20 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Jo urna l o f Ma g ne tism a nd Ma g ne tic Ma te ria ls X5 t IYYO) 20L7h No rth- Ho lla nd PHASE SEPARATION OF MAGNETIC COLLOIDS AND CONCENTRATION DOMAIN PATTERNS A. CEBERS The concept of magnetic condensation a a mechanism of formation o f p e rio d tc c o nc e ntra tto n p a tte rns o f m a g ne ttc c o llo td s is d e ve lo p e d . The ir c ha ra c te ristic s in pla ne slots have been determined. It has been shown tha t a fa m ily o f e q uilib rium c o nc e ntra tio n p a tte rns can be found a s e ig e nfunc tio na of a se lf- a d jo int tnte g ro - d iffe re ntia l o pe ra to r. Typ e s of p a tte rn tra nsfo rm a tio n c a use d b y m a g ne tic insta b ilitte s o f m ic ro d ro p s of the condensed phase have been described. It should be noted that there is an analogy of the dynamics o f two - d im e nsio na l m a g ne tic p a tte rn fo rm a tio n with d iffusio n- lim ite d g ro wth p ro c e sse s. The d a ta o f num e ric a l e xp e rim e nts ind ic a ting a possible fo rm a tio n o f fra c ta l m a g ne tic o b je c ts a rc g we n. 1. Magnetic condensation The existing thermodynamical models of mag- netic colloids indicate the possibility of their phase separation in a certain range of temperature. con- centration and field strength [1.2]. Since in the magnetic field there might occur phase separation of colloid which is stable at its absence [l]. the above phenomena could be named magnetic con- densation. Its basic difference from condensation of common media lies in the fact that microdrops of the condensed phase interact via long-range magnetic forces. As a result, for example, at mag- netic condensation of colloid in the field trans- verse to the boundaries of plane slot. periodical patterns of condensed phase [3,4] are formed. The cause of the appearance of the above con- centration pattern is basically the same as that of development of the domain pattern of ferromag- netics. If the latter occurs due to the tendency of decreasing energy of the self-magnetic field of ferromagnetics, then magnetic colloid condensa- tion, for the same reason, results in forming a periodical pattern. The given concept can be employed in describ- ing a number of experimental data obtained for magnetic colloids of different composition. The connection between the origin of a periodical pat- tern of magnetic colloid in a flat slot and its instability in the field with respect to phase sep- aration allows one to have the following relation for the temperature dependence of critical field strength [5] As seen from fig. 1, the experimental data [3] agree well with the linear dependence (1). The value of magnetic moment of colloidal particle of magnetite nz found from the slope of the straight line (fig. 1) corresponds to its radius of about 14 nm, which is in good agreement with electron microscopic data for the given octane-based mag- netic colloid. 1 H,3 1 L \ 0,005 i \ \ \ \ k \ \ \ -\ \ t,c zyxwvutsrqponm 0 zyxwvutsrqponmlkjihgfedcbaZYXWVU 20 Fig . 1. De p e nd e nc e o f c ritic a l no rm a l fie ld stre ng th o f p e rio d t- c a l m ic ro dro p p a tte rn in fla t slo t fo rm a tio n on the tempera- ture Do ts ind ic a te e xp e rim e nta l data 131. 0304-X853/ 90/ $03.50 :(:I1990 - Elsevier Sc ie nc e Pub lishe rs B.V. (North-Holland)