indian Journal of Pure & App l icd Phys i c~ Vol. 40. May 2002. pp. 370-374

Recent advances in understanding magnetic emulsion stability John Philip, R Kiran, T Jayakumar, P Kalyanas undaram & Baldev Raj

Mctal lurgy & Matcri als Group, Indira Gandhi Centrc (o r A tomi c Research. Kalpakkam, Tamilnadu 603 102

Rcceivcd 15 May 200 I; revised 3 December 200 I ; accepted 6 Fcbruary 200:2

The recent advances in the undcrstanding of magneti c cmulsion stabi I it y, from force mcasurcmcn t stud ies and somc of the intercstin g applicat ions of stabi l izedmagnetie cmu lsions have becn prcsented. A ncwly dcvelopcd forcc mcasurcmcnttechnique. which facilitatcs thc measurement of the forces bctween individual co lloidal particles has been presented. The rcsu lts on the forces net ween indi vidua l co lloidal liquid droplets in the presence of differel1l t ype~ of ~urfactants and a weak polye lectro lytc, po ly acryl ic aciLl (PAA) and one of thc potellli :ll appl ication of magnet ic emulsiom for non-dest ructive tes tin g have been presented.

1 Introduction

App li cat ion s of co ll oida l sc ience are in constant progress in many aspects of li fe (lsrelachvili '). Coll oids are used in a huge number of processes such as ex tractin g o il from geo logica l deposits , aeroso ls for dispensing domest ic products like shav ing creams and deodorants, sprays conta ining in secti c ides, emul sions, ge ls, food process ing, packaging industry etc. Colloids genera ll y require repulsive surface forces to become meta-s table. The net force acting between the colloida l particles determines the stability of the colloida l system. A net attracti ve force leads to the aggregati on or c luste ring of particles or for liquid particle coa lescence may occur and the sys tem wi II be unstable (Philip J el of). If the net force is repulsi ve, the sys tem will re main stab le. The he igh t of the potenti a l barrie r decides the stab i lity of the co ll o ida l system.

The e lect rostati c , ste ri c or e lectro-steri c are the ~ommonly used stabili zati on techniques . Most of the time, e lectrostati ca ll y stabili zed colloids become un stab le when the ionic strength of the medium is increased suffi cientl y, due to the reduction in the spatial extension of electrical double layers (e.g., clays, so ls, latt ices). Therefore, one major advantage of using macromo lecules as stabili zers is that the ir double layers are less sensiti lie to e lectrolyte concentration. As a result , steri c stab ili zation of co ll oida l dispersions has triggered a lot of inte res t in recent years due to its advantages over e lectrostatic counterpart and more importantly due to its numerous indu strial app lications ( appel-" Pate l & Tirrell-l). Stabi li zat ion of the co lloida l d ispersions with macromolecules has been

exp lored in various techno logical applicat ions for many years , however, the interacti ons between polymer bearing surfaces cou ld be stud ied directly onl y a fter the int roduct ion of the surface force measuremen t apparatu s (Israe lachvili & Tabol-" Israe lachvili & Adams!', Lukham & Klien7). The e luc idati on of the structure of the adsorbed layer on the co ll o ida l emul sion is important ' ince it p lays a major ro le in the stab ili zation of the colloids. Steric stab ili zat ion of the co ll oida l di sp rsions has tri ggered a lot of inte rest due to its several advantages over its e lectrostatic counterpart and more importantl y due to its numerous industria l applicati ons (Sato & Ruch s\ Napper'\ Tadors~).

Force measurement techniques have been widely used to understand the interact ions th at give insight into the co ll o ida l stabi lity (lsrae lach vi Ii I) . The measurement of forces acting between solid surfaces has been a topic of intense research over the last two decades (Israelachvi li I). Recently, a new techni que (Lea l el al. lO, Philip ' 1.l 2, M onvalu. '-l), ca lled magnetic field induced Chaining Technique (MeT ) has been introduced which a ll ows the measurement of forces between two indi vidua l emul sion drop lets. In contras t to other older techniques , thi s method leads to the direc t in situ measurement at the collo ida l scale. Using this technique, one can measure the co ll oida l forces in a vari ety of materi a ls encou ntered in emulsions and di spersions. The present work is mainl y a imed to understand the stability of emul sion d rople ts in the presence of po lyelectro lytes .

PHILIP et al. :MAGNETIC EMULSION STABILITY 371

2 Experimental Details

Ferrofluid consists of a collection of ferrolferri magnetic domains dispersed in a liquid carrier e.g. octane (RosensweigI5). The typical size of the oxide particles is about 10 nm. The technique used to make the stock emulsion is the classical inversion method. The first step in this method is to mix slowly an anionic surfactant, sodium dodecyl sulphate (SDS) in water and then ferrofluid oil is added. This first step leads to water in oil (W/O) emulsion with very large droplets of sizes ranging from 10 micron or more. This emulsion is then inverted to an oil-in-water (OIW) emulsion using a colloidal mixer. After proper mixing, a polydispersed emulsion with a droplet size ranging from 0.1 to I micron is obtained. The polydispersed emulsion is diluted with water and then more surfactant (SDS) is added to the emulsion. The concentration of the SDS is just above the critical micellar concentration (CMC of SDS = 0.008 moillitre) . After proper mixing, the emulsion is transferred into a cylindrical beaker and the fractionation technique (Bibette I6) is used to select emulsion droplets with a diameter of about 200 nm.

An applied field induces a magnetic dipole in each drop, causing them to form chains. Without external field , these droplets have no permanent magnetic moments because of the random orientation of the magnetic grains within the droplets, due to thermal motion. An external magnetic field orients these magnetic grains slightly toward the field direction, which results in a dipole moment in each droplet. The magnitude of the magnetic dipole moment increases with the strength of the applied field until saturation is reached. At low concentration, one droplet thick chains are well separated and oriented along the field direction. Due to the presence of the one-dimensional ordered structure, Bragg peak can be observed, from which the inter-droplet separation is estimated precisely. The condition for forming a linear chain is that the repulsive force between the droplets must exactly balance the attractive force between the droplets induced by the applied magnetic field . The spacing between droplets is directly measured from the determination of the spectral distribution of the scattered light at a constant angle.

3 Results and Discussion

The repulsive force between the colloidal droplets stabilized with an anionic surfactant (SDS) at two

concentrations 8 x 1O.3 M and 8 x 1O.4 M are shown in Fig. I . Here, the electrostatic repulsion arises from the presence of anionic surfactant (SDS) molecules adsorbed on the colloidal droplet. The estimated values of Debye lengths at these concentrations are respectively 3.4 nm and 10.7 nm respectiyely. Due to the changes in the surfactant concentration, the corresponding surface potential and Debye length changes. For the lower concentration, the counter ion layer extents weB beyond the surface and become diffused. This results in long-range electrostatic repulsion. At a higher surfactant concentration (CMC of SDS = 8 x 10 .. 1 M), most of the counter ions are located within a few nanometers from the surface of the droplets that lead to a short-range repulsion . This observation shows that the spatial extension of the double layer is very sensitive to the concentration (or ionic strength) of surfactant.

The minimum force shown in Fig. I corresponds to the force, sufficient to hold the particles within a chain, and the corresponding interaction energy is necessarily greater than the thermal energy, kT. The maximum force is determined by the saturation field of the ferrofluid , which may eventually be increased by an order of magnitude by using a ferrofluid with larger susceptibility. The minimum and maximum force measured using this technique is 10.13 Nand 10.11 N respectively and the resolution of interfacial separation is about 1 nm.

Fig. 2 shows the variation of force profiles as a function of distance for PAA solutions of four different concentrations 0.01 and 0.1, 0.3 and 0.5%(wt.). In all these cases, the pH of the PAA solutions was about 4.0. The colloidal droplets are stabilized by a non-ionic surfactant (NPI 0). Without adsorbed polyelectrolyte, the hard wall profile observed is shown by the circles. Due to the presence of the non-ionic surfactant at the colloidal droplet interface, the system remains perfectly stable even after it was brought down to the hard wall contact.

At low PAA concentration (0.01 %), the repulsive force profile was long-range. Repulsion is first noted at an inter droplet separation of 109 nm. This distance corresponds to more than 3 Rg of PAA. As the

372 INDfAN J PURE & APPL PHYS , VOL 40, MAY 2002

o [80S J - B.OE-3M -• [SDS] - B.OE-4M -

- % ::: 0 0 0 M • .-.J 0 M ~ x a x Q) x Z 0 • '-' 0

Q)10- 12 ()

0

'" 0 0 '"

0

0 0 0

I I I I I I I

16 20 26 !l0 3~ 40 46 60 55 60

h (rm)

Fig I - Force-distance proli les wi th ionic surfactant SDS o f two d ifferent concentrati ons

di "tance between the droplets is reduced, an inc rease in the repulsive force that becomes stronger as the adsorbed PAA begins to overlap. The net repulsion conta in s contributi ons due to e lectrostati c and steri c forces. The hard wall contact occurs at a few nanomete rs furthe r from the surface (i. e., a lmost close to the one without PAA). Thi s observation suggests that the polye lectro lyte adsorbed at the inte rface of the droplet is e ither desorbi ng from the droplet interface or the tail s prefe rs to fl ow out from the inte r-droplet spacing as the separation is reduced . Therefore, at shorter inte r-droplet spacing, onl y a few " trains" remain on the surface of the collo idal d roplets. Thi s cou ld be probably because PAA has no specific anchoring groups to get adsorbed irreversibly on the drop lets. Due to thi s weak binding, the layers may be hi gh ly dynamic, w ith indi vidual segments continua lly attaching and detaching from the drople ts. Up to a concentrati on up to 0 .01 % the colloida l system was perfec tl y stable and the force curves were reversible. At hi gher concent rations e .g. 0 . 1 %, when

10-11 oNP10 only Y~.Ol%

l j .10% 00.30% -~ 60.50% 0 +l o I ~ ~ ,: i \~ ~" ~ ~0-12 . • lie

K j Xx

Q) ~. x~ 0 , \ t.. 6 \ ~ 0 Dl x

'" •

~ ~ x x ~ rf! x x

At ) JD-13 " x >it

I 0 20 .0 eo 10 100

h (rm)

Fig. 2 - Force-di stance pro il le with PAA (l'vIw=250 K) at di fferent co ncentrations

the droplet separati on was about 25 nm from the hard wall touching, the authors have noticed a jump (instability region shown by two vertical lines in Fig. 2) on the force profil e. In F ig. 2, only two PAA concentration s 0 .0 I and O. I % were shown , however experimental results at intennediate concentrati on were similar to what was observed with 0. 1 % PAA concentration. Microscopic obs rvat ion of the co llo ida l system just be fore the instability region shows that doublets or chains are not formed until thi s leve l. Just after the jump, the formati on of dou blets and sing lets in the system was observed . Further reducti on in the separation (with larger magneti c fi e ld) between the droplets inc reases the number of these doub lets. It also leads to the formati on of longer chains w ith severa l droplets attached. Probabl y, thi s is d ue to the ' bridging' of two or more polymer-covered surfaces, leading to permanent chains even afte r removin g the fi e ld . Just after the instability region, only a few tra in s are remaining on each droplet. As the trai n length vari es from one droplet to another, the fi rs t double t is seen, when the longes t train connects two droplets.

PHILIP el a/.:MAGNETIC EMULSION STABILITY 373

When the separati on is reduced further, the remaining smaller trains start to join and therefore the number of drop lets in each chains and the length of the chai n increases progress ively.

10- 11

-c 0

,.J

~ ~ z

'-"

Cl lO-12

0 ~ 0

tz.

o

i oNP10 onl?a xO.001M aCI

0 '~ . Ol ~ ~aCl :\ o .1 aC 1 ·q. o M NaCl o 0

L.o o ~ , 0

x f:l .. 0 0 66 , 0

~ ~A , 0 x (:, . 0

x t, 0 x 6 ' ~ t. , DO

6 ,

I

20

.x 6

, I

.0 eo h (!In)

1

80

Do 0

JOO

Fig. 3-Force pro fI le with PAA (250 K, 0.0 1% Mw] for various NaCI concent rat ions

Fig. 3 shows the fo rce profile fo r different sa lt concentrati on at a fixed PAA concentrati on. In a ll these cases, the p H of the soluti ons was fixed at 4. Let us firs t consider a situ ati on where there is no PAA in the system. Under thi s conditi on, if we simply increase the salt concentrati on from 0.00 I M to 1.0 M , the di ffused double layer (Debye length) is reduced from a value of about 10 nm to 0 .3 nm. Thi s means that, the range of repulsion should reduce considerably when the salt concentration increases from 0.00 1 M to 1.0 M. However, what is seen in Fig. 3, is different from what is expected according to the concept of electrostati c screening theory. Without any salt in the system, fo r a PAA concentration of 0.0 I %, the fo rce profile is a hard sphere profil e (shown by the circles). As the salt concentrati on increases fro m 0.00 1 M to 0. 1 M NaC I, the range of repulsion increases. On further increase in the salt concentrati on it reduces the

range of repul sion.

The hydrodynamic di ameter of the droplet with and without PAA at the i ntelface was measured from dynamic li ght scattering (DLS) using an Amtec Spectrometer in associati on a Brook Haven Instrument Di gital Correlator (BI 2030A T , 72 Channel cOITelator). The hydrodynamic di ameter of the droplet for a sa lt concentration of 0 .00 1 M is increased to about 2 1 0 nm fro m the bare droplet (di ameter=200 nm), without any salt. At 0 . 1 M NaCI, the hydrodynamic radius is increased to 400 nm. Further increase in the salt concentration leads to a reducti on in the hydrodynamic thickness to 300 nm. These resu lts suggest that the addition of salt in the sys tem has a dramati c influence on the adsorption isotherm and the stability of the emu lsions.

4 Application of Ferrofluid Emulsions for Defect Detection

Magnetic flu x leakage technique (M FL) is often used in industries for in-service inspecti on of ferromagnetic materials. Thi s technique has been used to locate and assess the di scontinuities such as fati oue b cracks, corrosion, eros ion and abras ive wear in materials during in-service inspecti on. T he commonly used flu x leakage sensors are magnetic particles, pickup coils, Hall probes, magneto diodes and Forester micro probes. At IGCAR, they have developed a new optical technique, which can be used fo r the measurement of magnetic flu x leakage from ferromagnetic specimens or components (Philip ' 7.'~) .

The new flu x leakage probe consists of monodi spersed ferrofluid confined between two thin transparent sheets or in a cuvette and a white li oht

b

source for illuminati on. By empl oyin g ferrofl uid d roplets of suitable size and surfactant concentrati on and mounting the ce ll on the test specimen surface, one can detect the region where the defect is located in the tes t specimen by visually observing a colour change in the ferrofluid cell , in the vicinity of the defec t. The origin of thi s colour change in the back-scattering direction is due to Bragg scattering fro m the droplet chains, formed by the leaked magnetic fl ux in the presence of a defect. Also, by grabbing the colour pattern using digital camera, it is poss ible to obtai n the exact wavelengths corresponding to di fferent co lours. Such an approach would fac ilitate the defect sizi no with an improved sensiti vity in the defect detection. b

374 INDIAN.J PURE & APPL PHYS , VOL 40, M A Y 2002

Usi ng the felTofluid probe, the authors could detect a defect of 0.6 mm width , which was 5 mm inside a mild steel specimens (rear side MFL measurement). Since they had used ferrofluid in a cuvette of I mm gap with a total thickness of 3 mm ( I mm thick glass plates), the minimum possible lift-off was 2 mm. It is very much possible to detect defects of smaller dimens ions using thin ferrofluid probe (ferrofluid encapsul ated between thin mylar sheets). Compared with magnetic particle tes ting, the new technique is si mple to operate and interpret the results . The colour change or wavelength shift is directl y related to the defect size and shape. The defect identifica ti on and characteri zation can be done qualitatively by using co lour mapping. Preparation of the ferrofluid cells of any geometry according to the requirement is easy and is much less expensive compared to other MFL probes. Also, the ferrofluid probe is immune to electrica l interference contrary to other ex isting MFL probes.

5 Conclusions

A new ex perimental technique suitable for the measurement of fo rces between magnetic emul sion droplets has been di scussed. The forces in the presence of surfactant and polyelectrolytes adsorbed at the interface of colloidal droplets using magnetic chaining technique have been studi ed. These ex perimental results on the force measurement studies give us insights into the stab ility of emulsions under vari ous conditi ons, and it would be useful for tailoring emul sions of des ired stability. A new meth od for detection of defects in ferromagnetic components , usin g ferrofluid emulsion has been developed.

Acknowledgement

The auth ors woul d like to thank Dr S L Mannan, Assoc iate Director, MDG and Shri S B Bhoje, Director, lndira Gandhi Centre for Atomic Research fo r support and encouragement. They thank, Shri 0 Mondain Monval , Shri Lea l F Calderon and Shri J Bibette, for fruitful di scussions. This work was supported by the Indo-French Centre for the promotion

of advanced Research/Centre, Franco-lndien Pour 1a Promotion de la Rechereche.

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