Problem No 4 (2010). “SOAP FILM” Alexander BARNAVELI

I.Vekua Phys-Math school No 42. Tbilisi, GEORGIA ( s.barnaveli@gmail.com )

Problem: Create a soap film in a circular wire loop. The soap film deforms when a charged body is placed next to it. Investigate how the shape of the soap film depends on the position and nature of the charge. 1. The Soap Film Structure Let’s start investigation of this problem by considering the soap film structure[1]

briefly. Soap film is formed by surface active agents (Surfactants). There are two layers of soap molecules, between which there is water (see Figure 1[1,2]). A soap molecule is divided into positive Na+ and negative C17H35COO - ions. Negative ions collect on the surface of film and they form surface structure. Between two layers of film there are water and Na+ ions.

Figure 1. Soap film structure. Soap molecules play the important role in the film formation[1,2]:

Negative ions C17H35COO - form the surface structure of soap film; Soap molecules decrease the evaporation of water; They also decrease the surface tension.

Here we want to note that this difference in the positive and the negative ion distribution in a soap film leads to a very interesting phenomenon, which will be discussed below. In some of our experiments we added a small amount of glycerin to soap, because the glycerin decreases the water evaporation from the surface and also stabilizes the soap film[1,2]. So, due to the glycerin a soap film gets an "extra duration of life" and becomes more elastic. 2. The experimental Setup Now let us describe our experimental setup (for better quality color photos please see our presentation [3]). We used: • A voltage source - an old CRT monitor, which

provides 27 000 V; • Electrodes of different forms; • Different distances between the electrodes and the

soap film; • A metal loop (frame) for the soap film - grounded

one and also not grounded; Figure 2. Experimental setup. • A dielectric loop (frame); • Different sign of body charge (and this gave different results!!!).

3. A spWe besoap fismall ftoo largIt was o– As la

the l– In th

defofilm

– For Filmfilm

Let us chargemake tredistrisigned film wichargecomplicinteracassumsegme In suchThe latforce bmethoddependapproxdistanc ElectricThus o• The

the f• The • The

grouprespoin

• The mus

• We • AC –• Q – • q’ –

pherical cgan experlm. In our rames werge – to getobserved, arger the less the infhe case oormation inremained the small

m continuedexploded. explain th

es on the lathe film eqbuted (Indcharges w

ll stretch es are gathcated, bu

ction” can e the exte

ent.

h interactiotter can b

between thd". Equatindence of hximation, wce to the ch

c force. our assump

external fifilm; film surfacinteractio

unded sphsented as tnt charges;

value andst give equassume th– the loop the extern

– the "effec

harged boriments witexperimen

re not convt stable filmthat: distance dfluence (de

of d~5 cmn the centestable; d~h, the

d to stretch

his theoretatter will beual-potent

duced) chawill gather towards it

hered. Gent approximbe solved

ernal charg

on the elee calculate

he charge ng the Couh on the

when the dharged sph

ptions are tield redistr

ce is equaln betweenhere film the interac

d the place al-potentia

hat the film (frame) of

nal charge,ctive" charg

ody th an interants we usevenient forms and to a

d between eformation

m, the heiger was ab

stable stah, discharg

tically. Whe redistribuial. Electric

arges. On t(they are

t – see Fnerally, themately thed by usinge as a p

ctrostatic fed by meaand the fi

ulomb and external s

deformatiohere.

the followinributes the

l-potential;n the char

can be ction betwe

of "effectival film surfa is a sphera radius a at the dist

ge at the di

action betwed the framr making obavoid influe

sphere ann); ght h of bout 1 cm

ate did noge develop

en the chuted (sincec field on tthe film, neattracted igure 3), w

e distributioe problem

ng the “Eloint charg

force is baans of Laplm we calthe surfacsphere chon is much

ng: charges o

rge and th"effectivelyeen the tw

ve" chargeace; re segmena; tance d>>istance x f

ween a spme for soapbservationence of eve

nd film -

the film and the

ot occur. ped and

arged bod

e the film isthe film wilear the extby this extwhile far fon of chargm of “soalectrostatice and the

alanced byplace equaculated us

ce tension arge Q ah less tha

on

he y"

wo

e

t ABC of th

>h ; from the lo

herical chap film of ras and meaen small a

dy approacs a conducll be compternal charternal charfrom it – ges on theap film - c Image Me film shap

y the surfaation whilesing an "elforces we

and on d. an the loop

he height h

op.

3

arged bodyadius a~5 casurementsir flows.

Figuches the fictor) in thepensated brge, the oprge and ththe same

e film will bexternal

Method”. Wpe as a sp

ace tensione the electlectrostaticcan calcuWe will u

p radius a

h;

3 cm

y and a cm. Too s, while

re 3. ilm, the

e way to y these

pposite-hus the -signed

be quite charge

We will pherical

n force. trostatic c image late the use the and the

From tfollowin

and the

This fo SurfaceUsing t

we obt

If we w

BalancUsing t

It is quwhen h

Figure 5The depstates: theory -

the Figureng values:

e effective

orce must b

e tension fthe Laplac

ain

write down

ce of Electrthe force e

uite a comph << a; h

5. pendence h((experiment- curve).

e 4 the co

electrosta

be balance

force. ce formula P

Fl = pS, w

rostatic andequilibrium

plicated forh << d.

(d) for the st - points;

ondition of

tic interact

ed by the s

Pl = 4σ0 /

we will get t

d Surface condition

rmula, so tWe can d

exra(1

wan

Lede

Fostable φ

Q

φB = φC

;

tion force w

urface tens

R and taki

that the fo

tension forFk=Fl for

to make it do it in ouxperimentsadius. In ),(2),(4) ge

x ≈ 2h –

here from nd d:

et's calcuependenceor a 1cm raφ = 27 00

= Cφ = 4

= 0 (a g

will be:

sion force.

ing into ac

.

rce of the s .

rces. (2) and (3)

clearer, leur case, sis was sma

this appet the form

d ; q’

kQ2 =

we get th

·

ulate h fe is given oadius char0 V, we ge

4πε0r· φ ≈ 3

rounded f

count, that

surface ten

) we get:

.

et us use thince the fiall compa

proximationm:

≈ -Q ;

32π hd

he depend

for our eon the graprged spheret:

3 · 10-8 Co

film) leads

,

.

t

nsion is

he approxilm stretchred to the

n the eq

F = kQ2/

d2 , dence of h

·

experimentph. re, at the p

oulomb.

s to the

(1)

(2)

(3)

(4)

mation, h in our e frame uations

/(4d)2 ;

(5)

h on Q

. (6)

t. This

potential

Noting that is Let us increassituatioexplod 4. A flaBy thdeformbecausHowevthe filmthat ca 6. An aTo obtparallewhile toThe filmloop. IstretchIn the sthe fonegativ 7. InteIn this but thewind observby so cstrong strengtplace welectrotype shit is vethe nastronglfilm anfact thaparticleexperimthe deta large.

that σ0 ≈ 2quite near

make somses with hon is oppoes.

at chargedhe flat

mation wase the fieldver withoum was sigase the ions

approximatain the ul large lidso the top lim was formn the uni, due to thstrong fieldorces active charges

eraction ofcase the s

e dip was ffrom abo

ved dip (inscalled "elecand this st

th of attrawhen the

ode is very harp electrery strong. ail polarizey repels td form theat this wines we pment. Instetector of che amount

25 · 10-3 Nto our exp

me notes. Fh faster thsite - Fk in

d body charged s larger

d was stront groundingnificantly s could not

ately unifouniform fies. The bottod the high med on a form field e force sym

d the film sng on th

s.

f film with soap film dformed, as

ove (see stead of a ctric wind",trength wa

action. Eledensity ohigh (like

rode) and eThe field

es the airthe electroe above-mend was cauproved byead of soaharged part of accel

N/m, for thperimental

From (3) ahan Fk. Soncreases w

body th(see Fig

nger. ng the str

less becat “leave” th

orm fieldeld we usom lid wasvoltage waplastic (diethe film

mmetry. simply washe positiv

charged ndid not stres if there b

Figure 7"hill") was , which ble

as greater tectric windof charge

the case electric fieat the bot

r moleculeons. They entioned dused by cy the foap film we rticles and erated pa

he distanceresults.

nd (2): Flo the stabwith h fast

he film gure 6)

retch of ause in he film.

sed two s groundedas appliedelectric) did not

s torn by ve and

nail. etch out, blew the 7). The

caused ew rather than the

d takes on the of nail-ld near ttom of es and hit the ip. The harged llowing placed it fixed

articles.

e d = 5 cm

l ~ h; Fk ~ble equilibrter than F

d .

m from (6) w

~ 1/(d-h)2.rium is re

Fl, so the f

Small-hea

3 cm

we get h ≈

If h << dached. If

film stretch

FFlat charge

FSharp-ende

Fig

aded charge

≈ 1 cm,

d, the Fl h ~ d,

hes and

Figure 6. ed body.

Figure 7. ed body.

gure 8. ed body.

8. InteIn this attracti

9. Thechan

As we article, the negcausesIt tookpoles, seems and no In our changethe bodof discof sparthe elewas chspreadelectrodischar What isAt highelectroFrequethe train atomconstalight. Iemits bluish-that whthe ligh(which is emitt The Nacceleremissionegativexterna In the film. Th

eraction ofcase the oon towards

e asymmetnge of pomentionedthe differe

gative ion s a very ink place wi.e. the chto be the

ow let’s car

experimee of the pody) affects harge on trk are depeectrode chharged pod and yeode was rge was bl

s the reasoh temperatomagnetic ency of emnsition ene

m: EPhoton=nt, and ν It is knowyellow liglilac light. Then electroht is emitteare prese

ted by Sod

Negative erated electon. Also, ve externaal body, co

case of phese ions

f film with observed bs electrode

try with reles. d in the beence in thedistributio

nteresting when we arge sign

e most interefully cons

ents we ooles (i.e. cthe shape

he soap filendent on harge. Whsitively, d

ellow, whcharged

luish and

on of this? tures the a

waves mitted lightergy betwe

=hν, where- frequen

wn, that Sght, whileThus we code is chared by the atnt in air) w

dium (Na)

external ctrons strikein this cal charge, m

oncentrate

positive exhit the film

small-heabehavior ree. Both – th

espect to

ginning of e positive n in soap phenomenchanged of the boderesting resider it.

observed, charge signe and the cm. Paramethe sign ofhen electrdischarge wile when

negativstraight.

atoms emit(photo

t dependseen two leve h – is Plancy of emiSodium (N Nitrogenan concludrged negattoms of Ni

while in the) atoms. W

charge ace the Nitr

ase the pomove alongthere and

xternal chm and punc

aded chareveals bothhe "dip" an

the

this and film

non. the y. It

esult

that n of

color eters f Disrode was the

vely,

the ons). s on vels

anck tted

(Na) n - de ively D

Nitrogen e case of p

Why it is so?

ccelerates rogen atomositive Na+

g the film fcause the

harge heavch out of it

rged bodyh processend the "hill"

scharge (ye

Discharge (b

positively?

the electrms of air a+ ions in freely towa straight fo

vy ions of t the positiv

3

y. s - the elec" are forme

llow). Positi

blue). Negat

charged e

rons towaand cause soap are

ards the poorm of disc

air acceleve Na+ ion

3 cm

4 cm

ctric wind aed (See Fig

Fively charge

Figureively charge

electrode, t

ard the filmtheir bluisattracted

oint nearescharge.

erate towans which fo

and the gure 8).

Figure 9. ed body.

e 10. ed body.

the light

m. The sh-lilac by the t to this

ards the orm the

“cloud” above the film. So the discharge proceeds in this “cloud” of positive Na+ ions. This gives the yellow color of discharge. Besides, in this case the punched out Na+ ions are attracted by the negative ions of soap (these negative ions form the soap film structure and are more or less “spread-fixed” around the film surface area). Thus the discharge becomes more "spread" (here we see the discharge shape asymmetry with respect to pole sign. It is caused by the soap film ionic structure – different “flaw freedom” degree of the positive and the negative ions). In this case we also saw that most upper part of the spark (which is above the cloud of positive Na+ ions) is blue, while the main yellow region is within the cloud of the punched out Na+ ions. As we observed, the discharge destroys the film. 10. Conclusion. In this work we studied the soap film interactions with the charged bodies. The different shapes of the charged bodies were tested and the resulting film deformations were examined We found out that film deformation strongly depends on:

– The shape of the charged body – The charge value – The distance from the film – The grounding of the loop on which the film is formed – The soap film structure

Also, a very interesting phenomenon of the discharge asymmetry with respect to the change of poles was observed. In the case of Negative external charge the straight bluish-lilac discharge takes place, while in the case of Positive external charge – the “spread” yellow discharge. We explained it by the specific ionic structure properties of a soap film. Finally I want to thank my grandfather Tengiz Barnaveli for his help in the experiments with the high voltage. References [1] Cyril Isenberg. The science of Soap Films and Soap Bubbles. Dover Publications, NY 1992. [2] http://www.soapbubble.dk/en/bubbles/ [3] http://solutions.iypt.org/uploads/2010_GE_Soap_film_Alexander_Barnaveli_1321459517.pptx