From soap bubbles to polymers, soft matter have accompanied humankind through countless centuries. In the past decade, the new paradigm of Industry4.0 has seen the introduction of new technologies and methods of processing materials, for example, direct printing, additive, and bottom-up manufacturing processes. They are expected to involve new ways of making products in future, and most innovative fabrication processes will be based on the manipulation of soft matter that will be shaped at the nanoscale. Therefore, high-precision quantitative characterization of soft matter turn to be the key to open the door of future manufacturing processing and technology.
In a new paper published in Light: Advanced Manufacturing, a team of scientists, led by Dr. Pietro Ferraro from Institute of Applied Sciences and Intelligent Systems "E. Caianiello" - Italian National Council of Research (ISASI-CNR) and Prof. Pier Luca Maffettone from University of Naples Federico II have reviewed the most significant examples to demonstrate the capability of digital holography (DH) to measure soft matter structures and the related fabrication techniques.
For the first time, a comprehensive overview of such a fascinating field of investigation is described where 3D-imaging and accurate measurements are merged into a single metrological tool to study soft matter for different applications. This review is divided into three sections for systematically expounding the application and development of DH technologies in soft matter measurement; different types of challenging cases are descripted and discussed, including thin liquid films and membranes, ink-jet printing process, self-assembling processes of polymer and/or liquids, microfluidic rheology, and study of solid-liquid interfaces.
The authors show that 3D dynamic monitoring in real-time or in situ using DH enables quantitative measurements and accurate characterization for soft matter.
In the first section of this review, the authors introduce the applications of DH in thin-film measurements and explore the feasibility of the technique in the measurement of dynamic process. The conventional DH recording geometries and strategies for retrieving thin-film thickness are discussed. Then, through a series of experimental datasets, the authors demonstrate that DH has a strong ability to characterize films of different materials and films with different motion states. Meanwhile, a side-by-side measurement between DH and white light interferometry (WLI) is shown off.
Owing to the combination of WLI and DH, it is possible to achieve high-precision real-time thickness measurements while expanding the imaging field of o view (FoV) and range. As the last part of this section, thickness mapping of film rupture process based on high-speed CMOS camera is demonstrated, which helps in modeling the thin film opening process using holographic technology for the first time.
In the second section, the authors show as DH can be useful for inkjet printing, additive manufacturing, and advanced fabrication techniques. Thanks to the non-contact, non-destructive, real-time measurement capability of DH, quantitative control of micro-nano fabrication process can be achieved. A representative example is DH used for monitoring the tethered pyro-electrospinning (TPES) process.
The authors, for the first time, shows the holographic 3D tracking of polymeric cone and beads-on-a-string (BOAS), which will allow the visualization and quantitative analysis of a complex fabrication process of electrospun polymeric fibers. Moreover, the holographic characterization of self-assembled liquid microlenses and polymeric lenses printed by EDH ink jet printing are discussed; based on the real-time 3D morphology of the lenses, the high-precision fabrication processes can be achieved. In fact, DH can be used to measure most of transparent or translucent soft matter with certain condition; in addition to the above examples, the authors also review the DH application in fabrication process of microchannels, micro-scaffold, two-photon polymerization, and surface relief gratings.
In the last section, the authors demonstrate the application of DH to measure the generalized soft matter. In fact, owing to the numerical refocusing process during DH reconstruction, the multi-layer information orthogonal to the illumination direction of object beam can be revealed. This will allow 3D positioning and tracking of moving objects in recording FoV. Herein, the authors review not only the ability of DH to monitoring the mechanical stress but also the 3D tracking of microparticles.
Results prove that DH is a powerful and flexible tool that can be used for measurement needs in a variety of scenarios. Furthermore, the combination of DH and deep learning in particle tracking is also discussed, which effectively improve the accuracy and range of DH in related measurement.
The results reviewed in this paper show that DH is robust in many challenging situations as it does not require frequent calibration while maintaining the attractive abovementioned features. The authors believe that DH has a bright future in the field of soft matter and advanced fabrication processes owing to its intrinsic capability to perform 3D imaging combined with quantitative analysis.