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The most important advantage arising from the invention of holography was realized with the development of lasers. The applications of holography has been exploited in many industrial fields as a very sensitive measurement tool, such as aerospace, automotive, energy and cultural heritage, owing to the discovery of holographic interferometry by Powell and Stetson1, 2. In fact, by employing different holographic modalities, such as holographic interferometry, time-average holography, or the so-called TV and/or digital speckle holography, it was possible to develop accurate optical measuring strategies for non-destructive testing systems and experimental mechanics3, 4. The above holography-based measurement systems were applied successfully to guarantee the quality of industrial products (e.g., turbine blades, tires, composite materials, photovoltaic solar panels, automobile components, and electronic circuits). Later, with the microelectronic revolution in the 80s and 90s as well as the surge of capabilities of micro-nanofabrication techniques, holography-based methods were successfully applied by scaling their probing-gauge down to microsystems (i.e., MEMS, MOEMS, and NEMS)5-10. In the past 20 years, the introduction of powerful PCs and high-performance solid-state light sensors has resulted in the achievement of further developments of holographic technology in digital-modality toward biological, biomedical, and industrial applications, which has led to the widespread use of such systems11-16.
Recent progress in bottom-up self-assembly fabrication approaches and direct printing for production processes, especially at the micro and nanoscales, has motivated the development of holographic optical tools for achieving accurate and full-field characterization of systems based on soft matter materials, such as liquids and polymers. These are expected to be the basis of the next revolution in fields of biotechnology and flexible electronics systems, among others. Indeed, the most common imaging modality for such microfabrication techniques is direct optical imaging, which is a video-based method. It enables the monitoring of the dynamic evolution of the processes and measuring features related to the geometrical dimensions of the microstructure17, 18. High-speed and high-resolution cameras were used for the detection of small particles during inkjet printing. Although such imaging systems can be complex, significant information is completely lost as in the case of the detection and measurement of out-of-focus structures and particles. Furthermore, not all of the information related to the phase shift introduced by the samples can be evaluated. Many microstructures are transparent or semi-transparent, and it is critical to measure the phase delay introduced in the light path, as in the case of the evaluation of the optical quality of optical components, such as microlenses and thin-film thickness mapping at high spatial resolution. For these reasons, the inspection of soft matter and related fabrication processes takes advantage of the noteworthy capabilities of DH as a metrology tool. As summarized below, the main attractive features of DH enable the achievement of the necessary capability and flexibility in the measurements:
1) The possibility of numerically managing the complex wavefront scattered or transmitted by the sample under investigation allows the extraction of all of the information (intensity and phase) through a full-digital modality.
2) Flexibility in achieving object imaging well-in-focus (or to obtain good focus of any portion of it) that enables correct DH microscopic measurements to be obtained, even though the object could have a three-dimensional (3D) shape behind the depth of focus of the optical imaging system.
3) Potential to retrieve phase-contrast maps that enable quantitative measurements of the sample in full-field mode and 3D.
4) The possibility to manage and remove aberrations in the optical system using simple and flexible methods, thus simplifying the optical apparatus and measurement operations.
Owing to the above unique features of DH, this paper reviews what are considered the most significant examples to demonstrate the capability of DH to measure soft matter structures and the related fabrication techniques. In particular, we focused on different types of challenging cases.
• thin liquid films and membranes;
• ink-jet printing process for directly printed structures;
• analysis and quantitative measurements of bottom-up self-assembling processes of polymer and/or liquids;
• microfluidic rheology and study of solid-liquid interfaces.
The four different above-mentioned typical cases enable us to demonstrate the aptitude of DH systems in combining metrology and 3D imaging capabilities and by furnishing quantitative measurements in challenging circumstances. We show that 3D dynamic monitoring in real-time or in situ using DH enables quantitative measurements and accurate characterization.
It should be noted that biological matter, both cell populations and tissues, benefits from the above-mentioned characteristics, so DH in recent years has been extensively developed, enabling its incorporation in life sciences19-26. However, we focus on and limit the present review to fabrication topics, thus supplying a complete overview of DH in this area without including cells of biological matter that have different peculiarities.
This review is organized into three sections. The first focuses on the measurement of the thin-film thickness over a wide field of view (FoV). Various experimental configurations are described, and their applicability to different materials is provided. For such applications, the thickness measurement ranges from ~50 nm to ~30 μm, with a resolution at the nanometer scale on a large FoV of a circular area with a radius of ~20 mm. The second section is centered on a specific microfabrication technique based on the exploitation of the pyroelectric effect of ferroelectric crystals, where liquids and polymers can be manipulated by electric fields. This enables us to provide many examples where DH has proven to be a suitable metrological tool not only for transparent solid microstructures but also for the full characterization of dynamic processes such as the self-assembly of liquids for the realization of microstructures. The main results show that high-resolution two-dimensional (2D) and 3D measurements can be achieved by exploiting the quantitative phase content encoded in the digital holograms. The third section presents DH to investigate the rheological properties, monitor stress, and track particles embedded in solution or liquid film. All three sections contain a detailed description of the related state-of-the-art. Different setups and target materials, in addition to diverse observables, are described and discussed in this manuscript.
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From soap bubbles for children to semiconductors for electronics, thin films have accompanied humankind through countless centuries, and are an important category of soft matter. Over the past century, with the improvement in industrial production technologies, a large number of products related to thin-film materials have been produced, especially in the field of daily chemistry. With the informatization and intelligence of human society, wearable and in-body devices are becoming the general direction of future technology applications. Because of its special physical and chemical properties, film material has become the best carrier to support future devices, and a detailed measurement of its characteristics is highly desirable.
The mapping of the thin-film thickness is an open and long-term challenge27. One of the well-known methods for mapping film thickness is the interferometric approach28, which uses the optical path difference between the beam’s reflection of the upper and lower surfaces of the film to create an interferometric pattern from which the thickness information of the thin film can be determined29, 30. The interferometry method has been used for a long time. However, conventional interferometric measurements have some shortcomings, such as a small measurement FoV, limited measuring range, and non-real-time measurement. To overcome these drawbacks, a series of interferometric techniques has been proposed in recent decades, namely colorimetric interferometry31, 32, multiwavelength interferometry33, phase-shift interferometry34, and scanning interferometry35, etc. However, for the dynamic thickness measurement of a thin liquid film under a large FoV, interferometric methods hardly meet all recording requirements, especially for films with high viscosity coefficients. In 2019, for the first time, Ferraro et al. implemented DH to map the thin-film thickness during the evolution of liquid bubbles36. In 2021, Wong et al. applied DH to measure beer bubble morphology37. The height of the beer bubbles and water surface can be revealed simultaneously by the back-propagation process of DH. In fact, most film materials are good samples for holographic measurements because of their transparent or semi-transparent properties38. However, the existence of micron-level deformation in the film evolution generates scattering and/or refraction for object beam, which significantly affects holographic recording. This problem was solved by introducing a telescope structure before the camera, which could collect the beams and adjust the magnification. As a result, real-time holographic measurements of thin-film thickness were successfully realized. Many thin-film measurement experiments have been conducted using holography in different scenarios. In the following subsections, we systematically introduce the applications of DH in thin-film measurements and explore the feasibility of the technique in the measurement of soft matter.
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The primary problem faced by DH in thin-film measurement is the limited thickness measurement range in a single shot39. There is a specific measurement range for the holographic recording of laser sources in the visible spectral range. Moreover, owing to the influence of surface tension, the thickness distribution of thin-film materials is usually continuous, enabling numerical holographic phase unwrapping. Considering the real-time requirements of film thickness measurement, the off-axis DH system could be a good candidate for testing14, 40. In preliminary optical geometry, an off-axis Mach-Zehnder recording system was used to measure the dynamic film thickness evolution process36, 41. A telescope structure was set before the camera to adjust the FoV and suppress the influence of film scattering (the setup and experimental results are shown in Fig. 1.
Fig. 1 Conventional holographic thin-film thickness mapping, setup, and preliminary results. a Digital holographic setup based on Mach-Zehnder structure. b Thickness change before and after rupture in the central area of a polymer film. c Full FoV thickness mapping of selected timepoints in b. Reproduced from Ref. 36 © 2021 Springer Nature.
Because thin films represent imaging objects with rich and varied high-frequency information, it is difficult for a lens-less system to collect all information during the recording process. Therefore, a telescope structure (composed of two lenses with different focal lengths) represents a strategy for collecting object information. As shown in Fig. 1a, lenses L1 and L2 (focal length of L1 is longer than that of L2) were set to construct a telescope lens group for zooming out. The light-collecting ability of the 4-Focus lens group allowed the collection of scattered object beams. In other words, this setup allowed the enlargement of the FoV and the simultaneous recording of high-frequency information. In the initial testing, a film close to rupture was selected as the sample. The thickness change in the central area of the film was measured41, as shown in Fig. 1b. It can be seen that for thin-film materials with a known refractive index (RI), DH is a good tool for measuring the thickness of the film, and it does not require other reference images. Generally, in interferometry, it is necessary to clearly observe the appearance of a common black film42 to obtain absolute thickness measurements. However, for holographic measurements, when the thickness of the film is less than twice the recording wavelength, the following phase-thickness conversion formula can be used directly to extract the thickness information from the phase map:
$$ h=\frac{\varphi \lambda }{2\pi n} $$ (1) where h is the thickness, φ is the phase, λ is the recording wavelength, and n is the RI of the film. For the range of thickness measurement in a holographic recording system with a specific laser wavelength, the lower limit depends on the wavelength, whereas the upper limit can be continuously increased owing to the holographic phase unwrapping algorithm. Here, PUMA43 phase unwrapping is a good candidate for extracting film thickness information because of its good phase-unwrapping capabilities for a continuously changing phase. However, in the case of films where the maximum thickness reaches a few micrometric levels or even higher, conventional holographic reconstruction will not be able to implement absolute thickness measurements owing to the limitation of the wrapping phase. To solve this problem, a full life cycle measurement method was proposed36. To do this, the entire thickness evolution process of the film, from the initial formation to the final rupture, needs to be recorded. Once the film is close to rupture, the starting area of the rupture tends to be a common black film, indicating that it has the lowest phase value in the full FoV. Taking advantage of this feature, a time variable can be added to the phase-unwrapping process to deal with the phase jump in all recorded frames, as shown in Fig. 2.
As a film thickness extraction scheme, the full life cycle recording method has certain limitations; in fact, the measured film must have a clear natural rupture process. For most free-standing thin liquid films, the natural rupture process will occur owing to gravitational drainage44, which enables us to use the proposed strategy for extracting film thickness information. Therefore, compared to other existing optical measurement methods, DH offers high precision, a large measurement range, and real-time characteristics, representing a good tool for thin liquid film thickness mapping, as discussed in detail in the following sections.
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DH is based on the numerical diffraction reconstruction of digitally recorded interference fringes to obtain the spatial information of the recorded object45,46, whereas white-light interferometry (WLI) uses the direct identification of the number and color of interference fringes to obtain thickness information28, 29. These two methods have certain limitations in real-time thin-film thickness mapping, but for some specific materials, both can be used, for example, surfactant film. Thus, the complementary characteristics of WLI and DH enable comparison and fusion. To implement simultaneous recording for one liquid film using the two methods, joint imaging was designed by combining the conventional off-axis DH geometry and WLI geometry. To ensure that the WLI recording did not affect the intensity contrast of the hologram, oblique illumination was implemented47. Owing to the combination of WLI and DH, it is possible to achieve high-precision real-time thickness measurements while expanding the imaging FoV and range48. The experimental setup, which combines the two interferometric geometries, is shown in Fig. 3a.
Fig. 3 Optical geometry and results of DH and WLI hybrid imaging. a Experimental setup for simultaneous recording. b Comparison of measurement results at two different time points. c The complementary characteristics of white light interferometry and digital holography will provide high-precision and wide-range thin-film thickness measurement. Adapted with permission from Ref. 48 Copyright 2021 American Chemical Society.
In the experiment reported in Fig. 3b, a surfactant film was selected as the experimental sample, and its RI was determined in a previous study49. Meanwhile, for the WLI measurement, the thickness-color correspondence of the surfactant film is known50. The surfactant film was formed in a fixed metal ring and simultaneously illuminated by white light and a red laser beam. It can be seen from the experimental results in Fig. 3b that when the film was close to rupture, both DH and WLI could map the thickness of the full FoV. Based on the comparison, it can also be found that the measurement results of DH and WLI maintain good consistency in the range below 1 μm. Moreover, for a thickness greater than 1 μm, WLI hardly provides the right thickness value, while DH still works. For the results at the early stage after film formation, the WLI interferogram showed complicated patterns; thus, it was difficult to interpret and retrieve the effective thickness. The colored interference fringes in such a range of film thicknesses were spatially and periodically modulated by the complex film structure, which led to an impossible reading. On the other hand, the thickness distribution information can be clearly identified in the results of DH. This is the greatest advantage of holographic measurement: for films with complex structures, holographic measurement can still provide an accurate thickness distribution. In conclusion, for surfactant films with a maximum thickness of less than 1 μm and a smooth surface structure, both DH and WLI can be used. However, for films with a higher thickness or complex surface structure, DH would be a better choice. Therefore, the reliability of the DH measurements can be verified through WLI experiments.
However, in the case of film with thickness less than half the wavelength, transmission holography has difficulties in providing accurate thickness measurements. Therefore, one strategy that has been effectively adopted is to use the complementary characteristics of WLI and DH for a new hybrid measurement modality48. Thus, phase correction was achieved by comparing the two different measurement results. From the results in Fig. 3c, the deviation in the low-thickness area was significantly improved; essentially, the hybrid recording and reconstruction completed the phase calibration of the film and achieved an accurate thickness distribution. As a tool for improving microscopic resolution, holographic imaging can be easily integrated with other technologies, creating a wider range of possibilities.
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In the above two sections, conventional imaging setups and methods of DH in thin-film imaging were discussed for the measurement of the thickness level. Furthermore, the difference between the results of WLI and DH for the same film was compared, proving that DH is a reliable measurement method for thin films. In conventional interferometric experiments, the measurement methods and systems are limited because of the need to keep the sample as smooth as possible. In addition, because the distance between the camera and the sample has certain requirements for creating interference fringes, it is difficult to measure the thickness distribution when the film is moving. However, these limitations can be overcome for DH recording. The DH geometry has good flexibility in structural settings; therefore, it can deal with different experimental scenarios51. Meanwhile, thanks to the numerical diffraction propagation process, DH can present real-time thickness mapping for the film in motion. Fig. 4 shows the time-lapse recording results obtained by DH in different film motion modes, including pumping bubble, thin film lifting, and thin film opening. Thickness mapping for these processes appeared to be difficult during interferometric recording, but could be completed in DH recording.
Fig. 4 Digital holography allows thin-film thickness measurements to be produced under different scenarios, breakthrough the limitation for film forming requirement in interferometry. a Measurements for pumping bubble. b Measurements for thin liquid film lifting in tube. c Measurement for film opening.
In an experiment on pumping liquid bubbles, a flat thin film was prepared at one end of a customized tube, and air was pumped into the tube to promote bubble growth. Finally, a semicircular bubble was formed36. The transmissive DH can effectively record the thickness of the bubble during the growth process, and then the thickness information extraction can be completed through numerical refocusing and phase distortion calibration. Fig. 4a shows the measurement results of related experiments. In this experiment, a polyacrylamide (PA) solution was used to create a thin liquid film. For this kind of film, the reconstruction of thickness mapping could be provided using the full life cycle recording method. In addition, owing to the phase distortion calibration, the hemispherical film thickness distribution can be projected as a horizontal mode, which is convenient for researchers to analyze the drainage process. For the lifting thin-film experiment, the film was prepared in the middle of a customized tube and then lifted by pumping air into the tube from the bottom. Because of the centimeter-level movement of the film inside the tube, conventional interferometric methods were unable to provide the thickness distribution during the movement; however, DH could solve this problem. Fig. 4b shows the results of related experiments. The drainage phenomenon in the process of film uplift is related to the velocity of movement, which introduces the potential for controllable drainage. This will help to better study the physical model of the drainage process. The results showed the distribution of the film structure at different moving speeds, and the difference in drainage patterns was revealed. For the film-opening experiments, an optical iris was conceived and manufactured to open the film52. Then, an adjusted telescope structure was used to reach a large FoV (~40 mm diameter), which ensured the recording of the entire film area during the film-opening process. This is also a significant advantage of holographic measurement; the FoV can be changed by adjusting the lens group, thereby realizing continuous holographic recording with a large FoV. The opening process of the film included a potential change in the overall film thickness from high to low. This experimental system ensured that the total amount of solution used to prepare the film remained constant, thereby facilitating the analysis of the hydrodynamic process. Fig. 4c shows the results of related experiments.
In the above three experimental scenarios, holographic measurements showed good thickness measurement capabilities. For most experimental environments, the holographic setup can be easily changed to meet the requirements of recording, enabling its application in the field of fluid mechanics testing.
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The term “thin film” is ubiquitous in our daily lives, and includes a broad range of materials, from solid to liquid films. In general, under the conditions of interferometric measurements, different configurations are used to complete experiments on different matters. For example, tilted WLI is mainly used for liquid films20, whereas a vertical interferometric system is required for solid films, such as semiconductors of the indium tin oxide (ITO) layer53. In this section, we show the measurement results for films made of different matter to demonstrate the good adaptability of the DH technology in film measurements.
The main problem with conventional interferometric measurements is that it is difficult to realize real-time imaging with a large FoV. In recent years, some methods have been proposed to solve this problem54, but certain restrictions on the RI and film thickness have been added. However, DH recordings are not affected by these limitations. Fig. 5 shows liquid films prepared using three different solutions: surfactants48, polyacrylamide (PA)36, and polydimethylsiloxane (PDMS)52.
Fig. 5 Digital holography has excellent adaptability to the measurement of transparent thin liquid films with different thickness and surface morphology.
Although we can observe differences in the thickness and morphology of the different films, as shown in Fig. 5, the DH system successfully recorded thickness information. For transmissive holographic recording, light transmittance and polarization of the recording object must be considered. For thin-film materials, most liquid films have good light transmittance, and their polarization effects are so small that they can be ignored. Therefore, holographic measurements can be used for most transparent thin liquid films. Compared with liquid films, solid films are more difficult to image; a main reason is the molecular spacing of the solid films is closer than that of the liquid film, which means that the RI of solid films is generally greater than that of liquid films. Currently, the measurement technology for solid nonuniform thin films remains an unresolved challenge. Many factors limit conventional measurement methods55, although some initial results have been obtained30. From the above experiments, holography has shown excellent measurement capabilities for different materials, is not limited to physical properties, and can successfully provide measurements for most of thin-film materials. This is a very interesting feature that challenges the stringent requirements of interferometry for samples.
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In the previous sections, the film imaging process at a standard recording speed was discussed. However, for holographic recording systems, owing to the excellent real-time imaging capabilities in off-axis geometry, it is possible to use a high-speed CMOS camera to dynamically follow the thickness evolution. Among these, the rupture process of films is a widely studied issue. For a long time, there has been no reliable measurement technology to capture the edge morphology during film rupture, and DH technology can therefore fill this gap.
The film rupture process can be easily imaged using a conventional off-axis holographic setup with a high-speed camera (980 fps). The related process was modelled to describe the rupture mechanism33. From the time–space model in Fig. 6c, it can be observed that after using the needle to break the film, it resists the rupture process. These results were revealed using holographic technology for the first time. We believe that relevant experimental data will provide important data support for researchers in fluid mechanics. In fact, many microfluidic phenomena occur very quickly and can be analyzed with the help of DH.
Fig. 6 Digital holographic high-speed imaging for recording thin-film rupture process, experimental scenarios, results, and modeling. a Implementation of the film rupture process. b Description of parameters of rupture area. c Holographic thickness reconstruction of the fracture process. d Modeling the process of film resistance to rupture. Reproduced from Ref. 36 © 2021 Springer Nature.
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We have presented, reviewed, and extensively discussed how DH can be a suitable metrology tool for use in a variety of challenging cases dealing with soft matter, that is, with liquid/polymer matter and fast dynamically evolving processes such as inkjet printing. DH can successfully provide engineering solutions for many applications and basic scientific questions. Even though this review is not exhaustive, we believe that we have reported the most significant cases in which such measurement techniques can be effectively applied to soft matter. We have intentionally excluded “biological matter”, as it is outside the scope of this review. In particular, we reported on the applications of DH measurement for different types of soft matter, such as thin films, EHD drawn fibers, polymeric lenses, micro-channels, hydrogel micro-scaffolds, photoresists, surface relief gratings, and particles. Table 1 lists the implementation conditions and parameters of the relevant measurements, where we identified the optimal measurement approaches for different materials/processes.
Soft matter Method Light Source Measurement range Sample feature Thin-film WLI White light ~50 nm to ~1.5 μm Smooth liquid film with thickness less than 1.5 microns. Off-axis DH Visible laser ~300 nm to ~100 μm Transparent film with continuous morphology. Hybrid approach White light
Visible laser~50 nm to ~100 μm All types of liquid film. EHD drawn fiber Off-axis DH Visible laser According to morphology
and phase unwrapping.Transparent or translucent soft- matter micro-structures with continuous thickness distribution and no-hollow structure. Polymeric lens Micro-channel Hydrogels micro-scaffold Photoresist Surface Relief Grating Particles In-line DH Visible laser Nanoscale particles moving
in microscale.Solution embedded particles. Off-axis DH Visible laser Microscale particles moving
in milliscale.Solution or films embedded particles. Table 1. Cross-overview over the different materials and DH presented approaches
For most transparent soft matter, DH is one of the best candidates for implementing measurements. Among them, the special case is for ultra-thin liquid films, meaning liquid films with an average thickness of less than 1 μm; WLI will be the best method in theory because it has excellent resolution for smooth sub-micron thickness distribution. Meanwhile, we indicate the possible measurement ranges for different materials, but these ranges are only reasonable references. In soft matter holographic measurements, the precise measurement range is related to the choice of recording wavelength and sample morphology. For an interferometric imaging system, the horizontal resolution depends on the selected magnification lens, while the axial resolution depends on the recording wavelength and the minimum pixel size of the camera. Therefore, one of the advantages of holographic measurement is its adjustable measurement range and optimized measurement accuracy. Although holographic phase errors can be analyzed and calibrated through specific methods, the main measurement uncertainty still comes from the light source itself133, which is different from interferometric measurement. The measurement error of interferometry arises from the number of observed interference fringes and the position of the sample134-136. This also implies that one of the reasons for choosing DH as the main measurement method for soft matter is that the placement of the sample does not affect the measurement accuracy.
The results reviewed here show that DH is robust in many challenging situations as it does not require frequent calibration while maintaining the attractive features of DH, that is, non-invasive, no-contrast agent, full-field imaging, high spatial resolution, and temporal high-frequency, up to the MHz range. We 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 analysis137. This idea is also supported by the increase in technological applications of thin films, from electronic to cosmic protective clothing and packaged water in gravity-free environments. Several other examples and published papers can be found discussing other challenging applications, we apologize if it was not possible to include all of them in this review.