Monitoring sessile droplet evaporation on a micromechanical device

Optomechanical measurement of single nanodroplet evaporation with millisecond time-resolution

Tracking the evolution of an individual nanodroplet of liquid in real-time remains an outstanding challenge. Here a miniature optomechanical resonator detects a single nanodroplet landing on a surface and measures its subsequent evaporation down to a volume of twenty attoliters. The ultra-high mechanical frequency and sensitivity of the device enable a time resolution below the millisecond, sufficient to resolve the fast evaporation dynamics under ambient conditions. Using the device dual optical and mechanical capability, we determine the evaporation in the first ten milliseconds to occur at constant contact radius with a dynamics ruled by the mere Kelvin effect, producing evaporation despite a saturated surrounding gas. Over the following hundred of milliseconds, the droplet further shrinks while being accompanied by the spreading of an underlying puddle. In the final steady-state after evaporation, an extended thin liquid film is stabilized on the surface. Our optomechanical technique opens the unique possibility of monitoring all these stages in real-time.

A Quartz Crystal Microbalance, Which Tracks Four Overtones in Parallel with a Time Resolution of 10 Milliseconds: Application to Inkjet Printing

A quartz crystal microbalance (QCM) is described, which simultaneously determines resonance frequency and bandwidth on four different overtones. The time resolution is 10 milliseconds. This fast, multi-overtone QCM is based on multi-frequency lockin amplification. Synchronous interrogation of overtones is needed, when the sample changes quickly and when information on the sample is to be extracted from the comparison between overtones. The application example is thermal inkjet-printing. At impact, the resonance frequencies change over a time shorter than 10 milliseconds. There is a further increase in the contact area, evidenced by an increasing common prefactor to the shifts in frequency, Δf, and half-bandwidth, ΔΓ. The ratio ΔΓ/(-Δf), which quantifies the energy dissipated per time and unit area, decreases with time. Often, there is a fast initial decrease, lasting for about 100 milliseconds, followed by a slower decrease, persisting over the entire drying time (a few seconds). Fitting the overtone dependence of Δf(n) and ΔΓ(n) with power laws, one finds power-law exponents of about 1/2, characteristic of semi-infinite Newtonian liquids. The power-law exponents corresponding to Δf(n) slightly increase with time. The decrease of ΔΓ/(-Δf) and the increase of the exponents are explained by evaporation and formation of a solid film at the resonator surface.

Technique and Circuit for Contactless Readout of Piezoelectric MEMS Resonator Sensors

A technique and electronic circuit for contactless electromagnetic interrogation of piezoelectric micro-electromechanical system (MEMS) resonator sensors are proposed. The adopted resonator is an aluminum-nitride (AlN) thin-film piezoelectric-on-silicon (TPoS) disk vibrating in radial contour mode at about 6.3 MHz. The MEMS resonator is operated in one-port configuration and it is connected to a spiral coil, forming the sensor unit. A proximate electronic interrogation unit is electromagnetically coupled through a readout coil to the sensor unit. The proposed technique exploits interleaved excitation and detection phases of the MEMS resonator. A tailored electronic circuit manages the periodic switching between the excitation phase, where it generates the excitation signal driving the readout coil, and the detection phase, where it senses the transient decaying response of the resonator by measuring through a high-impedance amplifier the voltage induced back across the readout coil. This approach advantageously ensures that the readout frequency of the MEMS resonator is first order independent of the interrogation distance between the readout and sensor coils. The reported experimental results show successful contactless readout of the MEMS resonator independently from the interrogation distance over a range of 12 mm, and the application as a resonant sensor for ambient temperature and as a resonant acoustic-load sensor to detect and track the deposition and evaporation processes of water microdroplets on the MEMS resonator surface.

Single cell imaging with near-field terahertz scanning microscopy

OBJECTIVES

Terahertz (THz)-based imaging techniques hold great potential for biological and biomedical applications, which nevertheless are hampered by the low spatial resolution of conventional THz imaging systems. In this work, we report a high-performance photoconductive antenna microprobe-based near-field THz time-domain spectroscopy scanning microscope.

MATERIALS AND METHODS

A single watermelon pulp cell was prepared on a clean quartz slide and covered by a thin polyethylene film. The high performance near-field THz microscope was developed based on a coherent THz time-domain spectroscopy system coupled with a photoconductive antenna microprobe. The sample was imaged in transmission mode.

RESULTS

We demonstrate the direct imaging of the morphology of single watermelon pulp cells in the natural dehydration process with our near-field THz microscope.

CONCLUSIONS

Given the label-free and non-destructive nature of THz detection techniques, our near-field microscopy-based single-cell imaging approach sheds new light on studying biological samples with THz.

Fully Differential Piezoelectric Button-Like Mode Disk Resonator for Liquid Phase Sensing

We present a unique lateral shear resonance mode excited in a microelectromechanical (MEM) disk resonator. We refer to this proposed mode as the button-like (BL) mode. The BL mode has a characteristic lateral strain profile (based on the sum of orthogonal strain components in the plane of fabrication) which resembles a shirt button, hence our choice of name for this mode. The strain profile of the BL mode is highly suited for piezoelectric transduction. Like the more widely reported wine-glass (WG) or elliptical mode, the BL mode offers feedthrough cancellation through fully differential transduction. However, compared to the WG mode, the BL mode possesses a higher coupling coefficient ( [Formula: see text]) and a higher quality ( Q ) factor for the same disk radius. These advantages make the BL mode highly attractive for realizing electrically addressed MEM resonators for liquid-phase sensing. This paper examines various design aspects pertaining to the BL mode: tether geometry, characterization setup, size of disk, and even the effect of the gap around the disk on the Q factor. The highest Q factor measured in water is 410 based on a disk with a radius of [Formula: see text]. The lowest motional resistance in water is 1.36 [Formula: see text] based on a disk with a radius of [Formula: see text].

Acoustic biosensors

Resonant and acoustic wave devices have been researched for several decades for application in the gravimetric sensing of a variety of biological and chemical analytes. These devices operate by coupling the measurand (e.g. analyte adsorption) as a modulation in the physical properties of the acoustic wave (e.g. resonant frequency, acoustic velocity, dissipation) that can then be correlated with the amount of adsorbed analyte. These devices can also be miniaturized with advantages in terms of cost, size and scalability, as well as potential additional features including integration with microfluidics and electronics, scaled sensitivities associated with smaller dimensions and higher operational frequencies, the ability to multiplex detection across arrays of hundreds of devices embedded in a single chip, increased throughput and the ability to interrogate a wider range of modes including within the same device. Additionally, device fabrication is often compatible with semiconductor volume batch manufacturing techniques enabling cost scalability and a high degree of precision and reproducibility in the manufacturing process. Integration with microfluidics handling also enables suitable sample pre-processing/separation/purification/amplification steps that could improve selectivity and the overall signal-to-noise ratio. Three device types are reviewed here: (i) bulk acoustic wave sensors, (ii) surface acoustic wave sensors, and (iii) micro/nano-electromechanical system (MEMS/NEMS) sensors.

Liquid identification by using a micro-electro-mechanical interdigital transducer

A surface-acoustic-mode aluminum nitride (AlN) transducer is utilized to determine the type of liquid dropped on the propagation path. It is based on tracking the shrinking droplet radius and observing stagnant liquid molecules during and after the liquid evaporation process. The device configuration is suitable to test small amounts of liquids, in the microliter range. According to both mass loading and physical property mechanisms, eight samples of liquids, isopropanol (IPA), ethanol (ETH), deionized-water (DW), tap water (TW), heptane (HEP), propylene glycol monomethyl ether acetate (PGMEA), hexamethyldisilazane (HMDS) and acetone (ACE), which have different equilibrium vapor pressures, molecular weights and boiling points, are accurately detected. The experimental results show that the rate of the change in the energy loss including a slow and fast attenuation region depends on the change of physical properties, such as density, sound speed in liquids and evaporation rate, during the evaporation process. As the evaporation rate of the DW is rather slow, the slow attenuation region occurs for a longer time than the fast one. Consequently, the whole oscillation duration of the attenuation occurs for a longer time, whereas that of the other liquids studied, like ACE, ETH, and IPA, having a faster evaporation rate is shorter. Sensitivities of the surface-acoustic-mode transducer to the evaporation process of liquids such as DW, TW, PGMEA, HMDS, HEP, IPA, ETH and ACE are -29.39, -29.53, -31.79, -34.12, -33.62, -32.87, -32.67, and -32.82 dB μm^-2, respectively. The concentration of stagnant liquid molecules causes a change in the surface mass of the micro-electro-mechanical transducer, which causes a frequency shift and increases the signal noise at the receiver after the liquid evaporation process. The average frequency shifts of ACE, HEP, HMDS, ETH, IPA, PGMEA, TW and DW are 241, 206, 172, 117, 76, 27.3, 11.6 and 0 kHz, respectively, coherent with the type of formed liquid pattern on the device surface, thus allowing to detect liquid samples effectively.

Ultrasonic monitoring of droplets' evaporation: Application to human whole blood

During a colloidal droplet evaporation, a sol-gel transition can be observed and is described by the desiccation time τD and the gelation time τG. These characteristic times, which can be linked to viscoelastic properties of the droplet and to its composition, are classically rated by analysis of mass droplet evolution during evaporation. Even if monitoring mass evolution versus time seems straightforward, this approach is very sensitive to environmental conditions (vibrations, air flow…) as mass has to be evaluated very accurately using ultra-sensitive weighing scales. In this study we investigated the potentialities of ultrasonic shear reflectometry to assess τD and τG in a simple and reliable manner. In order to validate this approach, our study has focused on blood droplets evaporation on which a great deal of work has recently been published. Desiccation and gelation times measured with shear ultrasonic reflectometry have been perfectly correlated to values obtained from mass versus time analysis. This ultrasonic method which is not very sensitive to environmental perturbations is therefore very interesting to monitor the drying of blood droplets in a simple manner and is more generally suitable for complex fluid droplets evaporation investigation.

Low internal pressure in femtoliter water capillary bridges reduces evaporation rates

Capillary bridges are usually formed by a small liquid volume in a confined space between two solid surfaces. They can have a lower internal pressure than the surrounding pressure for volumes of the order of femtoliters. Femtoliter capillary bridges with relatively rapid evaporation rates are difficult to explore experimentally. To understand in detail the evaporation of femtoliter capillary bridges, we present a feasible experimental method to directly visualize how water bridges evaporate between a microsphere and a flat substrate in still air using transmission X-ray microscopy. Precise measurements of evaporation rates for water bridges show that lower water pressure than surrounding pressure can significantly decrease evaporation through the suppression of vapor diffusion. This finding provides insight into the evaporation of ultrasmall capillary bridges.