Superhydrophobic Substrates Research Articles

Experimental investigation of contact time of bouncing droplet on vibrating substrates

The observation of an elastic substrate self-driving droplet to produce a “springboard effect” provides new enlightenment to the application of elastic materials in the anti-icing area. The droplet–substrate dynamic of a water drop impacting a superhydrophobic elastic substrate is experimentally investigated at different Weber (We) numbers and beam stiffness. For water drop, the spreading dynamic is not affected by the We number and beam stiffness since the inertial action is dominant, and the elastic action of the beam is relatively small, while the receding dynamic is closely related to the parameters. For elastic substrate, the vibrating deflection increases with the increase in the We number and reduction of the stiffness, while the vibrating frequency is only dependent on its stiffness. Based on this, the rebound dynamic of the droplet is discovered dependent on the scale relationship between the droplet and substrate oscillation period. Finally, a relation of the contact time of a droplet impacting elastic substrates, which is verified to hold for a large range of We numbers, beam stiffness, and droplet sizes, is established. The discoveries may contribute to the design of a droplet–elastic substrate system to achieve desirable contact time, providing a theoretical basis to forecast the performance of droplet–substrate systems by employing elastic materials.

Rational MOF Membrane Design for Gas Detection in Complex Environments.

Metal-organic frameworks (MOFs) hold significant promise in the realm of gas sensing. However, current understanding of their sensing mechanisms remains limited. Furthermore, the large-scale fabrication of MOFs is hampered by their inadequate mechanical properties. These two challenges contribute to the sluggish development of MOF-based gas-sensing materials. In this review, the selection of metal ions and organic ligands for designing MOFs is first presented, deepening the understanding of the interactions between different metal ions/organic ligands and target gases. Subsequently, the typical interfacial synthesis strategies (gas-solid, gas-liquid, solid-liquid interfaces) are provided, highlighting the potential for constructing MOF membranes on superhydrophobic and/or superhydrophilic substrates. Then, a multi-scale structure design strategies is proposed, including multi-dimensional membrane design and heterogeneous membrane design, to improve sensing performance through enhanced interfacial mass transfer and specific gas sieving. This strategy is anticipated to augment the task-specific capabilities of MOF-based materials in complex environments. Finally, several key future research directions are outlined with the aim not only to further investigate the underlying sensing principles of MOF membranes but also to achieve efficient detection of target gases amidst interfering gases and elevated moisture levels.

Coalescence and Rebound Dynamics in Two Droplets Train Impacting on a Heterogeneous Wettability Surface.

Droplets that can be steered and rebound off surfaces are fundamentally interesting and important due to their promising potential in numerous applications, such as anti-icing and -fogging, spray coating, and self-cleaning. Heterogeneous wettability surfaces have been shown to be an effective means of droplet manipulation. This paper combines numerical simulation with theoretical analysis to investigate the dynamics of two droplets training impacting on and bouncing off a heterogeneous surface (superhydrophobic substrate decorated with a hydrophilic strip). First, the time evolutions of the droplet morphology and velocity vectors are examined to explore the particular dynamic behaviors. At different ratios of the impact velocity, three distinct rebound patterns are observed, and a regime diagram is established. After that, the effects of the impact conditions and surface wettability on the rebound performance of the coalesced droplet are studied systematically. Special attention is paid to the variations of the rebound height and the lateral transportation distance with the Weber number of two droplets and the distance between the impacting point and the hydrophilic stripe. Moreover, a theoretical analysis of two droplets' impact is performed based on the energy conservation. The obtained scaling laws match well with the numerical data in the trend. Our research may strengthen the understanding of the interactions between droplets, which is valuable for the manipulation of multiple droplets in related fields.

Superwettable microchip for visual detection of date-rape drug in beverages

Gamma-hydroxybutyric acid (GHB), which is used in drug-facilitated sexual assault, is typically delivered in the form of gamma-butyrolactone (GBL) in beverage carriers. In response to the growing need of methods for rapid and on-site sensing of the drugs, we carried out an investigation aimed at developing a superwettable microchip for visual detection of the date-rape drug GBL in adulterated beverages. The superwettable microchip is comprised of Fe3O4 nanoparticles which aggregate to form a reactive spot on a superhydrophobic fabric. The spot captures and confines microdroplets containing GBL on the superhydrophobic substrate. Fe(III), generated by HCl promoted reaction of the Fe3O4 spot, then participates in a colorimetric reaction with GBL to form a naked eye observable colored complex. Quantitative determination of GBL in beverages is achieved by analyzing the red proportion of a smartphone camera derived image. The detection range of this assay protocol is ca. 0.1–10 mg mL−1, which meets the requirements to detect potentially dangerous concentration of GBL in drinks. The detection stability of the superwettable microchip for GBL in water and common beverages is maintained over at least 30 cycles.

A comprehensive study of thermocapillary rupture of liquid layer

There is a lack of understanding of the physical mechanisms involved in the rupture of a liquid film on a solid surface. This is in part due to the large number of parameters of the liquid film—substrate system on which this complex process is dependent. Due to the thermocapillary effect, local heating can have a destabilizing effect on the stability of the film. In this work, using a numerical model verified by the experimental data, the influence of parameters not available for variation in the experiment has been studied. It was found that the formation of dry spots in a locally heated liquid film occurs only under the influence of thermocapillary forces. In turn, the expansion of the dry spots takes place under the influence of the capillary forces. Thus, until the dry spot appears, the rupture does not differ on either hydrophilic or superhydrophobic substrates, but the contact line velocity during dry spot growth is extremely sensitive to the contact angle.

Off-Stoichiometry Thiol-Ene (OSTE) Micro Mushroom Forest: A Superhydrophobic Substrate.

Superhydrophobic surfaces have been used in various fields of engineering due to their resistance to corrosion and fouling and their ability to control fluid movement. Traditionally, superhydrophobic surfaces are fabricated via chemical methods of changing the surface energy or mechanical methods of controlling the surface topology. Many of the conventional mechanical methods use a top-to-bottom scheme to control the surface topolopy. Here, we develop a novel fabrication method of superhydrophobic substrates using a bottom-to-top scheme via polymer OSTE, which is a prototyping polymer material developed for the fabrication of microchips due to its superior photocuring ability, mechanical properties, and surface modification ability. We fabricate a superhydrophobic substrate by OSTE-OSTE micro mushroom forest via a two-step lithography process. At first, we fabricate an OSTE pillar forest as the mushroom stems; then, we fabricate the mushroom heads via backside lithography with diffused UV light. Such topology and surface properties of OSTE render these structures superhydrophobic, with water droplets reaching a contact angle of 152.9 ± 0.2°, a sliding angle of 4.1°, and a contact angle hysteresis of less than 0.5°. These characteristics indicate the promising potential of this substrate for superhydrophobic applications.

Steerable droplet precise bouncing on a superhydrophobic surface with superhydrophilic stripes

The precise rebound of a droplet upon hitting a solid surface has garnered significant attention in recent years due to its critical applications in self-cleaning, printing industries, and the design of heat exchanger surfaces, among others. This study introduces an innovative approach that combines femtosecond laser processing with a high-temperature stearic acid modification to create surfaces that feature superhydrophilic (SHL) stripes on a superhydrophobic substrate. By controlling the offset distance between the droplet's impact point and the SHL stripe, we achieved a directional and precise rebound of the droplets. Our findings indicate that the lateral displacement of the droplet increases with the offset distance and always tilts toward the direction of the SHL stripe. This study also incorporates numerical simulations to validate the findings, shedding light on the energy conversion mechanisms at the liquid–solid interface during the impact, particularly during the retraction phase. This discovery is significant for more accurately predicting the specific landing spots of rebounding droplets.

Cauliflower-like super-hydrophobic SERS substrate via two-step laser processing for acetamiprid detection

Residues of organic pesticides continue to accumulate in the human body through the food chain, posing a serious threat to health. Therefore, the efficient and sensitive detection of pesticide residues is crucial. Surface-enhanced Raman scattering (SERS), as a powerful spectroscopic technique, holds great potential for the trace detection of pesticides. In this study, a cauliflower-like SERS substrate fabricated via a two-step laser processing is proposed for highly sensitive detection of acetamiprid. The cauliflower-like structure features a large surface area, providing numerous hotspots and abundant adsorption sites for the target molecules. Furthermore, the rich micro/nano structures on the surface confer super-hydrophobicity properties to the substrate, effectively enriching the analyte and greatly enhancing the SERS performance. As a result, the cauliflower-like super-hydrophobic substrate demonstrates a detection limit as 10-15 M for the typical dye analyte (R6G), with an enhancement factor of 2.6 × 108. Additionally, the substrate exhibits excellent signal reproducibility with a relative standard deviation of 8.33 %. In practical pesticide detection, the detection limit for acetamiprid on orange is 10-4 g/L, significantly surpassing the national standard. In conclusion, the cauliflower-like super-hydrophobic substrate enhances the sensitivity, uniformity, reproducibility, and reliability of SERS signals in detection process, showing tremendous potential in the field of food safety.

Experimental study of dynamic behavior of impacting droplets on vibrating super-hydrophobic surfaces

The dynamic behaviors of droplets impacting on a vibrating solid surface are complex and interesting, as the differences in the initial phase angle can result in various droplet behaviors. The dynamic behavior of free-falling droplets impacting vibrating superhydrophobic substrates was investigated via high-speed photography. The effects of the initial phase angle (φ), Weber number (We), and vibration frequency (f) on the morphology evolution and energy dissipation were analyzed. Herein, 12 initial phase angles were selected as variables to investigate the evolution of droplet morphology with various initial phase angles. The effect of different initial phase angles on the maximum spreading diameter of droplets can cause variations of over 10%. The initial phase angles enhancing or restricting the droplet spreading were quantitatively defined. The Weber number and initial phase angle which can produce daughter droplets are obtained quantitatively. Meanwhile, a correlation between dimensionless spreading coefficient and dimensionless time was established for various vibration frequencies (f), revealing the effects of f on the maximum spreading diameter of droplet. Furthermore, a mathematical relationship for predicting the maximum spreading diameter of droplet impacting on a vibrating substrate was established based on the derivation of energy conservation. The error of the prediction mathematical model was proved to be less than 2% by the experimental results. These results provide fundamental understanding of droplet impacting on a vibration wall and could be useful for related engineering applications.

Detection of water pollutants using super-hydrophobic porous silicon-based SERS substrates.

Super hydrophobic porous silicon surface is prepared using a wet chemical synthesis route. Scanning electron microscopic investigation confirms a correlation between pore size and reaction time. SERS substrates are prepared by silver nanoparticle deposition on porous silicon surface. They exhibit excellent characteristics in terms of sensitivity, reproducibility, stability, and uniformity. They could detect rhodamine 6G in femtomolar range with SERS enhancement factor of ~ 6.1 × 1012, which is best ever reported for these substrates. Molecule-specific sensing of water pollutants such as methylene blue, glyphosate, andchlorpyrifos, is demonstrated for concentrations well below their permissible limits along with excellent enhancement factors. Porous silicon substrate functionalized with Ag nanoparticles demonstrates to be a promising candidate for low-cost, long-life, reliable sensors for environmental conservation applications.

Condensation or Desublimation: Nanolevel Structural Look on Two Frost Formation Pathways on Surfaces with Different Wettabilities.

Processes of water condensation and desublimation on solid surfaces are ubiquitous in nature and essential for various industrial applications, which are crucial for their performance. Despite their significance, these processes are not well understood due to the lack of methods that can provide insight at the nanolevel into the very first stages of phase transitions. Taking advantage of synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) and environmental scanning electron microscopy (ESEM), two pathways of the frosting process from supersaturated vapors were studied in real time for substrates with different wettabilities ranging from highly hydrophilic to superhydrophobic. Within GIWAXS, a fully quantitative structural and orientational characterization of the undergoing phase transition reveals the information on degree of crystallinity of the new phase and determines the ordering at the surfaces and inside the films at the initial stages of water/ice nucleation from vapor onto the substrates. The diversity of frosting scenarios, including direct desublimation from the vapor and two-stage condensation-freezing processes, was observed by both GIWAXS and ESEM for different combinations of substrate wettability and vapor supersaturations. The classical nucleation theory straightforwardly predicts the pathway of the phase transition for hydrophobic and superhydrophobic substrates. The case of hydrophilic substrates is more intricate because the barriers in Gibbs free energy for nucleating both liquid and solid embryos are close to each other and comparable to thermal energy kBT. At that end, classical nucleation theory allows concluding a relation between contact angles for ice and water embryos on the basis of the observed frosting pathway.

Boiling of binary mixtures on superhydrophobic surfaces with artificial cavities

This study experimentally investigates bubble behaviour from an isolated artificial cavity on a superhydrophobic substrate, the working fluid is a binary mixture of Novec649 and Novec7000 with mole fractions between 0.05 – 99.5 as well as their pure counterparts. The results show an earlier onset of bubble departure as well as deceasing trends in bubble departure diameter in the presence of the binary mixture compared to pure cases. The bubble departure diameter is at its minimum and fastest growth rate at a mole fraction of approximately 0.50. Theoretical models from literature were tested and satisfactory agreement with different models were achieved depending on the concentration.

Dynamics of microdroplet generation via drop impact on a superhydrophobic micropore

This study delves into the dynamics of generating microdroplets by impacting a droplet onto a micropore on superhydrophobic copper substrates. It identifies the necessary impact velocities for single microdroplet formation for each micropore and characterizes microdroplet size in relation to micropore diameter. The results underscore the significant role of viscosity, especially as the diameter of the micropore decreases. For micropores measuring 400 μm, an increase in viscosity up to 8 cP does not alter the critical impact velocities, while smaller diameters of 50 and 100 μm see a notable change in critical velocities with even minor increases in viscosity. Remarkably, the diameter of the microdroplet remains consistent regardless of changes in the liquid viscosity or impact velocity. This research showcases two practical uses of single microdroplets: printing on paper and fabricating microbeads. The insights gained from these findings pave the way for advancements in printing technology and microfabrication techniques.

A novel asymptotically consistent approximation for integral evaporation from a spherical cap droplet

The total evaporation rate due to a volatile capillarity-dominated droplet diffusively evaporating into the surrounding gas is a critically important quantity in industrial and engineering applications such as Q/OLED screen manufacturing. However, the analytical expression in terms of integrals in toroidal coordinates can be unwieldy in applications, as well as expensive to compute. Therefore, simple yet highly accurate approximate solutions are frequently used in practical settings. Herein we present a new approximate form that is both accurate and fast to compute, but also retains the correct asymptotic behaviour in the key physical regimes, namely hydrophilic and superhydrophobic substrates, and a hemispherical droplet. We illustrate this by comparison to several previous approximations and, in particular, illustrate its use in calculating droplet lifetimes, as well as approximating the local evaporative flux.

Bioinspired superhydrophobic surfaces with silver and nitric oxide-releasing capabilities to prevent device-associated infections and thrombosis

Bacteria-associated infections and thrombus formation are the two major complications plaguing the application of blood-contacting medical devices. Therefore, functionalized surfaces and drug delivery for passive and active antifouling strategies have been employed. Herein, we report the novel integration of bio-inspired superhydrophobicity with nitric oxide release to obtain a functional polymeric material with anti-thrombogenic and antimicrobial characteristics. The nitric oxide release acts as an antimicrobial agent and platelet inhibitor, while the superhydrophobic components prevent non-specific biofouling. Widely used medical-grade silicone rubber (SR) substrates that are known to be susceptible to biofilm and thrombus formation were dip-coated with fluorinated silicon dioxide (SiO2) and silver (Ag) nanoparticles (NPs) using an adhesive polymer as a binder. Thereafter, the resulting superhydrophobic (SH) SR substrates were impregnated with S-nitroso-N-acetylpenicillamine (SNAP, an NO donor) to obtain a superhydrophobic, Ag-bound, NO-releasing (SH-SiAgNO) surface. The SH-SiAgNO surfaces had the lowest amount of viable adhered E. coli (> 99.9 % reduction), S. aureus (> 99.8 % reduction), and platelets (> 96.1 % reduction) as compared to controls while demonstrating no cytotoxic effects on fibroblast cells. Thus, this innovative approach is the first to combine SNAP with an antifouling SH polymer surface that possesses the immense potential to minimize medical device-associated complications without using conventional systemic anticoagulation and antibiotic treatments.

Wetting, droplet evaporation and corrosion behavior of various composite and textured materials

Experimental studies were conducted on the evaporation rate of a droplet located on the surfaces of various composite and textured materials with different wettability at varying surface temperatures. The maximum droplet evaporation rate was recorded on a textured Cu–SiC composite material with superhydrophilic properties. The minimum droplet evaporation rate corresponded to a superhydrophobic AlMg3 substrate obtained using laser texturing with subsequent hydrophobization procedure. An increase in surface temperature led to a significant deterioration of hydrophobicity for a superhydrophobic substrate and a deterioration of hydrophilicity for a superhydrophilic substrate. As a result, with increasing temperature, the influence of the droplet geometry determined by surface wettability on the droplet evaporation rate for these substrates sharply decreases. Advancing and receding dynamic contact angles, as well as contact angle hysteresis were obtained on all studied substrates. It is generally accepted that a hydrophobic surface exhibits less contact angle hysteresis than a hydrophilic one. However, Cu-SiС composite substrate with a textured hydrophobic surface exhibited the opposite behavior. The contact angle hysteresis on the hydrophobic textured surface was found to be four times higher than that on the hydrophilic polished one. A mechanism was proposed that explains the high contact angle hysteresis and four hysteresis time modes. The minimum hysteresis value corresponded to the superhydrophobic AlMg3 substrate. Anti-corrosion properties were studied for all substrates under consideration. The minimum corrosion current corresponded to a copper substrate with a graphene coating, as well as a superhydrophobic AlMg3 substrate. For a given substrate, there are two corrosion modes with a three-fold difference in the corrosion current. In addition, the wettability behavior of different surfaces was examined using molecular dynamics simulations. The simulation results were found to be consistent with the experimental data.

Spontaneous motion of solid object on open channel

Spontaneous motion of a solid object floating on an unsymmetrical geometric open channel is investigated. The open channel was created by selectively fabricating hydrophilic patterns on a superhydrophobic substrate, confining water within the hydrophilic region, thus forming a distinct open channel. As a rectangular foam block was placed on a triangular open channel, the variation of the spanwise width of the wetted area leads to a gradient of surface tension force along the centerline that is able to drive the foam block directionally. What is more, two open channels could be parallelly arranged together to drive even larger object. A theoretical model was developed to explain the mechanism, which agrees well with the experimental results. The findings of this work extend the application of capillary force, which could be used in areas such as self-driven microfluidics, surface lubrication, and open microchannels.

Aerodynamic characteristics of water droplets on superhydrophobic surfaces

The primary objective of this study is to examine droplet dynamics on superhydrophobic surfaces in order to develop strategies to reduce droplet adhesion. The investigation utilized computational fluid dynamics simulations, employing the unsteady Reynolds-averaged Navier–Stokes equations in conjunction with the volume of fluid method. The central focus of this study pertains to the behavior of two droplets on a substrate characterized by a contact angle of 155° within a two-dimensional computational domain. The parametric studies include analyzing the dynamics of droplets with different freestream velocities, droplet sizes, distances between droplets, and the order in which droplets are arranged. Several key findings emerge from this study, notably the observation of an attractive force between two droplets prior to their coalescence. An attraction force between two water droplets was seen in many cases examined due to flow separation, where negative pressure gradients and recirculation flows affected the droplet farthest from the inlet moving upstream. Additional droplet dynamics include the detachment of droplets from the superhydrophobic substrate, the vorticity development after the droplets, and the subsequent wall forces influenced by parametric studies. These findings highlight the inherent capabilities of treated substrates, including self-cleaning attributes, hydrophobicity, and reduced friction. The potential applications based on this research can influence diverse fields, notably materials science, medicine, and engineering.

Fabrication of a Low Cost Superhydrophobic Substrate for Surface Enhanced Laser-Induced Breakdown Spectroscopy and Its Utility through Identification of Electrolyte Variation for Oral Cancer Detection.

Ultratrace elemental detections from a limited volume of samples can offer significant benefits in biomedical fields. However, it can be challenging to concentrate the particles being analyzed in a small area to improve the accuracy of detection. Ring-like deposits on the edges of colloidal droplets are a vexing problem in many applications. Herein, we report ultratrace elemental detection using a superhydrophobic surface-enhanced laser-induced breakdown spectroscopy (SELIBS) substrate fabricated by laser ablation followed by a soft lithography technique. In this work, the SELIBS spectra on a superhydrophobic polydimethylsiloxane (PDMS) substrate replicated from a laser-patterned master Teflon substrate are investigated. This work highlights the application of this newly created superhydrophobic substrate for detecting trace elements in body fluids using SELIBS. The developed PDMS substrate was successfully adopted to investigate the electrolyte variation in serum samples of oral cancer patients and normal volunteers. Principal component analysis (PCA) and match-no-match analysis were used to distinguish the elemental variation in cancer and control groups.

Design superhydrophobic no-noble metal substrates for highly sensitive and signal stable SERS sensing

Surface-enhanced Raman spectroscopy (SERS) is an analytical technique with a broad range of potential applications in fields such as biomedicine, electrochemistry, and hazardous chemicals. However, it is challenging to develop SERS substrates that are both good sensitive and signal stable. Here we designed a superhydrophobic Nd doped MoS2 uniformly assembled on the activated carbon fiber cloth (CFC) to avoid the coffee ring effect and enrich the analyte, improving the enhancement factor (EF) to 3.9 × 109 and pesticide endosulfan (<10−10) analyte detection. We demonstrate our strategy by density-functional theory (DFT) calculations confirming that both adsorption energy and density of states are enhanced after doping Nd leading to SERS enhancement. Beside DFT calculations, our experiments also provide an effective means to demonstrate that the high SERS sensitivity is based on multiple charge transfer processes combined with the activated carbon cloth. Our results address the limitations of low sensitivity and limit of detection (LOD) of semiconductor SERS substrates for trace analysis and are a step towards practical applications.