Fluorescent Image Correlation for Nanoscale Deformation Measurements

Abstract

Recent advances in nanotechnology have enabled the fabrication of a new generation of materials with highly complex structures. The characteristic length scale of these materials has now outpaced the ability of current techniques to make full-field, nanoscale mechanical property measurements in real time. In addition, biological materials also possess complex structures at the nanoscale, which can affect their resulting larger-scale response. Measurements of bulk properties, while important, offer little information about how the nanostructure influences performance. Localized measurements, such as nanoindentation, provide data that are often difficult to extend to larger length scales. High-resolution imaging techniques such as atomic force or scanning tunneling microscopy (AFM/STM) [1] and near-field scanning optical microscopy (NFSOM), [2] despite being full-field techniques, involve bringing a tip on or very near the surface and thus are not entirely suitable for use with soft (e.g., biological) materials. In addition, these methods require rastering, which limits real-time visualization capability (e.g., of fracture, of the motion and growth of cells, and so on). Fluorescent dyes and particles have enabled a different set of experimental tools for imaging displacements. Fluorescent particles have been widely used in flow visualization and measurement techniques. [3,4] In biological studies, fluorescent dyes indicate the presence of particular microorganisms or track the growth and development of cellular structures. [5][6][7][8] Recent advances in optical techniques have improved discrete fluorescent particle imaging, [9][10][11][12] making laboratory nanoscale measurements feasible. Single-particle tracking (SPT) via confocal laser scanning microscopy is a tool used in biophysical research to observe trajectories of small fluorescent particles with nanometer-scale precision. [13,14] Here, we present a powerful full-field technique, fluorescencebased digital image correlation (FDIC), to measure nanoscale deformation in materials using fluorescent nanoparticles. To demonstrate the capabilities of the method, we characterize the complex deformation fields generated around silica microspheres embedded in an elastomer under tensile loading. Displacement resolutions of 20 nm are obtained over a 100 100 mm field of view.Digital image correlation (DIC) is a data analysis method, which applies a mathematical correlation algorithm to obtain kinematic information from digital images acquired during deformation. [15,16] For conventional two-dimensional (2D) DIC, samples are prepared for testing by the application of a random speckle pattern to their surface. Comparison of successive images reveals a deformed speckle pattern relative to the initial, nondeformed one. The correlation works by matching small square subsets of the nondeformed image (Figure 1 a) to locations in the deformed image (Figure 1 b). The core of the DIC method lies in the optimization of a correlation coefficient between the two subsets over six ...

Novel method for mechanical characterization of polymeric nanofibers

“…The sample grips were subject to rigid body motions that can be resolved with resolution significantly better than the pixel size by using DIC. 32 Specifically, the rigid body motion of each grip was computed as the average of the motion of an area of 100ϫ 100 pixels.…”

“…32,38 The displacement accuracy depends on the imaging conditions, the signal-to-noise ratio of the imaging system, and the displacement distribution ͑gradient͒. Large correlation squares in DIC application favored better accuracy in capturing rigid body motions.…”

“…The 65 nm uncertainty is approximately 1 / 8 of the average wavelength of light used in the experiments which correlates well with previous reports about the resolution of DIC. 32 Since the uncertainty is not cumulative, measurement of large rigid body motions minimizes its relative contribution to the overall displacement. Thus, longer fibers improve the accuracy in determining stress-strain curves.…”

“…However, even in the case of SEM imaging, it is the effective "cross-head" displacements that are recorded, which reduces the problem to that of monitoring the motion of the sample grips. To this effect, rigid body motions can be resolved from optical images with an accuracy of a few tens of nanometers 32 by the application of digital image correlation ͑DIC͒. 33,34 It is, thus, not a prerequisite to use electron microscopy to obtain accurate engineering stress-strain curves from nanomechanical experiments.…”

Micro- and Nanoscale Deformation Measurement of Surface and Internal Planes via Digital Image Correlation

“…1(b)], while for nanoscale experiments fluorescent nanoparticle tracers are employed [ Fig. 1(c)] [12]. Rigid body translation and uniaxial tension experiments are carried out separately to establish the displacement resolutions at each scale.…”