1 | r""" |
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2 | This model provides the scattering intensity, $I(q) = P(q) S(q)$, for a |
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3 | lamellar phase where a random distribution in solution are assumed. |
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4 | Here a Caille $S(q)$ is used for the lamellar stacks. |
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5 | |
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6 | Definition |
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7 | ---------- |
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8 | |
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9 | The scattering intensity $I(q)$ is |
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10 | |
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11 | .. math:: |
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12 | |
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13 | I(q) = 2\pi \frac{P(q)S(q)}{q^2\delta } |
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14 | |
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15 | The form factor is |
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16 | |
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17 | .. math:: |
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18 | |
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19 | P(q) = \frac{2\Delta\rho^2}{q^2}\left(1-\cos q\delta \right) |
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20 | |
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21 | and the structure factor is |
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22 | |
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23 | .. math:: |
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24 | |
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25 | S(q) = 1 + 2 \sum_1^{N-1}\left(1-\frac{n}{N}\right) |
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26 | \cos(qdn)\exp\left(-\frac{2q^2d^2\alpha(n)}{2}\right) |
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27 | |
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28 | where |
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29 | |
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30 | .. math:: |
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31 | :nowrap: |
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32 | |
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33 | \begin{align*} |
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34 | \alpha(n) &= \frac{\eta_{cp}}{4\pi^2} \left(\ln(\pi n)+\gamma_E\right) |
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35 | && \\ |
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36 | \gamma_E &= 0.5772156649 |
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37 | && \text{Euler's constant} \\ |
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38 | \eta_{cp} &= \frac{q_o^2k_B T}{8\pi\sqrt{K\overline{B}}} |
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39 | && \text{Caille constant} |
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40 | \end{align*} |
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41 | |
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42 | Here $d$ = (repeat) spacing, $\delta$ = bilayer thickness, |
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43 | the contrast $\Delta\rho$ = SLD(headgroup) - SLD(solvent), |
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44 | $K$ = smectic bending elasticity, $B$ = compression modulus, and |
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45 | $N$ = number of lamellar plates (*n_plates*). |
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46 | |
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47 | NB: **When the Caille parameter is greater than approximately 0.8 to 1.0, the |
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48 | assumptions of the model are incorrect.** And due to a complication of the |
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49 | model function, users are responsible for making sure that all the assumptions |
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50 | are handled accurately (see the original reference below for more details). |
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51 | |
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52 | Non-integer numbers of stacks are calculated as a linear combination of |
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53 | results for the next lower and higher values. |
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54 | |
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55 | The 2D scattering intensity is calculated in the same way as 1D, where the |
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56 | $q$ vector is defined as |
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57 | |
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58 | .. math:: |
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59 | |
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60 | q = \sqrt{q_x^2 + q_y^2} |
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61 | |
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62 | |
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63 | References |
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64 | ---------- |
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65 | |
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66 | F Nallet, R Laversanne, and D Roux, J. Phys. II France, 3, (1993) 487-502 |
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67 | |
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68 | also in J. Phys. Chem. B, 105, (2001) 11081-11088 |
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69 | """ |
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70 | from numpy import inf |
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71 | |
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72 | name = "lamellar_stack_caille" |
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73 | title = "Random lamellar sheet with Caille structure factor" |
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74 | description = """\ |
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75 | [Random lamellar phase with Caille structure factor] |
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76 | randomly oriented stacks of infinite sheets |
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77 | with Caille S(Q), having polydisperse spacing. |
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78 | sld = sheet scattering length density |
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79 | sld_solvent = solvent scattering length density |
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80 | background = incoherent background |
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81 | scale = scale factor |
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82 | """ |
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83 | category = "shape:lamellae" |
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84 | |
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85 | single = False # TODO: check |
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86 | # pylint: disable=bad-whitespace, line-too-long |
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87 | # ["name", "units", default, [lower, upper], "type","description"], |
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88 | parameters = [ |
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89 | ["thickness", "Ang", 30.0, [0, inf], "volume", "sheet thickness"], |
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90 | ["Nlayers", "", 20, [0, inf], "", "Number of layers"], |
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91 | ["spacing", "Ang", 400., [0.0,inf], "volume", "d-spacing of Caille S(Q)"], |
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92 | ["Caille_parameter", "1/Ang^2", 0.1, [0.0,0.8], "", "Caille parameter"], |
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93 | ["sld", "1e-6/Ang^2", 6.3, [-inf,inf], "sld", "layer scattering length density"], |
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94 | ["sld_solvent", "1e-6/Ang^2", 1.0, [-inf,inf], "sld", "Solvent scattering length density"], |
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95 | ] |
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96 | # pylint: enable=bad-whitespace, line-too-long |
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97 | |
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98 | source = ["lamellar_stack_caille_kernel.c"] |
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99 | |
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100 | # No volume normalization despite having a volume parameter |
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101 | # This should perhaps be volume normalized? |
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102 | form_volume = """ |
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103 | return 1.0; |
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104 | """ |
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105 | |
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106 | # ER defaults to 0.0 |
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107 | # VR defaults to 1.0 |
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108 | |
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109 | demo = dict(scale=1, background=0, |
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110 | thickness=67., Nlayers=3.75, spacing=200., |
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111 | Caille_parameter=0.268, sld=1.0, sld_solvent=6.34, |
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112 | thickness_pd=0.1, thickness_pd_n=100, |
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113 | spacing_pd=0.05, spacing_pd_n=40) |
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114 | |
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115 | # |
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116 | tests = [ |
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117 | [{'scale': 1.0, 'background': 0.0, 'thickness': 30., 'Nlayers': 20.0, |
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118 | 'spacing': 400., 'Caille_parameter': 0.1, 'sld': 6.3, |
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119 | 'sld_solvent': 1.0, 'thickness_pd': 0.0, 'spacing_pd': 0.0}, |
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120 | [0.001], [28895.13397]] |
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121 | ] |
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122 | # ADDED by: RKH ON: 18Mar2016 converted from sasview previously, now renaming everything & sorting the docs |
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