Advances in Kriging-Based Autonomous X-Ray Scattering Experiments.

Autonomous experimentation is an emerging paradigm for scientific discovery, wherein measurement instruments are augmented with decision-making algorithms, allowing them to autonomously explore parameter spaces of interest. We have recently demonstrated a generalized approach to autonomous experimental control, based on generating a surrogate model to interpolate experimental data, and a corresponding uncertainty model, which are computed using a Gaussian process regression known as ordinary Kriging (OK). We demonstrated the successful application of this method to exploring materials science problems using x-ray scattering measurements at a synchrotron beamline. Here, we report several improvements to this methodology that overcome limitations of traditional Kriging methods. The variogram underlying OK is global and thus insensitive to local data variation. We augment the Kriging variance with model-based measures, for instance providing local sensitivity by including the gradient of the surrogate model. As with most statistical regression methods, OK minimizes the number of measurements required to achieve a particular model quality. However, in practice this may not be the most stringent experimental constraint; e.g. the goal may instead be to minimize experiment duration or material usage. We define an adaptive cost function, allowing the autonomous method to balance information gain against measured experimental cost. We provide synthetic and experimental demonstrations, validating that this improved algorithm yields more efficient autonomous data collection

Internal segregation and side chain ordering in hairy-rod polypeptide monolayers at the gas/water interface: An x-ray scattering study

We report studies of the structure and packing of Langmuirmonolayers (LMs) of polypeptide poly(γ-4-(n-hexadecyloxy)benzyl α,L-glutamate) (C16–O–PBLG) on the surface of water. The molecule is a “hairy rod” and consists of side attachments of hexadecyloxy chains (–O–C16) to the rigid rod-like core made up of α-helical poly(γ-benzyl L-glutamate) (PBLG). Measurements include surface pressure (Π) versus area/monomer (A) isotherms, x-ray specular reflectivity (XR), and grazing incidence diffraction(GID). In contrast to the LM of bare PBLG on water, which undergoes a monolayer/bilayer transition with increasing Π, monolayers of C16–O–PBLG remain stable up to the highest densities. On the basis of XR and GID results, the structure of the C16–O–PBLG monolayer is characterized by the following main features. First, hydrophobicity causes the –O–C16 chains to segregate towards the film/gas interface and away from water and the PBLG cores, which sit parallel to and near the water/film interface. Since the attachment position of some of the side chains is at the core/water interface, the segregation forces these chains into the space between neighboring core rods. Compression associated with increasing Π thickens the film but the internally segregated structure is maintained for all Π (i.e., >∼30 dyne/cm). Second, the C16–O–PBLG rods form domains in which the rods are aligned parallel to each other and to the interface. The correlation length for the interhelix positional order of the rods is short and typically comparable to or less than the length of the rods. With increasing Π the spacing d between nearest-neighbor rods decreases linearly with A at high Π, indicating a direct correspondence between the macroscopic compressibility and the microscopic interhelix compressibility. Third, as Π increases past ∼5 dyne/cm, the local packing of tethered –O–C16 chains displays the same herringbone (HB) order that is common for high-density bulk and monolayer phases of alkyl chains. Various features of the observed GID peaks also imply that the HB order of –O–C16 chains is oriented with respect to the helical axes of aligned PBLG cores. We propose that the HB order is established initially by one-dimensionally confined chains between aligned rods at low Π and grows laterally with compression

Internal Segregation and Side Chain Ordering in Hairy-Rod Polypeptide Monolayers at the Gas/Water Interface: An X-Ray Scattering Study

We report studies of the structure and packing of Langmuir monolayers (LMs) of polypeptide poly($\gamma$-4-(n-hexadecyloxy)benzyl $\alpha$,L-glutamate) (C16–O–PBLG) on the surface of water. The molecule is a “hairy rod” and consists of side attachments of hexadecyloxy chains (–O–C16) to the rigid rod-like core made up of $\alpha$-helical poly($\gamma$-benzyl L-glutamate) (PBLG). Measurements include surface pressure ($\Pi$) versus area/monomer (A) isotherms, x-ray specular reflectivity (XR), and grazing incidence diffraction (GID). In contrast to the LM of bare PBLG on water, which undergoes a monolayer/bilayer transition with increasing $\Pi$, monolayers of C16–O–PBLG remain stable up to the highest densities. On the basis of XR and GID results, the structure of the C16–O–PBLG monolayer is characterized by the following main features. First, hydrophobicity causes the –O–C16 chains to segregate towards the film/gas interface and away from water and the PBLG cores, which sit parallel to and near the water/film interface. Since the attachment position of some of the side chains is at the core/water interface, the segregation forces these chains into the space between neighboring core rods. Compression associated with increasing $\Pi$ thickens the film but the internally segregated structure is maintained for all $\Pi$ (i.e., >$\sim$30 dyne/cm). Second, the C16–O–PBLG rods form domains in which the rods are aligned parallel to each other and to the interface. The correlation length for the interhelix positional order of the rods is short and typically comparable to or less than the length of the rods. With increasing $\Pi$ the spacing d between nearest-neighbor rods decreases linearly with A at high $\Pi$, indicating a direct correspondence between the macroscopic compressibility and the microscopic interhelix compressibility. Third, as $\Pi$ increases past $\sim$5 dyne/cm, the local packing of tethered –O–C16 chains displays the same herringbone (HB) order that is common for high-density bulk and monolayer phases of alkyl chains. Various features of the observed GID peaks also imply that the HB order of –O–C16 chains is oriented with respect to the helical axes of aligned PBLG cores. We propose that the HB order is established initially by one-dimensionally confined chains between aligned rods at low Π and grows laterally with compression.Engineering and Applied Science

Autonomous Materials Discovery Driven by Gaussian Process Regression with Inhomogeneous Measurement Noise and Anisotropic Kernels

A majority of experimental disciplines face the challenge of exploring large and high-dimensional parameter spaces in search of new scientific discoveries. Materials science is no exception; the wide variety of synthesis, processing, and environmental conditions that influence material properties gives rise to particularly vast parameter spaces. Recent advances have led to an increase in efficiency of materials discovery by increasingly automating the exploration processes. Methods for autonomous experimentation have become more sophisticated recently, allowing for multi-dimensional parameter spaces to be explored efficiently and with minimal human intervention, thereby liberating the scientists to focus on interpretations and big-picture decisions. Gaussian process regression (GPR) techniques have emerged as the method of choice for steering many classes of experiments. We have recently demonstrated the positive impact of GPR-driven decision-making algorithms on autonomously steering experiments at a synchrotron beamline. However, due to the complexity of the experiments, GPR often cannot be used in its most basic form, but rather has to be tuned to account for the special requirements of the experiments. Two requirements seem to be of particular importance, namely inhomogeneous measurement noise (input dependent or non-i.i.d.) and anisotropic kernel functions, which are the two concepts that we tackle in this paper. Our synthetic and experimental tests demonstrate the importance of both concepts for experiments in materials science and the benefits that result from including them in the autonomous decision-making process

Surface layering of liquids: The role of surface tension

Recent measurements show that the free surfaces of liquid metals and alloys are always layered, regardless of composition and surface tension; a result supported by three decades of simulations and theory. Recent theoretical work claims, however, that at low enough temperatures the free surfaces of all liquids should become layered, unless preempted by bulk freezing. Using x-ray reflectivity and diffuse scattering measurements we show that there is no observable surface-induced layering in water at T=298 K, thus highlighting a fundamental difference between dielectric and metallic liquids. The implications of this result for the question in the title are discussed.Comment: 5 pages, 4 figures, to appear in Phys. Rev. B. 69 (2004