PHYSICAL REVIEW A VOLUME 42, NUMBER 8

Stability of cellular patterns in directional solidification

15 OCTOBER 1990

John D. Weeks* and Wim van SaarloostAT& T Bel/ Laboratories, Murray Hill, New Jersey 07974

(Received 19 July 1990)

Ideas from the theory of constitutional supercooling, as well as an approximate linear stabilityanalysis for small wavelength perturbations, lead to a simple stability criterion for the periodiccellular patterns seen in the directional solidification of alloys with small partition coefficient.There is good qualitative agreement with the limited experimental data available in the cellularregime. This criterion suggests that solutions found by using the analogy between directionalsolidification and Saffman-Taylor fingers may be unstable.

hu;cos8 ID(n Vu—); (2)

u; 1 —z;/lT —du@. , (3)

Recent analytical and numerical work has shown thatthe equations describing the directional solidification'(DS) of a binary mixture admit a continuous family ofperiodic steady-state cellular solutions with varying wave-lengths k. However, a steady-state analysis alone cangive no information about the stability of these solutions,and hence their relevance for real experiments. In this pa-per we present an approximate stability analysis that in

many cases will allow one to determine whether a givensteady-state solution is likely to be stable. The simplestprediction is based on ideas from the theory of constitu-tional supercooling, 's and we refer to it as the local con-stitutional supercooling (LCS) criterion. The LCS cri-terion relates the positions of the tips of stable periodiccellular arrays in the experimental cell to the pulling ve-locity V, and its predictions can easily be checked experi-mentally. A more general, but still approximate, linearstability analysis for arrays of cellular patterns suggeststhat in many cases corrections to the LCS criterion shouldbe small.

Equations similar to the LCS criterion have in fact beenused before, particularly in the dendritic regime found atstill higher pulling velocity, or smaller temperature gra-dient. However, the earlier derivations relied on some-what ad hoc matching arguments, and they attempted topredict a unique pattern spacing for a given set of experi-mental conditions, rather than determine which membersof a family of solutions might be stable. To ourknowledge, the derivation and interpretation of the LCScriterion and its corrections in terms of stability concepts,and its particular relevance in the cellular regime has notbeen realized before.

We consider the standard one-sided solutal model, '

where solidification is assumed to occur as matter in a thincell is pulled through a fixed linear temperature gradient,and impurity diffusion in the solid is ignored. The steady-state equations for DS in two dimensions in a frame mov-

ing at velocity Vcan be written as'1 Bu

az

where, using Eq. (3),

au; —= u; —u,'=1 —(1 —k)(z;/&T+do~) . (4)

which denotes the boundary of the stable single-phase re-gion in a modified phase diagram where the abscissa is thereduced concentration field u and the ordinate is the posi-tion z in the cell (by assumption linearly related to thetemperature T).

We note that in almost all cases, the curvature at thetips of experimentally relevant cellular patterns is smallenough that the term dux in (3) represents only a verysmall correction. '" (This will be discussed more quanti-tatively later. ) This suggests that we can think of the re-gion near the tips as locally planar, and use some of thesame stability ideas there that are used to examine thestability of the initial planar interface. The simplest ap-proximation leads directly to the LCS criterion.

Here do is the chemical capillary length, lT the thermallength, /D =D/ V a diffusion length, a' the interface curva-ture, and 8 the angle between the interface normal n andthe growth direction, taken parallel to the z axis. ' A sub-script i denotes a quantity evaluated at the interface, andthe superscript s denotes the solid phase. The dimension-less field u =—(e —c )/hen measures the impurity concen-tration c in the liquid relative to that far ahead of the in-terface c . It is normalized by the width of the planartwo-phase region, hco =—co —co =co(1 —k), with k thepartition coefficient and co the concentration at the planarsteady-state interface. Global conservation of impuritiesthen fixes the location of the steady-state planar interfacein the cell such that c =kco, or equivalently,uu duo 1. This defines the origin z =0 of our coordi-nate system.

Equation (2) expresses local conservation of impuritiesas the interface advances, with hu; in (4) the "impuritysurplus" rejected at the interface. For materials withsmall partition coefficient k, we see that h, u; has asignificant dependence on the position z; in the cell. Thiswill play an important role in what follows.

Equation (3) takes account of surface-tension effectsand imposes local equilibrium at the interface. For planarinterfaces with ~ 0, it describes the liquidus line

(5)

5056 1990 The American Physical Society

STABILITY OF CELLULAR PAL IERNS IN DIRECTIONAL. . . 5057

To begin the analysis, let us review the theory of consti-tutional supercooling (CS) for the planar interface. 'The basic idea is that in the steady state we can apply theconcept of local thermodynamic equilibrium to the melt

just in front of the moving planar interface. The interfaceis supposed to remain stable as long as the impurity con-centration in the melt just in front lies in the stablesingle-phase region as determined from the phase dia-gram. Since we assume local equilibrium at the interfacefrom (3), the CS stability criterion reduces to the follow-ing requirement. The (dynamical) gradient ~du/dz ~;

d u;/lp determined from the boundary condition (2)must not exceed the liquidus slope ~duL/dz

~I/lr in (5).

This criterion can be reexpressed as

veau; ~1,where we define the standard dimensionless control pa-rameter ' v = ir/lD. Note that v is directly proportional tothe velocity V.

The usual CS prediction vc 1 for the stability limitof the plane' follows from (6) where we note that globalconservation requirements force the plane to be located atz; 0, where hu; 1. This criterion is surprisingly accu-rate, given the simplicity of the CS argument. For ma-terials with small k, a linear stability analysis (see below)predicts only small changes in the critical value of v, i.e.,~c= 1.

However, the arguments leading to (6) seem nearly asplausible as before when applied locally to the melt infront of the tips of certain (nonplanar) patterns, providedthat the curvature corrections in Eq. (3) are small, andthat k is reasonably small, e.g., k 50.2. In particular, weneed to use only the boundary conditions (2) and (3) andthe local symmetry in the tip region (nearly planar, with8, 0) to arrive at (6), with hu; h,u„ the impuritysurplus at the tip. When curvature corrections to du& from(4) are small, Eq. (6) can be reexpressed as

g, = (v —1)—(1 —k)z, /iD ~ 0 .

Equation (7) predicts that the tip position z, of stable pat-terns with v & 1 must move up in the cell, relative to theplanar position at z 0, until the smaller impurity surplushu, allows (6) or (7) to hold true. It is natural to conjec-ture that the operating point in real experiments should beclose to the one that requires the least forward motion,i.e., the re~ion where g, =0. We refer to this as the LCScriterion. '

Of course, this argument is only suggestive. The CScondition (6) is not exact, even for the plane at z =0. It iseasy to see that corrections to the LCS picture must surelybe taken into account at large v & 1/k (usually in the den-dritic regime), since (7) would then predict an unphysical"concentration deficit" at the tip with u, & 0.

To determine the nature of the corrections to LCS, weextend the usual planar stability analysis' to the casewhere the wavelength of interface perturbations is smallcompared to A, , the wavelength of the (steady-state)profile z; (x). Representing the perturbed interface by

zf(x, t) z;(x)+zz exp(iqx+tot),

(8'u/8x '),co —p2 +

ur

p (pv/p—I+k)(veau, —1 —(v —l)crq ]

Qt(8)

Here p=A, /2lD is the Peclet number, which relates thescale of the pattern to the diffusion length, pv satisfies

pv(pv p) q 2, and

ks 4 vdplD0

g 9k (v 1)

where X, is the (lower) neutral stability wavelength forthe plane in the limit as k 0. cr provides a measure ofthe importance of surface-tension corrections due to thecurvature of the interface. Experimental values for a areusually very small. ""3We note that (8) reduces to thestandard result for the plane, ' where hu, duo 1, and(r1 u/8x ), 0. The expression in (9) for A,, then followsfrom (8) if we set k 0 and solve for the associated wavevector q, giving co 0.

In general, for small k, the first term on the right-handside of (8) is small, and important only in a small regionof v very near threshold. The quantity (8 u/8x )& ap-pearing in the second term on the right in (8) is negative(stabilizing) and numerical calculations indicate that ittoo is often small in magnitude. '

We can then obtain the LCS criterion from (8) byneglecting both these terms as well as all terms propor-tional to o, ' requiring that a stable solution with to ~ 0results. This derivation makes it clear that the LCS con-dition g, =0 gives an upper bound to the distance that thetips of patterns have to move to achieve stability, since allterms omitted in (8) are negative (stabilizing). Correc-tions due to the curvature of the interface are more impor-tant at large v, since they are proportional to (v —1)~.Indeed, for very large v in the dendritic regime, the tipsmove up very close to the line z 1~. The stability is thendetermined by curvature effects (solvability) and weak in-terdendritic interactions. However, in the cellular re-gime with v —1 of O(1), it appears that corrections to theLCS condition for patterns with small o are indeed small.

There is some limited experimental support for this con-clusion. The only data we found in the literature that al-lowed us to test the LCS picture for cellular arrays arethose of Esaka and Kurz on cells in succinonitrile-acetone mixtures with k 0.1. In Fig. 1 we plot the pre-dictions of Eq. (7) for the tip position as a function of thecontrol parameter v, along with results obtained from the

we assume the perturbed impurity field takes the form

uv(x, z, t) u(x, z)+u~exp[iqx —pv(z —z, )+totj

where u(x, z) is the field associated with the solutionz;(x). In these and the equations that follow, we take thehalf wavelength X/2 as the unit of length and A, /4D as theunit of time. We make the usual quasistationary approxi-mation, ' where we assume that Eqs. (1) and (3) still hold,but use the correct (time-dependent) normal coinponentof the interface velocity in Eq. (2). Substituting zf and uv

and linearizing, we find in the region near z, our basic re-sult

5058 JOHN D. WEEKS AND WIM VAN SAARLOOS

0.8

o~ 0.6

~~ 0.4

0.2

1 2 3 4 5 6 7 8 9

—1)ax. Here (; —= (v —1)—(1 —k)pz; is the LCS pa-rameter (7) evaluated at a general (dimensionless) posi-tion z;.

These equations are valid for any p, but we now takethe limit p~ 0 in the following natural way. ' We exam-ine the region near the tips, allowing for the possibilitythat pz& reaches a finite limit as p 0, but make the im-portant assumption, to be verified later, that (, remainsgreater than zero. In the fingertip region, of O(1) in ourunits, we can then ignore the z variation of g;, replacing itby g, . Then ignoring all other terms proportional to p, wesee the modified DS equations are identical in form to theST equations given above if we let p/(, p and

FIG. 1. Plot of the tip position z,/lr vs v. The LCS criterion0 for k 0. 1 yields the dashed line. The solid line gives the

k 0 limit of the LCS criterion. The dotted lines denote thepredictions of (11) for k 0. 1 and various values of e. The soliddot at v 4.2 and the square dot at v 8.7 represent two datapoints of Esaka and Kurz (Ref. 7), the latter showing some den-dritic features, while the data points for v 1.3 and 4.2 are ob-tained from Ref. 8.

mapping onto the Saffman-Taylor finger as discussedbelow, and compare with the experimental data. There is

good qualitative agreement between experiment and theLCS criterion for data in the cellular regime. Clearly,more experiments are called for, especially since wettingor surface preparation effects involving the cell thicknessmay be of importance. ''

If the LCS picture is accurate, it has important conse-quences for the applicability to real experiments of theoften used analogy between Saffman-Taylor fingers andDS at small Peclet number p. ' ' To see this, let us firstrecall that the steady-state equations describing Saff-man-Taylor (ST) fingering2 can be written as V2&sT 0,with boundary conditions (n Vga ); cos8 and

o rc, where p—b (P —Po)/(12pVa), and cr

b y/12@ Va z. Here b is the plate spacing, p the viscosi-

ty, V the finger velocity, P the pressure field, Po the pres-sure inside the finger, y the surface tension, and a the cellhalf width (taken as unit of length).

Dombre and Hakimz show that for "fat" fingers whosewidth approaches the cell width 2a, cr as defined here'approaches the maximum value 1.393, and they derive thelimiting behavior for small e of a (e), where e is the rel-ative groove width at the base of the finger. Karma andPelce' give a convenient interpolation formula fitting nu-merical results for larger e. One finds that viscous fin erswith narrow grooves are associated with values of o oforder unity.

Returning now to the DS problem, the analogy toviscous fingering can be seen most easily if we introduce amodified field2' p by the equation u 1 —p(z+P)/v.The field p measures the difference between the impurityconcentration field u and its dominant variation in the zdirection along a straight piece of the interface as dictatedby the phase diagram. By substituting p into Eqs.(1)-(3), when written in dimensionless units, we find ex-actly the modified DS equations V p+p8&/Bz —p, with

[(;—(1 —k)p(v —1)aa]cos8 (n Vp); and p; -(v

(v —1)o(p- 0)g, (p- o)

(lo)

Karma and Pelce'6 and Mashaal et al. ' use conserva-tion and matching arguments to arrive at an expressionfor (,. Using also (10), their result can be expressed as

1

1+(1 —k) e/k

Different members of the family of solutions satisfying(10) and (11) are parameterized by the relative groovewidth e of the DS finger, now measured in the "matchingregion, " a distance of O(1) behind z, . Equation (11)pre-dicts that for small e & k, both g, /(v —1) and o are O(1).This is consistent with a key assumption made in the map-ping to the ST problem: g, remains finite as p 0.

Weeks and van Saarloos showed that if the partitioncoefficient k is small, there is a natural extension of thematching arguments leading to (10) and (11) to systemswith finite p of O(1). The resulting finite p corrections tothe right-hand side of (11)are relatively small, at least forsmall e, and yield somewhat larger values for g, and o.

Unfortunately, this entire class of solutions appears tobe in qualitative disagreement with many experiments.Equation (11) implies that the small e solutions are asso-ciated with large values of ~. Note that solutions withe& n have wavelengths A, (11,0 lying outside the planarinstability band. However, most experimental patternshave narrow grooves, or small e, but with small cr valuessuch that X&A, "'i Small values of a can arise from(11),but only for large c.

Moreover, these solutions are not consistent with theLCS picture, where we expect g, =0. ' This is illustratedin Fig. 1, where we note that the ST curves for narrowgrooves, say t. ~0.1, lie far below that of the LCS cri-terion. Of course, this argument is not conclusive, espe-cially since corrections to the LCS criterion are more im-portant at large o and all these tend to increase the valueof (&. Still, it does suggest there could exist a different,and more physically relevant, branch of steady-state solu-tions where g, is small for small e. Note from (11) that forsmall p this would also imply small values of cr as seen in

experiments.This conclusion seems fully in accord with the work of

Brener, Geilikman, and Temkin on the related problemof crystal growth in a channel (essentially the v ~ limitof DS). When interface kinetics is taken into account,

STABILITY OF CELLULAR PA I I ERNS IN DIRECTIONAL. . . 5059

they find at least two branches of steady-state solutions.

Only one of these is related to ST fingers in the limit

p 0, but it exhibits several unphysical features. Thesecond branch properly described free dendritic behavioras the channel width tends to infinity. %e believe a simi-lar scenario may apply to DS.

These results point out the need for a systematic experi-mental test of the LCS criterion, and its corrections, forcellular arrays. If this turns out to be accurate, then there

are theoretical as well as practical implications. Manytheoretical steady-state patterns found in the literatureare predicted to be unstable by the LCS criterion. Theconsiderable body of work based on the mapping to theST finger should then be reexamined, and a more rigorousand systematic stability analysis for arrays of cellular pat-terns is called for. More generally, the question of"branch selection" for this entire class of pattern formingsystems needs more study.

'Present address: Institute for Physical Science and Technology,University of Maryland, College Park, MD 20742.

tAddress after 1 January 1991: Instituut Lorentz, University ofLeiden, Nieuwsteeg 18, 2311 SB Leiden, The Netherlands.

'For a general review, see J. S. Langer, Rev. Mod. Phys. 52, 1

(1980).2T. Dombre and V. Hakim, Phys. Rev. A 36, 2811 (1987). See

also M. Ben-Amar and B. Moussallam, Phys. Rev. Lett. 60,317 (1988).

See, e.g., D. A. Kessler and H. Levine, Phys. Rev. A 39, 30413208 (1989); N. Ramprasad, M. J. Bennett, and R. A.Brown, Phys. Rev. B 38, 583 (1988).

4J. D. Weeks and W. van Saarloos, Phys. Rev. A 39, 2772(1989). A more complete description of both the matchingmethods used in this reference, and the LCS ideas discussedherein can be found in J. D. Weeks, W. van Saarloos, and M.Grant (unpublished).

5W. A. Tiller, K. A. Jackson, J. W. Rutter, and B. Chalmers,Acta Metall. 1, 428 (1953).

See, e.g. , D. J. Allen and J. D. Hunt, Metall. Trans. 7A, 767(1976), and references therein.

7H. Esaka and W. Kurz, J. Cryst. Growth 72, 578 (1985).Some additional data from Esaka is reported in Ref. 8.

B. Billia, H. Jamgotchian, and R. Trivedi (unpublished).9This point was made by J. A. Warren and J. S. Langer (unpub-

lished), who studied the stability of dendritic arrays.' Here do: yTu/(LmLbco) with y the surface tension, L the la-

tent heat per unit volume, T~ the melting temperature of thepure solvent, and mt. the liquidus slope ~

d TL/dc ~; and

Ir =mLttco/6, with 6 the temperature gradient.''S. de Cheveigne, C. Guthmann, and M. Lebrun, J. Phys.

(Paris) 47, 2095 (1986).'2In general, let r, =—[x;(z),z] denote an arbitrary point on the

interface of a fingerlike pattern, and r, = r;+ al~n a point in

the adjacent fluid layer for a small normal displacement al&with a « l. A measure of the differential supercooling at r, isbu,"cs —= [uL(r, ) —u(r )]/a = ((z)cos8. Here g(z) is (7)evaluated at the general position z, and we have used (5) and(2), ignoring curvature corrections. It is easy to show fromthe Scheil equation (Ref. 4) that bu s 0 deep in the

grooves due to the cosa factor, but this fact places no con-straints on the absolute position of the finger. However, re-

quiring that the differential supercooling is small near the tip,where cosa 1, yields the LCS criterion, which does (approxi-mately) fix z, .

' For example, using the data shown in Fig. 2 of Ref. 11, aftersome misprints are corrected, we find o =0.006.

'4See, e.g., L. H. Ungar and R. A. Brown, Phys. Rev. B 29, 1367(1984); 31, 5931 (1985). By dilferentiating (3), we can showthat (8 u/Bx ), —(p/v)[veau, —1 —(v —

1 )tz(dx/dz), ]x„noting that du;/dz (Bu/Bz), +(Bu/Bx);/(dz;/dx) and thatnear x„(Bu/Bx), -(8'u/Bx'), (x; —x&), etc. Thus the ideathat veau, —1= /, is small is consistent with small (Bzu/8 '), .

' Although short-wavelength perturbations were assumed in

deriving (8), we expect it will remain at least qualitatively ac-curate for perturbation wavelengths of order A, , i.e., q =x.For such perturbations, the term (v —1)aq2 in (8) representsonly a small correction in the cellular regime if cr is small.However, since (8) does not properly incorporate the physicsof competition between different fingers generated by pertur-bations with still smaller wave vectors q, it cannot be used togive a quantitative treatment of the pattern selection mecha-nism.

'sSee A. Karma and P. Pelce, Phys. Rev. A 39, 4162 (1989),and references therein.

' M. Mashaal, M. Ben-Amar, and V. Hakim, Phys. Rev. A 41,4421 (1990).

' Our crsr is equal to do/x' in the notation of Ref. 2; 4/C in Eq.(22) of Ref. 16; and 4trsv in Ref. 17.

'9Indeed, if (, & 0, or even if it is O(p) as p 0, then the map-

ping to the ST equations could not be carried out. However,it seems more likely to us that the problem with the matchingresult (11) [and our generalization (Ref. 4) to finite p] lies in

a breakdown of the assumption that there is a single matchingregion whose width vanishes as e 0 where the inner (tail)and outer (tip) solutions can be joined together.E. A. Brener, M. Geilikman, and D. E. Temkin, Zh. Eksp.Teor. Fiz. 94, 241 (1988) [Sov. Phys. JETP 67, 565 (1988)].