
e o luid (see e.g. Fig. 4) hold ue
(
( )
1 1
A A
z z
ϕ
ϕ
µ µ
∂
   ∂ ∂ ∂
+ = −
   
∂ ∂ ∂ ∂
 
 
J
, (3)
A
ϕ
∈Ω
, oge he wi h bounda y condi ion and he
magne ic ield s eng h is
1 1
,
z
A A A
H H
z
ϕ ϕ ϕ
µ µ
∂ ∂
 
= − = − +
 
∂ ∂
 
(4)
Fu u e o elec o echnics: e o luids 11
F om gene alized Na ie -S okes equa ions o
con en ional luid mechanics may be deduced o
incomp essible iso opic e o luids and o he
mo ionless sys em he e odynamic Be noulli
equa ion. In he s eady s a e ha he o m [14], [15]::
0
cons .
p g h M H
ρ µ
∗
+ − =
,
p
∈Ω
(5)
wi h ollowing bounda y condi ion:
*
n c
p p p p
+ = +
(6)
we e
s m
p p p p
∗
= + +
is composi e p essu e, p is
he modynamic p essu e, p
s
is magne os ic i e p es-
su e,
m 0
p MH
µ
=
is luid-magne ic p essu e,
whe e
0
d
H
M M H
=

,
1
2
n 0 n
2
p M
µ
=
is magne ic
no mal ac ion, p
c
is capilla y p essu e, p
0
is
p essu e in nonmagne ic luid, ρ is pa icle mass
densi y and g = 9,8 m/s.
Fo some applica ions he dynamics o magne ic
iscosi y phenomenon is impo an (e.g. o
e ohyd odynamic dampe ). In his cases is impo -
an he de e mina ion o he esponse o iscosi y o
he change o he ou e magne ic ield. I s alue is
calcu- la ed in nanoseconds.
4. THE EFFECT OF THE MAGNETIC FIELD.
EXPERIMENTS
To unde s and he beha io o e o luids in he
magne ic ield, some simple expe imen s may be
p esen ed ha show his physically complex en i on-
men some imes beha es con a y o expec a ions.
Fe o luid wi h ee bounda y in nonhomoge-
nous magne ic ield. I B eaches ce ain c i ical
alue, su ace ins abili y o e o luids occu s and i s
su ace changes in a sys em o spikes di ec ed in he
cou se o magne ic lines o o ce. These spikes a e
he esul o complex s uc u al o ce a io in non-
linea aniso opic en i onmen o he liquid whe e
magne ic o ces apply as well as g a i a ional o ce
and su ace ension. In Fig. 3 he e is a Pe i dish wi h
e o luids and pe manen magne unde nea h.
Fig. 3. Fe o luid in a Pe i dish in he magne ic ield o a
pe manen magne : i s su ace changed in o a se o spikes.
Fe o luid a ound a cu en -ca ying wi e. In
Fig. 4 he e is a Pe i dish wi h e o luid.
Conduc o wi h cu en I, goes
h ough he dish,
which, as i is known, induces in i s en i onmen
magne ic ield o in ensi y H = I/2 , whe e is
pe pendicula dis ance om he conduc o .
Fe o luid is abso bed by he non-homogenous
magne ic ield, so i s o iginally la su ace changes
i s shape. In axial sec ion e o luid su ace is
bounded by he cu e ype y = cons ./ .
Fig. 4. Fe o luid nea a cu en -ca ying wi e.
Theo e ical solu ion. On Fig. 5 is 1D domain Ω,
2
,
R
∈
, o e o luid. Acco ding o Be noulli
eq. (5) is
* *
0 2 2 0 2
( ) ( )
p gh MH p gh MH
ρ µ ρ µ
+ − = − −
I
2
= → ∞
, hen
2
( ) 0,
H →
2
( ) 0,
M
→
2
0.
h h
= =
I we neglec he capilla i y, p
c
= 0,
hen acco ding eq. (6):
1
( ) cons . ( ) Cons .
h H
= = (7)
The bounda y be ween subdomains e o luid/ai
is no solid. He ewi h is cha ac e ized he coupled
p oblem.
h
h
R
1
2
Fig. 5. To he in luence o magne ic ield o he cu en -
ca ying wi e on he le el o e o luids
.
12 Ad ances in Elec ical and Elec onic Enginee ing
Le i a ion in e o luid.
I is known ha
pe ma-
nen magne canno ha e s able le i a ion in
s a iona y magne o s a ic ield. (so-called Ea nshaw
Theo em). The si ua ion changes when he medium,
in which le i a ion occu s is e o luid. I we place a
sealed dish wi h e o luid in a non-homogenous
magne os a ic ield, he e o luid will be d awn o he
places wi h inc easing ield in ensi y, Fig. 6.
Fe o luid ension appea s and i a non-magne ic
body is imme sed, he inne ension is b oken and he
body is exposed o o ces ha ( oge he wi h he g a-
i a ional o ce) caused s able le i a ion o he body -
so called passi e le i a ion o he non-magne ic body.
Fig. 6. To he le i a ion in e o luid.
5.
USAGE OF FERROFLUIDS
He e some ypical examples o use ul usage o
e o luids in echnical a eas.
Fe ohyd odynamic dampe .
Dampe s
used in
machine y enginee ing dissipa e kine ic ene gy o
ib a ing mechanism and so damp mechanic shocks
and consequen ib a ions. Con en ional hyd aulic
dampe s ha e cons an damping, only in special cases
hei damping can be changes by egula ion o luid
low wi h a h o le al e. Fe ohyd aulic dampe s
p o ide a mo e elegan solu ion. They a e illed wi h
e o luid ha is exposed o he magne ic ield o he
coil induced by a con olled cu en signal. I changes
he iscosi y o he e o luids and he dumping
inc eases. The cu en signal is, acco ding a pa icula
s a egy con olled by an on-line senso ha eads he
causes o ib a ions.
In Fig. 7 he e is one o cons uc ion a ian s o he
e ohyd odynamic dampe .
Fe ohyd odynamic dampe s we e used in di e -
en equipmen s om ine measu ing appa a uses o
washing machines, lo y sea s o he chassis dumpe
o means o anspo and a e e y p ospec i e. I a
ca mo es a he speed o 72 km/h, i makes 2 cm
dis ance in 1 ms. Con en ional dampe s eac a e 15
ms, he ca hen makes 30 cm be o e he dampe
eac s. The e omagne ic dampe eac s much as e ,
a e 5 ms. The dampe s eac s in 10 cm dis ance and
Fig. 7. Fe ohyd odynamic dampe : 1 - e o luid, 2 –
exci ing coil, 3 – plunge o dampe , 4 – dampe shell, 5
– gap, 6 – h o le al e.
he ca ge s o e a bump wi hou „bouncing“. Mo e
ansmission o he ib a ions o he d i e cabin,
limi s ansmission damping p e- en s he
bouncing o wheels and hus a loss o adhesion
be ween he i e and he su ace which inc eases he
s abili y o he ca , mainly in cu es. Magne ic
dampe s hus inc ease he sa e y and com o abili y
o he ide, sho en b aking eac ion, imp o e he
beha io o he ca and ex end i s du abili y, mainly
i s i es.
Ano he example is he dampe o lo y sea s. In
Fig. 8 he en elopes o sea ib a ions when using
con en ional (non-con olled) hyd aulic dampe
wi h bo h s ong a weak damping and magne ic
(con olled) dampe . Magne ic dampe hus inc e-
ases he com o and sa e y o d i ing.
sea
sp ing
dampe
displacemen
sou ce o ib a ions
equency
sligh ly damping, non-con olled
s ongly damping, non-con olled
con olled damping wi h e o luid
Fig. 8. Vib a ion o lo y sea wi h con en ional (non-
con olled) hyd aulic dampe and wi h a con olled
magne ic dampe .
Fu u e o elec o echnics: e o luids 13
Magne ic dampe s a e used also in house
appliances, such as washing machines. In con-
s uc ion o buildings in seismically ac i e a eas
buildings a e planned ha will ha e buil in magne ic
dampe s and so will be esis an agains ea hquake.
Fe ohyd odynamic sealing.
Using e o luids
enables pe ec sealing o a o a ing sha . The way o
sealing o a ing sha wi h e o luid o magne ic
ma e ial sha is in Fig. 9. In mu ually sepa a ed a eas
he e a e di e en p essu es p
1
,
p2. On a pe manen
magne in he shape o a sho cylinde s and pole
ods om magne ically so ma e ial. Be ween he
sha s and he pole ods he e is an ai gap o he size
o se e al en hs o cen ime e . In he ai gap a s ong
magne ic ield concen a es. To his a ea e o luid is
pushed ha is ixed by he in luence o he magne ic
ield and has he unc ion o sealing. Fo magne ic
induc ance in he ai gap B ∼ 1 T his sealing can keep
he p essu e di e ence | p
1
- p
2
| ∼ 0, 2 o 1 a m. Based
on he gi en p inciple mul i-laye sealing a e buil ,
ha enable o inc ease he o e p essu e| p
1
- p
2
| o he
alue o e 10 a m.
pe manen ní magne
pólo ý nás ec
e okapalina
hídel (magne icky odi ý)
p
1
p
2
pe manen magne
pole piece
e o luid
sha
Fig. 9. Sealing magne ic sha using e o luids.
In compa ison o con en ional (mechanical) sea-
ling he gi en p inciple has conside able ad an-
ages. I is simple and (and hus cheape ) mo e
eliable and has a lowe ic ion momen um ( e o-
luid unc ions as a lub ican ), high igh ness, long
du abili y (p oduce s claim up o10 yea s) and can
unc ion in a wide empe a u e ange om –100
o
C o
200
o
C. This way o sha sealing is used also o
iden ical p essu es (p
1
= p
2
), as e ec i e dus p oo
sealing e.g. o p o ec bea ing ope a ing in dus y,
chemically agg essi e, oxic o biologically ac i e
en i onmen .
Speake s wi h e o luid
a e cons uc ed as
common elec odynamic
speake s: in he magne ic
ci cui wi h pe manen magne he e is an ai gap, in
which a coil ib a es ed by acous ic signal and
connec ed wi h he memb ane, Fig. 10. Unlike he
con en- ional solu ion, a ound he coil he e is no ai ,
bu e o luid. By he in luence o he s ong magne ic
ield o pe manen magne he e o luid is pe ma-
nen ly kep in he ai gap. The acous ic pe o mance
is limi ed by he accep able cu en load o he coil.
As he he mal conduc i i y o e o luid is en
imes bigge han he mal conduc i i y o ai , i
enables o inc ease cu en densi y in he coil and
hus he acous ic pe o mance o he speake .
memb ane
coil
e o luid
pe manen
magne
Fig. 10. Speake wi h e o liud in he ai gap.
Elec ical machines wi h e o luids.
Fo
powe ans o me s e o luid is used as a cooling
medium. Unlike ans o me oil i has a highe
he mal conduc i i y, while i s elec ic s eng h is
basically he same. Fo o a ing elec ical machines
e o luid is applied in he ai gap be ween o o and
s a o . I makes cooling be e , bu i s o all i
lowe s he eluc ance o he magne ic ci cui o he
machine and hus he magne izing cu en o he
machine. On he o he hand, i inc eases he hyd-
aulic eluc ance o he o a ion. This equi es usage
o e o luid wi h low iscosi y and high pe me-
abili y. The men ioned way is limi ed o slowly
unning machines. Expe imen s show ha o
o a ing machines o 1000 u ns /min he ad an ages
p e ail ela ed wi h usage o e o luids.
6.
FUTURE OF FERROFLUIDS
E en i e ohyd odynamics ep esen s a young
science i enabled o ealize new appa a uses and
echnologies, pa o which has been p esen ed he e.
Many a e s ill in p og ess and do no ep esen
b eak h ough disco e ies.
Ne e heless i is
expec ed ha o he new machine y and elec o-
echnical componen s and new p oduc ion echno-
logies will a ise and ha is why a ious esea ch
eams and p oduc ion companies s udy e o luids.
Mo e de ailed in o ma ion is p o ided in he
bibliog aphy.
Acknowledgemen
This wo k has been suppo ed om he G an
Agency o he Czech Republic as a p ojec
No.102/07/0147 and he Resea ch P ojec MSM
4977751310.
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14 Ad ances in Elec ical and Elec onic Enginee ing
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