Transcript Document

Observation of Critical Casimir Effect in a Binary Wetting Film:
An X-ray Reflectivity Study
Masafumi Fukuto, Yohko F. Yano, and Peter S. Pershan
Department of Physics and DEAS, Harvard University, Cambridge, MA
• A long-range force between two macroscopic bodies
induced by some form of fluctuations between them.
Casimir forces in adsorbed fluid
films near bulk critical points
(i) Fluctuations: Local order parameter f(r,z)
[e.g., mole fraction x  xc in binary mixture]
(ii) B.C. : Surface fields, i.e., affinity of one component
over the other at wall/fluid and fluid/vapor interfaces.
Specular
Reflection
a
47.7 °C
L
(ii) Non-critical (van der Waals) disjoining pressure:
At Tfilm = 46.2 °C ~ Tc
(iii) Critical Casimir pressure:
• Intrinsic chemical potential  of the film relative to bulk
liquid/vapor coexistence was controlled by temperature offset
T between the substrate and liquid reservoir [10].
 L
k T
 L

 L  k BTc  L 
Casimir pressure: pc  L, t    c    3   
L  x 
L
x 
T = Trsv + T.
z
Saturated
MC + PFMC
vapor
Bulk reservoir:
Critical MC + PFMC
mixture (x ~ xc = 0.36)
at
T = Trsv.
d
 x
dx
For each B.C., scaling functions  and  are universal
in the critical regime (t  0, x  , and L  ) [2].
• Scaling functions have been calculated using mean field
theory (MFT) (Krech, 1997 [3]).
T =
0.50 °C
MC + PFMC wetting
film on Si(100) at
Inner cell
(0.001C)
B c
Casimir energy/area: c  L, t   c    2   
L
x 
x 
T =
0.10 °C
Total film thickness L [Å]
• Finite-size scaling and universal scaling functions
(Fisher & de Gennes, 1978 [1])
T =
0.020 °C
qz [Å1]
Outer cell
(0.03C)
Theoretical background
where   x   2  x   x
• Anti-symmetric (+,) B.C.: Previous study at 30°C [10] showed
that MC-rich liquid wets the liquid/Si interface and PFMC is
favored at the liquid/vapor interface.
2+,
T =
0.020 °C
T =
0.10 °C
T =
0.50 °C
  T = Tfilm – Trsv
+,
(+,+)
2+,+
MFT scaling functions
for Casimir pressure,
where the ordinate has
been rescaled so that
½+,±(0) = +,±(RG)
at y = 0. (Based on [3])
   
  l  v T
 T T 
 sv  sl T
• “Casimir amplitudes” at bulk Tc (t = 0),    0 
for 3D Ising systems:
1
2
 Qualitatively consistent with theoretically
expected repulsive Casimir forces for (+,).
0
Recent observations of Casimir effect in
critical fluid films
• Thickening of films of binary alcohol/alkane mixtures on Si near the
consolute point. (Mukhopadhyay & Law, 1999 [6])
+,
+,+
RG: Migdal-Kadanoff procedure [4]
0.279
0
RG: e = 4 – d expansion [3]
2.39
0.326
• Thinning of 4He films on Cu, near the superfluid transition.
(Garcia & Chan, 1999 [7])
Monte Carlo simulations [3]
2.450
3.1
0.345
0.42
• Thickening of binary 3He/4He films on Cu, near the triple point.
(Garcia & Chan, 2002 [8])
Method
“Local free-energy functional” [5]
y = (L/x)1/n = t(L/x0)1/n,
where n = 0.632 and x0+/x0 = 1.96 for 3D Ising systems [11], and
x0+ = 2.79 Å (T > Tc) for MC/PFMC [12].
• Scaling function can be extracted experimentally from the measured L, using:
1 
3 Aeff
 L 
,  y  
k BTc 
6



MFT
2+, (RG)
Symbols are based on the measured L,
 = (2.2  1022 J/Å3)T/T, and Aeff = 1.2
 1019 J estimated for a homogeneous
MC/PFMC film at bulk critical
concentration xc = 0.36. The red line (—)
is for T = 0.020 °C. The dashed red line
(---) for T < Tc is based on Aeff estimated
for the case in which the film is divided in
half into MC-rich and PFMC-rich layers at
concentrations given by bulk miscibility
gap.
At Tfilm = 46.2 °C ~ Tc
Summary:
• Both the extracted Casimir
amplitude +, and scaling function
+,(y) appear to converge with
decreasing T (or increasing L).
This is consistent with the
theoretical expectation of a
universal behavior in the critical
regime [2].
Tfilm [°C]
• Thickness enhancement near Tc for small
T, with a maximum slightly below Tc.
y = (L/x)1/n = t (L/x0)1/n
 Scaling variable:
y = (L/x)1/n = t (L/x0)1/n
  l  v
(+,)
pc = [kBTc/L3]+,(y)
 +, > 0  pc tends to increase film thickness.
Normalized Reflectivity R/RF
45.6 °C
P = Aeff/[6L3]
 Effective Hamaker constant Aeff > 0 for the MC/PFMC wetting films (T > Twet).
 P tends to increase film thickness.
 Aeff for mixed films can be estimated from densities in mixture and
constants Aij estimated previously for pairs of pure materials [10].
46.2 °C
• Studied a wetting film of binary mixture MC/PFMC on Si(100),
in equilibrium with the binary vapor and bulk liquid mixture at
critical concentration.
correlation length x = x0 tn ~ film thickness L
 Each wall starts to “feel” the presence of the other wall.
 “Casimir effect”: film thinning (attractive) for (+,+)
and film thickening (repulsive) for (+,) when t ~ 0.
  > 0 tends to reduce film thickness.
 Can be calculated from T and known latent heat of MC and PFMC.
PFMC
rich
From: Heady & Cahn, 1973 [9],
Tc = 46.13  0.01 °C
xc = 0.361  0.002
• For sufficiently small t = (T – Tc)/Tc,
i.e., a balance between:
(i) Chemical potential (per volume) of film relative to bulk liquid/vapor coexistence:
MC + PFMC
x (PFMC mole fraction)
• As T  Tc, critical adsorption at each wall.
• Film thickness L is determined by
a
Si (100)
MC
rich
Comparison with theory
 = P(L) + pc(L, t)
qz = (4/l)sin(a)
Incident X-rays
l = 1.54 Å
(Cu Ka)
Temperature [C]
Two necessary conditions:
(i) Fluctuating field
(ii) Boundary conditions (B.C.) at the walls
Methylcyclohexane
(MC)
Perfluoromethylcyclohexane
(PFMC)
Thickness measurements by
x-ray reflectivity
+, = ½ +, (y = 0)
System and experimental setup
+, = (kBTc)1 [ L3 – Aeff/6]
What is a Casimir force?
+, (RG)
T = Tfilm – Trsv [K]
• The Casimir amplitude +, extracted at Tc and small T agrees well with +, ~
2.4 based on the renormalization group (RG) and Monte Carlo calculations by
Krech [3].
• The range over which the Casimir effect (or the thickness enhancement) is
observed is narrower than the prediction based on mean field theory [3].
References:
[1] M. E. Fisher and P.-G. de Gennes, C. R. Acad. Sci. Paris, Ser. B 287, 209
(1978).
[2] M. Krech and S. Dietrich, Phys. Rev. Lett. 66, 345 (1991); Phys. Rev. A 46,
1922 (1992); Phys Rev. A 46, 1886 (1992).
[3] M. Krech, Phys. Rev. E 56, 1642 (1997).
[4] J. O. Indekeu, M. P. Nightingale, and W. V. Wang, Phys. Rev. B 34, 330
(1986).
[5] Z. Borjan and P. J. Upton, Phys. Rev. Lett. 81, 4911 (1998).
[6] A. Mukhopadhyay and B. M. Law, Phys. Rev. Lett. 83, 772 (1999); Phys. Rev. E
62, 5201 (2000).
[7] R. Garcia and M. H. W. Chan, Phys. Rev. Lett. 83, 1187 (1999).
[8] R. Garcia and M. H. W. Chan, Phys. Rev. Lett. 88, 086101 (2002).
[9] R. B. Heady and J. W. Cahn, J. Chem. Phys. 58, 896 (1973).
[10] R. K. Heilmann, M. Fukuto, and P. S. Pershan, Phys. Rev. B 63, 205405 (2001).
[11] A. J. Liu and M. E. Fisher, Physica A 156, 35 (1989).
[12] J. W. Schmidt, Phys. Rev. A 41, 885 (1990).
Work supported by Grant No. NSF-DMR-01-24936.