Measuring the Nonuniform Evaporation Dynamics of Sprayed Sessile Microdroplets with Quantitative Phase Imaging

Single-shot off-axis digital holographic system with extended field-of-view by using multiplexing method

We propose a new configuration of single-shot off-axis digital holographic system to realize double the camera field-of-view (FOV) of the existing off-axis Mech-Zehnder type holographic setup. The double FOV is obtained by double spatial frequency multiplexing of two different areas of an object beam by inserting a Fresnel bi-prism in it, which divides the object beam into two, both carrying different object information. The image sensor is placed at the plane where these two different FOVs overlap so as to record simultaneously two parts of the wavefront of the object in a single-shot. The multiplexed hologram is carrying two interferometric images corresponding to two different FOVs of the object which are modulated with two different spatial carrier frequencies. The feasibility of the proposed digital holographic system is experimentally demonstrated by imaging two different areas of a resolution test target. The limitation of the proposed system and a method to overcome it, are also discussed. The proposed system is useful in a wide range of applications including microscopy and optical metrology.

High spatial and temporal resolution synthetic aperture phase microscopy

A new optical microscopy technique, termed high spatial and temporal resolution synthetic aperture phase microscopy (HISTR-SAPM), is proposed to improve the lateral resolution of wide-field coherent imaging. Under plane wave illumination, the resolution is increased by twofold to around 260 nm, while achieving millisecond-level temporal resolution. In HISTR-SAPM, digital micromirror devices are used to actively change the sample illumination beam angle at high speed with high stability. An off-axis interferometer is used to measure the sample scattered complex fields, which are then processed to reconstruct high-resolution phase images. Using HISTR-SAPM, we are able to map the height profiles of subwavelength photonic structures and resolve the period structures that have 198 nm linewidth and 132 nm gap (i.e., a full pitch of 330 nm). As the reconstruction averages out laser speckle noise while maintaining high temporal resolution, HISTR-SAPM further enables imaging and quantification of nanoscale dynamics of live cells, such as red blood cell membrane fluctuations and subcellular structure dynamics within nucleated cells. We envision that HISTR-SAPM will broadly benefit research in material science and biology.

Lipid Bilayer Thickness Measured by Quantitative DIC Reveals Phase Transitions and Effects of Substrate Hydrophilicity

Quantitative differential interference contrast microscopy is demonstrated here as a label-free method, which is able to image and measure the thickness of lipid bilayers with 0.1 nm precision. We investigate the influence of the substrate on the thickness of fluid-phase 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)-supported lipid bilayers and find a thinning of up to 10%, depending on substrate hydrophilicity, local bilayer coverage, and ionic strength of the medium. With fluorescently labeled lipid bilayers, we also observe changes in the bilayer thickness depending on the choice of fluorophore. Furthermore, liquid-ordered domains in bilayers, formed from DOPC, cholesterol, and sphingomyelin, are measured, and the corresponding thickness change between the liquid-ordered and liquid-disordered phases is accurately determined. Again, the thickness difference is found to be dependent on the presence of the fluorophore label, highlighting the need for quantitative label-free techniques.

Enhanced Environmental Scanning Electron Microscopy Using Phase Reconstruction and Its Application in Condensation

Environmental scanning electron microscopy (ESEM) is a broadly utilized nanoscale inspection technique capable of imaging wet or insulating samples. It extends the application of conventional scanning electron microscopy (SEM) and has been extensively used to study the behavior of liquid, polymer, and biomaterials by allowing for a gaseous environment. However, the presence of gas in the chamber can severely degrade the image resolution and contrast. This typically limits the ESEM operating pressure below 1000 Pa. The dynamic interactions, which require even-higher sensitivity and resolution, are particularly challenging to resolve at high-pressure conditions. Here, we present an enhanced ESEM technique using phase reconstruction to extend the limits of the ESEM operating pressure while improving the image quality, which is useful for sensing weak scattering from transparent or nanoscale samples. We applied this method to investigate the dynamics of condensing droplets, as an example case, which is of fundamental importance and has many industrial applications. We visualized dynamic processes such as single-droplet growth and droplet coalescence where the operating pressure range was extended from 1000 to 2500 Pa. Moreover, we detected the distribution of nucleation sites on the nanostructured surfaces. Such nanoscale sensing has been challenging previously due to the limitation of resolution and sensitivity. Our work provides a simple approach for high-performance ESEM imaging at high-pressure conditions without changes to the hardware and can be widely applied to investigate a broad range of static and dynamic processes.

Development of a surface tension mediated technique for dry stabilization of mammalian cells

Dry state preservation at ambient temperatures (lyopreservation) is a biomimetic alternative to low temperature stabilization (cryopreservation) of biological materials. Lyopreservation is hypothesized to rely upon the creation of a glassy environment, which is commonly observed in desiccation-tolerant organisms. Non-uniformities in dried samples have been indicated as one of the reasons for instability in storage outcome. The current study presents a simple, fast, and uniform surface tension based technique that can be implemented for lyopreservation of mammalian cells. The technique involves withdrawing cells attached to rigid substrates to be submerged in a solution of lyoprotectant and then withdrawing the samples at a specific rate to an inert environment. This creates a uniform thin film of desiccated lyoprotectant due to sudden change of surface tension. The residual moisture contents at different locations in the desiccated film was quantified using a spatially resolved Raman microspectroscopy technique. Post-desiccation cellular viability and growth are quantified using fluorescent microscopy and dye exclusion assays. Cellular injury following desiccation is evaluated by bioenergetic quantification of metabolic functions using extracellular flux analysis and by a Raman microspectroscopic analysis of change in membrane structure. The technique developed here addresses an important bottleneck of lyoprocessing which requires the fast and uniform desiccation of cellular samples.

Compact single-shot four-wavelength quantitative phase microscopy with polarization- and frequency-division demultiplexing

We present a novel single-shot four-wavelength quantitative phase microscopy (FW-QPM). Four lasers operating at different wavelengths are multiplexed with a pair of dichroic mirrors and a polarization beam splitter in a three-mirror quasi-common-path interferometer. After a single-shot interference pattern is obtained with a monochrome camera, four holograms of different wavelengths were demultiplexed from it in the frequency domain with polarization- and frequency-division multiplexing. Polarization-division demultiplexing scheme uses polarization dependent visibility changes in an interference pattern, and it plays a critical role in making only two interference patterns exist within a single quadrant in the frequency domain. We have used a single-mode optical fiber as a phase object sample and demonstrated that a measured single-shot interference pattern can be successfully demultiplexed into four different interferograms of different wavelengths with our proposed scheme.

Diffraction phase microscopy imaging and multi-physics modeling of the nanoscale thermal expansion of a suspended resistor

We studied the nanoscale thermal expansion of a suspended resistor both theoretically and experimentally and obtained consistent results. In the theoretical analysis, we used a three-dimensional coupled electrical-thermal-mechanical simulation and obtained the temperature and displacement field of the suspended resistor under a direct current (DC) input voltage. In the experiment, we recorded a sequence of images of the axial thermal expansion of the central bridge region of the suspended resistor at a rate of 1.8 frames/s by using epi-illumination diffraction phase microscopy (epi-DPM). This method accurately measured nanometer level relative height changes of the resistor in a temporally and spatially resolved manner. Upon application of a 2 V step in voltage, the resistor exhibited a steady-state increase in resistance of 1.14 Ω and in relative height of 3.5 nm, which agreed reasonably well with the predicted values of 1.08 Ω and 4.4 nm, respectively.

Lateral shearing common-path digital holographic microscopy based on a slightly trapezoid Sagnac interferometer

We propose a compact and easy-to-align lateral shearing common-path digital holographic microscopy, which is based on a slightly trapezoid Sagnac interferometer to create two laterally sheared beams and form off-axis geometry. In this interferometer, the two beams pass through a set of identical optical elements in opposite directions and have nearly the same optical path difference. Without any vibration isolation, the temporal stability of the setup is found to be around 0.011 rad. Compared with highly simple lateral shearing interferometer, the off-axis angle of the setup can be easily adjusted and quantitatively controlled, meanwhile the image quality is not degraded. The experiments for measuring the static and dynamic specimens are performed to demonstrate the capability and applicability.

Realization of palladium-based optomechanical cantilever hydrogen sensor

Hydrogen has attracted attention as an alternative fuel source and as an energy storage medium. However, the flammability of hydrogen at low concentrations makes it a safety concern. Thus, gas concentration measurements are a vital safety issue. Here we present the experimental realization of a palladium thin film cantilever optomechanical hydrogen gas sensor. We measured the instantaneous shape of the cantilever to nanometer-level accuracy using diffraction phase microscopy. Thus, we were able to quantify changes in the curvature of the cantilever as a function of hydrogen concentration and observed that the sensor's minimum detection limit was well below the 250 p.p.m. limit of our test equipment. Using the change in curvature versus the hydrogen curve for calibration, we accurately determined the hydrogen concentrations for a random sequence of exposures. In addition, we calculated the change in film stress as a function of hydrogen concentration and observed a greater sensitivity at lower concentrations.

Insight into the Evaporation Dynamics of a Pair of Sessile Droplets on a Hydrophobic Substrate

In this work, we have demonstrated three unique regimes in the evaporation lifecycle of a pair of sessile droplets placed in variable proximity on a hydrophobic substrate. For small separation distance, the droplets undergo asymmetric spatiotemporal evaporation leading to contact angle hysteresis and suppressed vaporization. The reduced evaporation has been attributed quantitatively to the existence of a constrained vapor-rich dome between the two droplets. However, a dynamic decrease in the droplet radius due to solvent removal marks a return to symmetry in terms of evaporation and contact angle. We have described the variation in evaporation flux using a universal correction factor. We have also demonstrated the existence of a critical separation distance beyond which the droplets in the droplet pair do not affect each other. The results are crucial to a plethora of applications ranging from surface patterning to lab-on-a-chip devices.