Aqueous Drops Research Articles

Studies on wetting behaviour of pure and aqueous protic ionic liquids (PIL) solutions on different surfaces: ILphobicity to ILphilicity

In this study, the wetting behaviour of pure protic ionic liquids (PILs) and their mixture with water was studied at ambient conditions. Ethanolammonium acetate [EAAc], ethanolammonium butyrate [EABu], and ethanolammonium hexanoate [EAHx] were used as PILs for the wetting studies on various surfaces. The contact angle and wetting area of pure and mixed PIL solutions were determined on superhydrophobic, hydrophilic, and total wetting surfaces. Smaller alkyl chain PIL, ethanolammonium acetate (EAAc) displayed the highest contact angle (CA) of 141° on superhydrophobic surfaces. On hydrophilic polycarbonate (PC), shorter anionic chain PILs demonstrated CA < 90°, showing the ILphilic behaviour, and the variation of CA was PIL anionic chain length dependent. Total wetting glass exhibited the lowest CA for [EAAc], which increased with the PIL anionic chain. A binary mixture of PIL with water demonstrated unique wetting behaviour on the total wetting surface. The aqueous drops of EAHx exhibited a dynamic contact line on the glass surface, showing the greatest expansion in wetting area compared to drops of aqueous EAAc or aqueous EABu. This occurrence could be ascribed to the substantial decrease in water surface tension induced by the hydrophobic hexanoate anion, potentially leading to improved spreading characteristics.

Solutal Marangoni augmented levitation time of an aqueous surfactant drop over an immiscible oil pool

Droplets are found to exhibit a finite residence time over the volatile liquid pool due to the confined evaporation mediated sustained thin film draining. The levitated drops are quite critical in biomedical applications such as for the biophysical micro reactors. It is hypothesized that the presence of surfactants can induce solutal Marangoni and can further enhance the levitation time. Moreover, the surfactant drops can closely mimic the bio drop assay. Droplets of surfactant solutions are allowed to impact over a series of volatile hydrocarbon oils to understand the effect of surfactant concentration, impact height and the properties of the oil pool using high speed imaging technique. Flow visualization is employed in the pendant droplet mode to understand the Marangoni induced internal circulation. The droplets of surfactant solutions are found to exhibit enhanced residence time compared to the case of non-surfactant drops. The levitation time is a function of the surfactant concentration and the volatility of the oil pool. A mathematical model is developed incorporating the effect of solutal Marangoni to test the hypothesis and is found to predict the levitation time with reasonable accuracy. A universal floating–sinking regime map was developed incorporating the influence of governing parameters.

Additional Guidance on the Use of the PRESERFLO™ MicroShunt in the Treatment of Glaucoma: Insights from a Second Delphi Consensus Panel.

The PRESERFLO™ MicroShunt (PMS) has been proven to significantly lower intraocular pressure (IOP) in patients with glaucoma and has been available for use since 2019. With increasing published evidence and growing experience of glaucoma surgeons, the aim of this modified Delphi panel was to build on the findings of a previous Delphi panel conducted in 2021 and provide further guidance on the role of the PMS to treat patients with glaucoma in Europe. Thirteen European glaucoma surgeons experienced in the PMS procedure participated in a 3-round modified Delphi panel. A targeted literature review and expert steering committee guided Round 1 questionnaire development. Consensus was pre-defined at a threshold of ≥ 70% of panellists selecting 'strongly agree'/'agree' or 'strongly disagree'/'disagree' for 6-point Likert scale questions or ≥ 70% selecting the same option for multiple or single-choice questions. Questions not reaching consensus were restated/revised for the next round, following guidance from free-text responses/scoping questions. In total, 28% (n = 9/32), 52% (n = 16/31) and 91% (n = 10/11) of statements reached consensus in Rounds 1, 2 and 3, respectively. There was agreement that the PMS may be used in patients with pigmentary, post-trauma or post-vitrectomy glaucoma and for patients with uveitic glaucoma without active inflammation. The PMS may be more suitable for patients with contact lenses than other subconjunctival filtering surgeries, without eliminating bleb-associated risks. Consensus was reached that combining PMS implantation and phacoemulsification may be as safe as standalone PMS surgery, but further efficacy data are required. Following a late rise in IOP ≥ 4months post-surgery, topical aqueous suppressant drops or bleb revision may be suitable management options. This Delphi panel builds on the considerations explored in the 2021 Delphi panel and provides further detailed guidance for glaucoma surgeons on the use of the PMS, reflecting the availability of novel evidence and surgical experience. Videos are available for this article.

NONLINEAR EFFECTS IN VISCOELASTIC DROP SHAPE OSCILLATIONS

A study of axisymmetric shape oscillations of viscoelastic drops in a vacuum is conducted, using the method of weakly nonlinear analysis. The motivation is the relevance of the shape oscillations for transport processes across the drop surface, as well as fundamental interest. The study is performed for, but not limited to, the two-lobed mode of initial drop deformation. The Oldroyd-B model is used for characterizing the liquid rheological behavior. The method applied yields a set of governing equations, as well as boundary and initial conditions, for different orders of approximation. In the present paper, the equations and solutions up to second order are presented, together with the characteristic equation for the viscoelastic drop. The characteristic equation has an infinite number of roots, which determine the time dependency of the oscillations. Solutions of the characteristic equation are validated against experiments on acoustically levitated individual viscoelastic aqueous polymer solution drops. Experimental data consist of decay rate and oscillation frequency of free damped drop shape oscillations. With these data, solutions of the characteristic equation dominating the oscillations are identified. The theoretical analysis reveals nonlinear effects, such as the excess time in the prolate shape and frequency change for varying initial deformation amplitude. The influences of elasticity, measured by the stress relaxation and deformation retardation time scales, are quantified, and the effects are compared to the Newtonian case in the moderate-amplitude regime.

Hydration Energy-Dependent Ion Intercalation on Graphite and the Asymmetric Electrowetting.

Ion intercalation in graphite is widely used in desalination, batteries, and graphene stripping; it has high value in the fields of industry and research. However, selective ion transport, particularly (de)hydration energy and the hydration shell effect on the intercalation of ions into the graphite interlayer spaces, is still unclear. Here, we report low-voltage ion intercalation as observed by electrowetting on highly oriented pyrolytic graphite of an aqueous drop containing various inorganic salts. The electrowetting response exhibits asymmetric behavior with no contact angle change for the negative polarity and a threshold voltage for the onset of the contact angle change for the positive polarity. To explain the asymmetric electrowetting behavior and quantitatively predict the threshold voltage, we developed a physical model based on the hydration shell energy and size of the ion that undergoes partial breaking/deformation during the co-intercalation into the spaces between graphite layers. Electrowetting experiments using ions with various hydration energies and hydration radii were performed to confirm the prediction of the model. Further, we show a strategy to make the electrowetting response of LiCl drops symmetric via tuning the hydration energy of the Li+ ions using a binary solvent of a glycerol-water mixture. This article will provide an understanding of the hydration (solvation) energy dependence intercalation mechanism in graphite for electrowetting, which underpins various processes such as ion battery applications and the graphene exfoliation process.

Celecoxib/Cyclodextrin Eye Drop Microsuspensions: Evaluation of In Vitro Cytotoxicity and Anti-VEGF Efficacy for Retinal Diseases.

Celecoxib (CCB), a cyclooxygenase-2 inhibitor, is capable of reducing oxidative stress and vascular endothelial growth factor (VEGF) expression in retinal cells and has been shown to be effective in the treatment of diabetic retinopathy and age-related macular degeneration. However, the ocular bioavailability of CCB is hampered due to its very low aqueous solubility. In a previous study, we developed 0.5% (w/v) aqueous CCB eye drop microsuspensions (MS) containing randomly methylated β-cyclodextrin (RMβCD) or γ-cyclodextrin (γCD) and hyaluronic acid (HA) as ternary CCB/CD/HA nanoaggregates. Both formulations exhibited good physicochemical properties. Therefore, we further investigated their cytotoxicity and efficacy in a human retina cell line in this study. At a CCB concentration of 1000 μg/mL, both CCB/RMβCD and CCB/γCD eye drop MS showed low hemolysis activity (11.1 ± 0.3% or 4.9 ± 0.2%, respectively). They revealed no signs of causing irritation and were nontoxic to retinal pigment epithelial cells. Moreover, the CCB eye drop MS exhibited significant anti-VEGF activity by reducing VEGF mRNA and protein levels compared to CCB suspended in phosphate buffer saline. The ex vivo transscleral diffusion demonstrated that a high quantity of CCB (112.47 ± 37.27 μg/mL) from CCB/γCD eye drop MS was deposited in the porcine sclera. Our new findings suggest that CCB/CD eye drop MS could be safely delivered to the ocular tissues and demonstrate promising eye drop formulations for retinal disease treatment.

Formulation and Evaluation of Topically Retained Hydrogels Loaded with Timolol Maleate Microspheres for Extended Release in Treating Glaucoma

Conventional aqueous eye drop formulations have a shorter retention period and wash out of the precorneal region more easily. In this study, biodegradable microsphere-loaded hydrogels were developed to extend the time of residence and improve the drug bioavailability. The prepared microspheres were fabricated using PLGA as the biodegradable polymer and PVA as the surfactant. Hydrogels were prepared using Carbopol 940 and Gellan gum as gelling agents. Methods: The microspheres were created through a technique known as double emulsion solvent evaporation method. Evaluation included for its percentage yield, entrapment efficiency, particle size and surface morphology of the prepared microspheres. Timolol Maleate microsphere loaded hydrogel underwent assessments for pH, spreadability, rheological behaviour, sterility, antimicrobial efficacy, in vitro release studies, drug release kinetics and ocular irritation. Results: FTIR analysis confirmed compatibility between the drug and polymer. SEM observations indicated spherical microspheres with sizes ranging from 1 to 12 µm, suitable for ocular application. All formulations exhibited pH values between 4.2 and 5.0, with gelation occurring at 35°C to 38°C. Formulation F3 demonstrated optimal viscosity (1486 – 1850 cps) and good spreadability. Sterility tests were successful, and F3 displayed a drug release of 82.6% after 24 hours. Antimicrobial studies exhibited inhibition zones, and ocular tests on rabbits revealed no irritation. Stability analysis indicated no significant changes in pH or clarity. Conclusion: Based on physiological, rheological, and in vitro evaluations F3 was identified as the most suitable formulation for extended Timolol Maleate release via hydrogel, holding promise for effective glaucoma treatment.

Strategy for Patterning Titania Dendrites by Gas-Solution Interaction at Droplet Surfaces.

Interaction of the solution droplet surface with gaseous components of the environment can lead to the formation of highly ordered patterns, such as dendrites. Here, we show that these structures can be spontaneously created during the open-air interaction of aqueous solution drop of titanium(III) salt with gaseous NH3 at the contact boundary thereof. The conditions have been identified under which radially ordered dendritic patterns can form on the surface of the TiCl3 solution droplet. The formation of these self-organized dendrite patterns can be attributed to the surface instability manifesting in Marangoni thermal flows in a droplet occurring during open-air fabrication. The composition of as-synthesized structures corresponds to coprecipitated crystalline NH4Cl and amorphous TiO2nH2O. After thermal treatment at 450 °C, TiO2 with the anatase crystal lattice is formed; meanwhile, the ordered dendrite patterns are preserved.

A promising synthetic citric crosslinked β-cyclodextrin derivative for antifungal drugs: Solubilization, cytotoxicity, and antifungal activity

Effective antifungal therapy for the treatment of fungal keratitis requires a high drug concentration at the corneal surface. However, the use of natural β-cyclodextrin (βCD) in the preparation of aqueous eye drop formulations for treating fungal keratitis is limited by its low aqueous solubility. Here, we synthesized water-soluble anionic βCD derivatives capable of forming water-soluble complexes and evaluated the solubility, cytotoxicity, and antifungal efficacy of drug prepared using the βCD derivative. To achieve this, a citric acid crosslinked βCD (polyCTR-βCD) was successfully synthesized, and the aqueous solubilities of selected antifungal drugs, including voriconazole, miconazole (MCZ), itraconazole, and amphotericin B, in polyCTR-βCD and analogous βCD solutions were evaluated. Among the drugs tested, complexation of MCZ with polyCTR-βCD (MCZ/polyCTR-βCD) increased MCZ aqueous solubility by 95-fold compared with that of MCZ/βCD. The inclusion complex formation of MCZ/βCD and MCZ/polyCTR-βCD was confirmed by spectroscopic techniques. Additionally, the nanoaggregates of saturated MCZ/polyCTR-βCD and MCZ/βCD solutions were observed using dynamic light scattering and transmission electron microscopy. Moreover, MCZ/polyCTR-βCD solution exhibited good mucoadhesion, sustained drug release, and high drug permeation of porcine cornea ex vivo. Hen’s Egg test-chorioallantoic membrane assay and cell viability study using Statens Seruminstitut Rabbit Cornea cell line showed that both MCZ/polyCTR-βCD and MCZ/βCD exhibited no sign of irritation and non-toxic to cell line. Additionally, antifungal activity evaluation demonstrated that all isolated fungi, including Candida albicans, Aspergillus flavus, and Fusarium solani, were susceptible to MCZ/polyCTR-βCD. Overall, the results showed that polyCTR-βCD could be a promising nanocarrier for the ocular delivery of MCZ.

Cerium oxide bentonite nanocomposite-based colorimetric paper sensor for aflatoxins detection in cereal nuts, oilseed and legumes

Cerium oxide nanoparticles (NanoCeO2) and bentonite (Bent) based nanocomposite (CeO2-Bent) aqueous sensor drop casted on paper substrate has been used to detect food borne Aflatoxins (AFs) by Curcumin (Cur) functionalized redox reaction mediated colorimetric detection. Optical studies reveal that when chromophore-like Cur is first added to Nano CeO2/Bent, it creates a chelate complex that serves as a mediator for enhanced binding with AFs. Cur fictionalized CeO2-Bent in the presence of AFB1 result in a rapid electronic π - π* transition and showed change in color of the dye from red (pH 9.54) to yellow (pH 7). The Hypochromic shift from ~510 nm to ~433 nm was plotted linearly at 433 nm for increased AFB1 level in foods. The calculated LOD indicated values as 0.5 µg/Kg in corn, 2.4 µg/Kg in rice, 0.2 µg/Kg in peanut, 0.7 µg/Kg in almond, 7.8 µg/Kg in arhar and 9.2 µg/Kg in chana respectively. The results were validated with HPLC-FLD obtaining high accuracy and recovery. The sensor reagent further drop casted on paper showed a quick determination for presence of AFs.

Effectiveness and limitations of two drop studies to explain emulsion dehydration under modulated electric fields

AbstractElectrocoalescence has long been known for the separation of a water‐in‐oil emulsion. An associated challenge with electrocoalescers is the undesired noncoalescence and consequently chain formation of aqueous phase droplets. This leads to low separation efficiency and damage of electrical equipment. Recently Hasib et al. proposed an electric field modulated scheme that showed significant improvement in dehydration of water‐in‐oil emulsions. They investigated the range of modulation parameters when the scheme is most effective. The fundamental process in electrostatic dehydration of an emulsion is the interaction between a pair of water droplets. In the present study, two suspended aqueous drops in insulated oil experiments are compared for their behavior under unmodulated and modulated electric fields. Further, a model is developed and the experimental behavior under unmodulated and modulated electric fields is compared with numerical solutions. The model predicts the experimental observations accurately by balancing electrostatic, electrophoresis, dipolar, and resisting viscous drag forces, ignoring the end (bridge) effect during the contact. The study shows that the increase in the rate of dehydration of a water‐in‐oil emulsion under modulated electric fields with an increase in duty ratio and its near independence on the modulation time period can be explained by the two‐drop studies. However, several other processes such as multidrop interactions as well as scavenging of fine droplets by charged droplets created as intermediates in the interaction of two droplets cannot be explained by the two‐drop studies.

Biochar based self cleaning superhydrophobic surface with aqueous DESphobic properties

In this work, facile fabrication of chemically stable superhydrophobic surfaces based on biocompatible, fluorine free, carbon rich biochar has been reported. Biochar synthesized from dried rice straw by pyrolysis at 550˚C, which was characterized using Fourier Transform Infrared (FTIR) and Scanning Electron Microscopy (SEM). The biochar coated surface demonstrated superhydrophobic behaviour with static water CA > 150°. Aqueous drops remain in superhydrophobic state due to stability of Cassie-Baxter wetting regime and can shows very high-water contact angle (WCA > 160°) and self-cleaning behaviour. Such a very high WCA have been achieved typically by combination of low energy chemicals (fluorine based) and surface textures. However, no harsh chemical treatment of biochar was performed and a simple methodology of creating biochar layer onto the substate was adopted. Such surface also demonstrated repellent properties against aqueous mixtures of deep eutectic solvent (DES). A spherical aqueous DES drop can be easily formed onto the biochar surface for image analysis. The density of the liquid sample with a small sample volume can be assessed using this drop shape that was measured on the superhydrophobic biochar surface.

Influence of the Atmosphere on the Wettability of Polymer Brushes

Polymer brushes, i.e., end-tethered polymer chains on substrates, are sensitive to adaptation, e.g., swelling, adsorption, and reorientation of the surface molecules. This adaptation can originate from a contacting liquid or atmosphere for partially wetted substrates. The macroscopic contact angle of the aqueous drop can depend on both adaptation mechanisms. We analyze how the atmosphere around an aqueous droplet determines the resulting contact angle of the wetting droplet on polymer brush surfaces. Poly(N-isopropylacrylamide) (PNiPAAm)-based brushes are used due to their exceptional sensitivity to solvation and liquid mixture composition. We develop a method that reliably measures wetting properties when the drop and the surrounding atmosphere are not in equilibrium, e.g., when evaporation and condensation tend to contaminate the liquid of the drop and the atmosphere. For this purpose, we use a coaxial needle in the droplet, which continuously exchanges the wetting liquid, and in addition, we constantly exchange the almost saturated atmosphere. Depending on the wetting history, PNiPAAm can be prepared in two states, state A with a large water contact angle (∼65°) and state B with a small water contact angle (∼25°). With the coaxial needle, we can demonstrate that the water contact angle of a sample in state B significantly increases by ∼30° when a water-free atmosphere is almost saturated with ethanol, compared to an ethanol-free atmosphere at 50% relative humidity. For a sample in state A, the relative humidity has little influence on the water contact angle.

Engulfment of a drop on solids coated by thin and thick fluid films

When an aqueous drop contacts an immiscible oil film, it displays complex interfacial dynamics. When the spreading factor is positive, upon contact, the oil spreads onto the drop's liquid–air interface, first forming a liquid bridge whose curvature drives an apparent drop spreading motion and later engulfs the drop. We study this flow using both three-phase lattice Boltzmann simulations based on the conservative phase field model, and experiments. Inertially and viscously limited dynamics are explored using the Ohnesorge number $Oh$ and the ratio between the film height $H$ and the initial drop radius $R$ . Both regimes show that the radial growth of the liquid bridge $r$ is fairly insensitive to the film height $H$ , and scales with time $T$ as $r\sim T^{1/2}$ for $Oh\ll 1$ , and as $r\sim T^{2/5}$ for $Oh\gg 1$ . For $Oh\gg 1$ , we show experimentally that this immiscible liquid bridge growth is analogous with the miscible drop–film coalescence case. Contrary to the growth of the liquid bridge, however, we find that the late-time engulfment dynamics and final interface profiles are significantly affected by the ratio $H/R$ .

Topical dexamethasone delivery to the retina: An aqueous cyclodextrin-based microsuspension

Topical drug delivery to the eye is challenging and with conventional eye drops only 0.07%–4% of the dose reaches the aqueous humor and less than 0.001% the retina. Consequently, all currently available drug treatments for retinal diseases are based on invasive drug administration such as intravitreal injections and inserts. Six obstacles or barriers to drug permeation from the ocular surface into the eye have been identified and characterized. This review describes how each of these obstacles was addressed through cyclodextrin-based aqueous eye drop microsuspension and how novel physiochemical formulation techniques made it possible to deliver therapeutic drug concentrations topically to the retina. Finally, it is demonstrated that the therapeutic efficacy of aqueous 1.5% (w/v) dexamethasone eye drop microsuspension in humans for diabetic macular edema is comparable to that of a 0.7 mg dexamethasone/polymer intravitreal insert.

Electric Field Driven Reversible Spinodal Dewetting of Thin Liquid Films on Slippery Surfaces

AbstractThe stability of thin liquid films on a surface depends on the excess free energy of the system involving various short‐range and long‐range interactions. When forced to wet an unfavorable surface, thin liquid films dewet into multiple small‐sized droplets via spinodal, homogeneous, or heterogeneous nucleation process. However, if the total excess free energy of the system can be manipulated using external stimuli, for example, electric field, temperature, or light, one can control the stability of thin liquid films on demand. Here the electric‐field induced reversible dewetting and rewetting of thin liquid films underneath aqueous drops on slippery surfaces is studied. Upon applying voltage, hundreds of nanometer thick stable liquid films dewet in a manner identical to the spinodal dewetting of few nanometer‐thick liquid films following the linear stability analysis. Upon removing the applied voltage, the dewetted droplets spread, coalesce with neighboring ones, and form a uniform film again, albeit taking significantly longer times. The characteristic features defined via spatial and temporal evolution of the dewetting and rewetting processes are present over multiple cycles suggesting the complete reversibility of the process.

Numerical and experimental investigation of static wetting morphologies of aqueous drops on lubricated slippery surfaces using a quasi-static approach.

The static wetting behavior of drops on surfaces with thin lubricating films is very different compared to solid surfaces. Due to the slow dynamics of the wetting ridge, it is challenging to predict the apparent contact angles of such drops. It is hypothesized that for a sinking drop on a lubricated surface, quasi-static wetting morphology can be numerically computed from the knowledge of interfacial energies, lubricant thickness, and drop volume. In this study, we use Surface Evolver to numerically compute the static wetting morphology for the four-phase system using a quasi-static approach with a sinking time similar to the early-intermediate times, and the results agree well with the corresponding experiments. We find that the apparent contact angles depend significantly on the lubricant thickness and substrate wettability compared to other parameters.

Aqueous Prostaglandin Eye Drop Formulations.

Glaucoma is one of the leading causes of irreversible blindness worldwide. It is characterized by progressive optic neuropathy in association with damage to the optic nerve head and, subsequently, visual loss if it is left untreated. Among the drug classes used for the long-term treatment of open-angle glaucoma, prostaglandin analogues (PGAs) are the first-line treatment and are available as marketed eye drop formulations for intraocular pressure (IOP) reduction by increasing the trabecular and uveoscleral outflow. PGAs have low aqueous solubility and are very unstable (i.e., hydrolysis) in aqueous solutions, which may hamper their ocular bioavailability and decrease their chemical stability. Additionally, treatment with PGA in conventional eye drops is associated with adverse effects, such as conjunctival hyperemia and trichiasis. It has been a very challenging for formulation scientists to develop stable aqueous eye drop formulations that increase the PGAs’ solubility and enhance their therapeutic efficacy while simultaneously lowering their ocular side effects. Here the physiochemical properties and chemical stabilities of the commercially available PGAs are reviewed, and the compositions of their eye drop formulations are discussed. Furthermore, the novel PGA formulations for glaucoma treatment are reviewed.

Perioperative corneal abrasions: A report of 42 cases from the webAIRS database.

Corneal abrasions are an uncommon complication of anaesthesia. The aim of this study was to identify potential risk factors, treatment and outcomes associated with corneal abrasions reported to the web-based anaesthesia incident reporting system (webAIRS), a voluntary de-identified anaesthesia incident reporting system in Australia and New Zealand, from 2009 to 2021. There were 43 such cases of corneal abrasions reported to webAIRS over this period. The most common postoperative finding was a painful eye. Common features included older patients, individuals with pre-existing eye conditions, general anaesthesia and procedures longer than 60 minutes. Most cases were treated with a combination of lubricating eye drops or aqueous antibiotic eye drops. The findings indicate that patients who sustain a perioperative corneal abrasion can be reassured that in many cases it will heal within 48 hours, but they should seek earlier review if symptoms persist or deteriorate. None of the cases in this series resulted in permanent harm. Well established eye protective measures are important to utilise throughout the perioperative period, including the time until the patient has recovered in the post-anaesthesia care unit.

Stimulated Raman Scattering of an Organic Liquid in a Spherical-Shell Optical Cavity around an Aqueous Pendant Drop.

Stimulated Raman scattering (SRS) in a liquid droplet enhances the Raman scattering intensity through optical resonances. We have previously used a pendant drop at the tip of a capillary as the Raman-enhancing medium. In this study, we develop a new optical cavity for SRS measurement that consists of a spherical shell of organic liquid. This enables us to extend the applicability of the pendant-drop SRS method to liquids with low viscosity or low interfacial tension. This method is used to observe low-frequency modes of liquid benzene. The results indicate that the SRS emerges locally with respect to the drop size. The developed method extends the study of liquid structures based on vibrational spectroscopy.