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[Mizuki Tenjimbayashi](https://orcid.org/0000-0002-8107-8285)

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[Wetting phenomena and design of liquid-repellent surfaces](https://mdr.nims.go.jp/datasets/f9e233f4-1101-4984-bb5a-cd9e0d92037c)

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Wetting phenomena and design of liquid-repellent surfacesTutorial ReviewWetting phenomena and design of liquid-repellent surfacesMizuki TenjimbayashiNational Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, JapanE-mail: TENJIMBAYASHI.Mizuki@nims.go.jpWetting is ubiquitous in nature and contributes significantly to industrial processes. This commentary focuses on solid/fluidinteraction, which summarizes the fundamentals of wetting phenomena (surface tension, droplet shape, capillary force, andwetting models), the design strategy for liquid-repellent interfaces (superhydrophobic/superoleophobic surfaces and liquidmarble), and the associated cutting-edge applications. The relevant equations utilize only thermodynamics concepts taught inhigh school and are explained concisely such that even beginners can understand them sensibly. Additionally, typical examplesare provided.Received March 7, 2024; Accepted April 18, 2024Translated from Oyo Buturi 93, 519 (2024) DOI: https://doi.org/10.11470/oubutsu.93.9_519Content from this work may be used under the terms of the Creative Commons Attribution 4.0 license. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.1. IntroductionConsider the following scenario: You take a shower first thingin the morning. The steam fogs up the mirror, various dropletsand puddles form on the bathroom floor, and your hair getswet and clumps up. You foam up your shampoo and washyour hair, and then dry your body and apply moisturizingcream. Next, you prepare breakfast. You spread jam on breadand prepare fried eggs in a frying pan with oil. You makecoffee through a filter. When you leave the house, it is rainingoutside. You enter your waterproof-coating car and drive towork. These daily actions can be explained by the interactionbetween solids and fluids, i.e., the wetting phenomenon. Infact, the wetting phenomenon dominates all the industrialfields that pertain to our lives, including the cosmetics, textile,food, chemical, automotive, and glass industries. Therefore,understanding the physics behind this phenomenon is crucial.Simple wetting experiments can be performed using only tapwater. The topic of wetting has been investigated in academiafor more than 200 years; consequently, classical theories havebeen established that explain wetting phenomena well despitetheir simplicity [1,2]. Therefore, many researchers explainwetting phenomena using classical theories of wetting.Parallel with the establishment of wetting theory, materialsand surface-modification technologies for controlling wett-ability has been developed. In the early 1900s, synthesis tech-niques for hydrophobic materials such as silicone and Teflon,as well as surface chemical-modification techniques were re-ported, and by the 1990s, the development of nanotechnologyhas enabled the realization of surfaces with extreme wetta-bility, such as superhydrophobic and superhydrophilic sur-faces [3,4]. The number of articles pertaining to superhydro-phobicity has increased exponentially since 2000, and thesearticles report various hydrophobic/oleophobic processingtechniques and materials, as well as application examples [5].Herein, we first explain the basics of wetting phenomena(surface energy and capillary force). Next, we introduce therelationship between surface structure and wettability. Sub-sequently, we explain a method to design superhydrophobic/superoleophobic surfaces (materials) and their applications.2. Wetting phenomena2.1 Surface free energySurface free energy is the most fundamental element inunderstanding wetting phenomena. An example (althoughnot in the strict sense) is provided herein to illustrate surfacefree energy to the readers (Fig. 1A). Imagine childrenholding hands and forming a line. The child in the middleis happy because he/she is holding hands with the adjacentchildren. However, the child at the edge is only holdinghands with the adjacent child with one hand; thus, he/she isnot happy. The closer the friendship between the children, thegreater is the difference in mood between the children in themiddle and at the edges. This inequality can be eliminated byhaving the children at the edges holding hands with eachother and forming a circle.A similar phenomenon occurs at the molecular scale(Fig. 1B). Water droplets are formed by the accumulation ofwater molecules; however, adjacent water molecules in theinterior of the droplet are bonded to each other. Meanwhile,water molecules on the surface of the droplet have no otherwater molecules at the air side (in fact, they exist as watervapor); thus, an energy difference occurs with the moleculesin the interior of the droplet. This is the surface free energy(typically denoted as γ). In reference to Fig. 1B, the sphericalshape of the water droplet is caused by the minimization of thecontact area with the air to reduce the surface free energy andachieve an energetically stable state. However, unlike the caseof people holding hands, when the droplet becomes spherical,the interface with the air cannot be eliminated, and theexistence of a surface continuously creates an energydifference with the interior molecules. In other words, thesurface free energy (expressed in units of J/m2) is the energyper unit area that increases with the droplet surface area.The magnitude of the surface free energy is determined bythe interaction strength between molecules (analogous to thecloseness in friendship in the human example) (in fact, theexposed surface area of the molecules is a contributing factor).For example, as summarized in Fig. 1C, nonpolar liquids suchas hexane have a relatively low surface free energy owing tointermolecular forces being the only van der Waals inter-action. Glycerol and water have high surface free energiesowing to the presence of both intermolecular forces andhydrogen bonds. Liquid metals such as mercury have asurface free energy that is one order of magnitude higher thanthose of other liquids due to metallic bonds. Consequently,mercury droplets tend to form a sphere and are less affected bythe contact surface.JSAP ReviewJSAP Rev. 2024, 240212https://doi.org/10.11470/jsaprev.240212240212-1 © 2024 The Author(s)https://doi.org/10.11470/oubutsu.93.9_519https://creativecommons.org/licenses/by/4.0/https://doi.org/10.11470/jsaprev.2402122.2 Droplet shapeThe energy difference between molecules occurs not only atthe liquid surface (interface with air) but at all interfaces. Forexample, tone can easily imagine the existence of an energydifference between the interior of water and oil as well as atthe water–oil interface. In other words, free energy exists atthe surfaces and interfaces of different materials. Thus, thesurface free energy can be applied to all surfaces and inter-faces, and the surface (interfacial) free energy γ is generallyexpressed (in subscript form) as the first letter of the Englishname of the state of the material that forms the interface.Therefore, the surface free energy of solids and liquids as wellas the interfacial free energy of the solid–liquid interface areexpressed as γS, γL, and γSL, respectively. Next, we consider asystem in which a solid surface is completely wetted by aliquid (Fig. 1A). In this case, the surface of the system is thesolid surface before wetting, and the free energy is expressedas γS. After wetting, the solid surface disappears, and a solid–liquid interface and liquid surface form in its place. Therefore,the free energy of the system is expressed as �SL þ �L. Thechange in surface free energy due to wetting, S ¼ �S �ð�SL þ �LÞ, is represented by the spreading coefficient.When S > 0, the energy of the system decreases as the solidsurface becomes wet, and the liquid disperses. Meanwhile,when S < 0, the liquid cannot completely wet the solidsurface and remains as a droplet on the solid surface. Thus, theshape of the droplet can be defined by the contact angle θ,which is the angle between the solid–liquid interface andliquid surface.Next, we consider the relationship between θ and the surfacefree energy. As shown in Fig. 2D, consider a change in freeenergy dE when the solid–liquid interface is displaced by dR inthe wetting direction. By denoting R as the contact radius of thedroplet, the area of the liquid wetting the surface is 2�R dR, andthe surface area of the liquid increases by 2�R cos � dR. Here,dE ¼ 2�Rð�SL � �SÞ dR þ 2�R�L cos � dR. When the dropletis stationary, dE ¼ 0. Therefore, the Young’s equation can bewritten in as follows [6]:cos � ¼ �S � �SL�LHere, substituting S ¼ �S � ð�SL þ �LÞ into Young’s equationyields S ¼ �Lð�1 þ cos �Þ. By considering cos � < 1, thedroplet shape can only be defined when S < 0.Figure 2E shows the droplet shape under different θ. Atlarger θ, the droplet become more spherical, and a constantvolume resulted in a smaller apparent contact area with thesolid. Therefore, in many cases, the adhesive force of the liquidwas assumed to decrease as θ increased, thereby increasing thehydrophobicity/oleophobicity. Based on Young’s equation, θincreases as the liquid surface free energy and solid–liquidinterfacial energy increase, and as the solid surface free energydecreases. For example, the θ of a water droplet on fluororesinis larger than that on a glass substrate. Even on fluororesin, theθ of vegetable oil is smaller than that of water.2.3 Surface tension and capillary forceIn Sect. 2.1, we explained that the surface free energy is theenergy per unit area, which increases with the water dropletsurface area, and that it is expressed in units of J/m2. Notably,J/m2 = N/m, and the surface free energy has the same unitsof tension as the spring constant. In other words, analogousto the force that attempts to contract a rubber band whenstretched being characterized by a spring constant, surfacetension acts similarly on water droplets to reduce their surfaceIn the middle Hold both hands → HappyAt the Edge Hold only one hands → Angry Degree of friendship  energy differenceWater droplet Molecules at the air side has lower bonding energy than those in the interior→Unit surface free energy γ [J/m2]ABLiquid Hexane Glycerol Water Mercuryγ [mJ/m2] 18.4 63 72 480Molecular interaction van der Waals Hydrogen bond metallic bondCMinimize angry childrenFig. 1. (A) Example to understand the surface free energy. A group ofchildren stand side by side, holding hands. The children at the ends and thosein the middle are unequal because they hold different numbers of hands. Thecloser the friendship between the children is, the greater the difference inmood becomes. To eliminate this inequality, the children at the ends can joinhands to form a circle. (B) A drop of water on a leaf causes unfairness in thenumber of bonds between the water molecules inside and at the air boundary.The energy difference between these molecules is the surface free energy. Tominimize the number of surface molecules, the droplet becomes spherical.(C) Examples of liquid species and their surface free energy. The magnitudeof the surface free energy is determined by the strength of the interactionbetween the molecules (degree of friendship).ABFree energy : γSS: SolidL: LiquidInterface configuration : Solid surfaceγSL + γLSpreading coefficient S = γS − (γSL + γL)Change in surface free energy by wettingS > 0 S < 0Solid − Liquid interface + liquid surfaceθcosθ dRdRθDefinable liquid shapeCD EYoung’s equation cosθ = (γS − γSL) / γLγL γS HighLowLowHighθHigh LowR dRFig. 2. (A) Spreading coefficient S: change in surface free energy whenwetting a solid surface. (B) Complete wetting of the liquid when S > 0.(C) Incomplete wetting of the liquid when S < 0. The contact angle θquantifies the droplet shape. (D) Young’s equation: the relationship betweenthe droplet’s static shape and surface free energy. (E) Relationship betweencontact angle and surface free energy.JSAP ReviewJSAP Rev. 2024, 240212M. Tenjimbayashi240212-2 © 2024 The Author(s)area and is used as a vector quantity of surface free energy.Here, we consider the difference between a rubber band andwater droplet. Even when a water droplet becomes spherical(Fig. 1B) and its surface area is minimized, tension will con-tinuously reduce the surface area. This results in the internalpressure Pw of the water droplet increasing more than theatmospheric pressure Pair until it balances with the surfacetension (Fig. 3A). The pressure increase Pw � Pair at this timeis known as the Laplace pressure. Next, we examine therelationship between the Laplace pressure and surface freeenergy. When a water droplet with radius R expands by dR, itsvolume and surface area increase by 4�R2 dR and 8�R dR;therefore, its energy change dE is expressed as dE ¼�ðPw � PairÞð4�R2 dRÞ þ �Lð8�R dRÞ. By considering a staticstate (dE ¼ 0), we obtain Laplace’s equation as follows:Pw � Pair ¼ 2�=R. Here, the surface-tension direction isdetermined by the surface curvature 2=R of the liquid. Forexample, a comparison of a bubble in water with a droplet inair shows that the curvature of the water surface is reversed,which implies that the surface-tension is reversed as well.Consequently, the bubble pressure is greater than the waterpressure. This internal pressure increases as the bubble sizedecreases. Therefore, when connecting two bubbles of dif-ferent sizes by a capillary tube, as shown in Fig. 3B, thesmaller bubble is drawn into the larger bubble and mergeswith it, which is a phenomenon known as Ostwald ripening.This is caused by the macroscopic force of the bubblestraversing through the capillary tube due to surface tension,and this force is known as the capillary force. In this case, thecapillary force is obtained by scaling the surface tension(N/m) by the length (m) of the contact line among the threephases, i.e., the water, bubble, and solid (capillary tube)phases. The capillary force acts similarly in any system withthree phases in contact.Thus, a question arises: Can the direction of the capillaryforce be controlled by the material? Figure 3C shows thatthe immersion of a capillary in liquid causes the liquid levelto ascend and descend, which is known as the capillaryphenomenon. When the inner wall of the capillary, the liquid,and the gas form a three-phase interface, the surface curvatureof the liquid changes depending on θ. For example, in caseswhere the liquid phase is water, a hydrophobic capillary(θ > 90°) will cause the liquid level to exhibit an upwardconvex shape, whereas the capillary force causes the waterlevel to descend (buoyancy and capillary forces are inbalance). Meanwhile, a hydrophilic capillary (θ < 90°) causesthe water level to ascend to a height where gravity andcapillary forces are in balance.Capillary adhesion is a typical example of capillary force(Fig. 3D). The phenomenon of wet hair clumping together orwetted sheets adhering together is caused by water dropletsstretching in the lateral direction via capillary force, thusresulting in the action of stress bringing the solid surfacescloser together.3. Surface structure and wetting phenomenon3.1 Wenzel model [7] and Cassie–Baxter model [8]In Sect. 2, we explained the effects of droplets and solidmaterials on wetting phenomena. Next, we explain the effectsof the surface shape of a solid on wetting. Various modelshave been developed for this topic; however, the two mostcommonly used models in modern studies are the Wenzel andCassie–Baxter models, which can be explained via classicaltheory. In the Wenzel model, we consider a Wenzel state ofliquid penetrating and adhering to a surface with surfaceroughness r (ratio of surface area of rough surface to that of flatsurface) without any gaps (Fig. 4A). Here, we consider theapparent contact angle θw, which changes depending onsurface roughness, using the same derivation process as thatfor Young’s equation. As shown in Fig. 4B, we consider thechange in free energy when the solid–liquid interface is dis-placed by dR in the wetting direction. By denoting the contactradius of the droplet as R, the area wetted by the liquid on thesubstrate is expressed as r � 2�R dR, and the surface area ofthe liquid increases by 2�R cos �w dR. The change in freeenergy at this time is expressed as dE ¼ 2�Rð�SL � �SÞr dR þ2�R�L cos �w dR. Combining the fact that dE ¼ 0 when theAWater dropletAtmospheric pressure PairInner pressure PwRBC DCapillary forcePw−Pair= 2γL/Rθθθθ>90 θ 90Fig. 3. (A) Laplace pressure: increase in internal pressure of a droplet dueto surface tension. (B) Bubble coalescence underwater owing to Ostwaldripening. (C) Ascend and descend of the liquid level due to capillaryphenomenon. (D) Capillary adhesion phenomenon.Cassie−Baxter modelCθCBcosθCB dRdRDcosθCB = f cosθ − (1 − f)Solid − liquid contact fraction : fWenzel modelAθwcosθw dRdRBcosθw= r cosθSolid surface roughness : r Fig. 4. (A) Wenzel model: state in which liquid on the rough surface fillsthe rough surface without gaps. (B) Relationship between rough surfacetopography and surface free energy change in the Wenzel model. (C) Cassie–Baxter model: state in which an air layer is entrapped in the gap between therough surface and the liquid. (D) Relationship between rough surfacetopography and surface free energy change in the Cassie–Baxter model.JSAP ReviewJSAP Rev. 2024, 240212M. Tenjimbayashi240212-3 © 2024 The Author(s)droplet is static and Young’s equation cos � ¼ ð�S � �SLÞ=�L,we obtain Wenzel’s equation as follows:cos �w ¼ r cos �;where θ is the contact angle determined by the material of thesolid and liquid type, with �w > � when � > 90° and �w < �when � < 90°. Unlike the case of a flat surface, in the Wenzelstate, the contact angle becomes larger, and the apparentcontact area between the droplet and rough surface decreases(Fig. 2E). However, the actual contact area of the solid–liquidinterface increases by r; thus, the adhesive force of the dropletdoes not decrease [9].Meanwhile, the Cassie–Baxter model primarily explains theapparent contact angle θCB in the Cassie–Baxter state where anair layer is included between the liquid and rough surface, andthe liquid is left stationary (Fig. 4C). The contact ratio of the airlayer is expressed as 1 � f, where f is the contact ratio of thesolid–liquid surface between the liquid and rough surface. Asin the example, we consider the change in free energy when thesolid–liquid interface is displaced in the wetting direction bydR, as shown in Fig. 4D. By denoting the contact radius of thedroplet as R, he area where the liquid wets the substrate is2�Rf dR, and the surface area of the liquid increases by2�R cos �CB dR þ 2�Rð1 � f Þ dR. The change in free energyat this time is expressed as dE ¼ 2�Rð�SL � �SÞ f dR þ2�R�L cos �CB dx þ 2�R�L dRð1 � f Þ. Combining the fact thatdE ¼ 0 when the droplet is static and Young’s equationcos � ¼ ð�S � �SLÞ=�L, we obtain Cassie–Baxter’s equation asfollows:cos �CB ¼ f cos � � ð1 � f Þ;where a smaller f results in a larger apparent contact angle θCBand a smaller apparent contact area between the droplet andrough surface (Fig. 2E) [9]. The actual contact area of thesolid–liquid interface is the apparent contact area multiplied byf. This results in a liquid adhesive force that is significantlysmaller than that of a flat surface or rough surface in theWenzelstate. In particular, the droplet rolls off the solid surface whenthe contact angle of a droplet exceeds 150° in the Cassie–Baxter state. This phenomenon is known as superhydropho-bicity when the droplet is water, and superoleophobicity whenthe droplet is vegetable oil or an organic solvent.3.2 Superhydrophobic surfaces and liquid marblesHow do we design a surface structure that maintains theCassie–Baxter state? Here, we once again consider the exampleof capillary-tube action (Fig. 5A). When we immerse a hydro-phobic (� > 90°, i.e., cos � < 0) capillary tube in water, thecapillary force Fcap causes the water level inside the capillarytube to be lower than the water surface, which balances withthe hydrostatic pressure (buoyancy). If we set the innerdiameter of the capillary tube to 2r, then the capillary force canbe expressed by scaling the surface tension by the contact-linelength as Fcap ¼ 2�rð�SL � �SÞ ¼ �2�r�L cos �. The capillaryforce per unit area (i.e., water-resistance pressure) is expressedas Fcap=�r2 ¼ �2�L cos �=r, which implies that a thinnercapillary tube results in a greater water-resistance pressure perunit area. If we set the decrease in the water level as h, thenthe balance between the hydrostatic pressure (buoyancy) andwater-resistance pressure (capillary force) yields the relation�2�L cos �=r � �gh (�: water density, g: gravitational accel-eration), with a thinner capillary tube resulting in a greaterdecrease in the water level. Additionally, the stability of theCassie–Baxter state is determined by the balance between thewater-resistance and hydrostatic pressures.Next, we consider the state in which a water droplet isdeposited on a fine hydrophobic lattice structure, as shown inFig. 5B. Here, the hydrostatic pressure resulting from gravita-tional force exerting on the water droplet can be obtained byreplacing the decrease in the water level h in Fig. 5A with thethickness of the water droplet. The thickness of the waterdroplet is limited by gravity, and its maximum value isapproximately 2ð�L=�gÞ0:5, where Lc ¼ ð�L=�gÞ0:5 is knownas the capillary length, which is approximately 2.8mm forwater. In other words, maintaining the Cassie–Baxter stateshould result in a water-pressure resistance that is significantlygreater than the hydrostatic pressure, which would be satisfiedif �2�L cos �=r � 2�gLc or Lc � r=ð� cos �Þ. Qualitatively,the stability of the Cassie–Baxter state can be improved usinga fine uneven structure (r → small) and a material with highhydrophobicity (θ→ large). When using actual hydrophobicsurfaces, the hydrodynamic pressure arising from the collisionof water droplets is applied; therefore, a higher water-pressureresistance is favored.Figure 5C shows the use of nanoparticles modified withhydrocarbon groups to form a nanometer-scale hydrophobicuneven structure on a substrate (Fiber), which achieves theCassie–Baxter state. Meanwhile, Fig. 5D shows the adsorp-tion of hydrophobic nanoparticles to the water droplet surfaceto achieve the Cassie–Baxter state. The former is known as asuperhydrophobic surface, whereas the latter hydrophobicwater droplet is known as a liquid marble [10]. In both cases,the water droplet rolls on the substrate surface.However, the fine uneven structure is fragile and easilydestroyed by mechanical stimuli. Therefore, various ap-proaches have been considered for creating a superhydro-phobic surface with high mechanical durability [11]. Minutewater droplets such as condensation water and mist canoccupy the gap between the uneven structures, thus inducingthe Wenzel state and increasing the adhesion force of water.Surface design methods for repelling minute water dropletshave been devised in recent years [12].3.3 Re-entrant shape and superoleophobic surfaceWhen the target liquid is water, surface modification withθnm - μmA BC D2rhPenetrationresisting pressureHydrostatic pressureFig. 5. (A) Relationship between the width of the capillary tube anddescent water level in the capillary phenomenon. (B) A water droplet on ahydrophobic lattice structured surface. (C) Water droplets on the superhydro-phobic nano-coated fabric. (D) Non-wetting droplets: liquid marble formed byadsorption of superhydrophobic nanoparticles on water droplet surface.JSAP ReviewJSAP Rev. 2024, 240212M. Tenjimbayashi240212-4 © 2024 The Author(s)hydrocarbons or fluorocarbon groups can achieve a surfacewith � > 90°. However, no material can afford a surface with� > 90° for liquids with low surface energy, such as vegetableoils (e.g., oleic acid) and organic solvents (e.g., hexane).Therefore, as shown in Fig. 6A, when depositing oleic acidonto a hydrophobic lattice structure (assuming � ¼ 60°),capillary force acts in the direction in which the liquidpenetrates the substrate, and the substrate transitions into theWenzel state. Maintaining the Cassie–Baxter state for liquidswith � < 90° requires a specific surface structure.A re-entrant structure (rat-guard structure) is formed on thesuperoleophobic surface [13]. Figure 6B shows oleic acidplaced on an inverted tapered structure that tapers toward thebottom of the substrate. Here, the shape of the re-entrant struc-ture is defined by the angle� of the tapered structure. Thus, thecondition for the capillary force to resist the liquid pressureis � þ � > 90°, since the gas–liquid interface must form adownward convex structure, as shown in Fig. 6B. This enablesthe Cassie-Baxter state to be maintained even for liquids with� < 90°. However, even if the material is superoleophobic, thecontact angle varies depending on the type of organic solventor oil. For example, applying hexane (assuming � ¼ 20°) to aninverted tapered structure as shown in Fig. 6C results in aWenzel transition when � þ � < 90°. However, depositinghexane onto a T-shaped structure with � ¼ 90° as shown inFig. 6D allows for the Cassie–Baxter state to be maintained.Achieving a re-entrant structure on a fine scale is ideal forreducing the contact area between the liquid and structure.Similarly, modification with fluorocarbon groups is ideal forachieving S < 0 for oils and organic solvents.The results above show that establishing a process tech-nology for forming re-entrant structures over a large area andthe use of fluorine materials, which pose cost and environ-mental issues, are challenging. Furthermore, the developmentof surfaces that repel liquids with lower surface energy isnecessitated. Figure 6E shows a surface modified with fluoro-carbon group-modified nanoparticles into an inverse taperedstructure. As shown, the structure exhibits superhydropho-bicity and superoleophobicity against water and oleic acid,although hexane droplets wetted the surface and spread on it.As with superhydrophobic surfaces, these surfaces presentissues such as mechanical durability and loss of repellencyagainst microdroplets.4. Hydro- and oleophobic surfaces and theirapplication to droplet-transport technologyNext, we introduce hydrophobic/oleophobic-surface appli-cation examples along with our recent results. Hydro- andoleophobic surfaces can reduce the adhesion loss of dropletsto almost zero, thus enabling highly efficient liquid transport.For example, when retrieving food from a container, someof the food remains on the container due to wetness. Thisresidue causes food waste, bacterial growth, and high clean-ing costs. Figure 7A shows that using a glass container thatunderwent oleophobic treatment allows one to pour out theBBQ sauce completely without any adhesion loss. Liquidrepellency is not limited to food, and this technology may beable to eliminate the adhesion loss of liquids in diverseenvironments.Meanwhile, the lack of water adhesion would significantlybenefit the industrial field. For example, metal corrosiondue to salt water, power-line freezing, and reduced efficiencydue to condensation on heat exchangers are caused by wateradhesion. Researchers have attempted to solve these issues byapplying hydrophobic surface treatments. Figure 7B showsthat flow resistance can be reduced via hydrophobic treatment.Subjecting hydrophobic treatment to the inner walls of asilicone tube resulted in a 50-fold increase in the water-delivery speed [14]. However, the wetting phenomenonchanged depending on the water-droplet size and contactstate; therefore, surface design methods must be furtherinvestigated. Figure 7C shows a surface that can slide mist,and this surface adopts a design that promotes the coalescenceof mist particles and the growth of water droplets to facilitateWaterOleic acidHexaneθθθΨA BC DEΨ + θ > 90Ψ + θ < 90Ψ + θ > 90ΨθΨFig. 6. (A) Oleic acid on a lattice structure. (B) Oleic acid on an invertedtapered structure. (C) Hexane on the inverted tapered structure. (D) Hexaneon a T-shaped structure. (E) Drops of water, oleic acid, and hexane on a glasssubstrate with the inverted tapered nanostructure.ACB Drag reduc onFood repellent containerFig. 7. (A) Repellency of BBQ sauce in the oleophobic glass container.(B) Enhanced water transportation inside the silicone tubes with water-repellent inner walls. (C) The mist-repellent coating on the glass substrate.(B and C) Reproduced with permission from Ref. 14. © 2022 Wiley-VCHGmbH.JSAP ReviewJSAP Rev. 2024, 240212M. Tenjimbayashi240212-5 © 2024 The Author(s)their sliding [14]. Recently, we published an open-accessreview that summarizes the applications of droplet-transporttechnology and surface design methods; interested readers canrefer to this paper [5].Finally, we introduce an application example of non-wetting droplet liquid marbles. The liquid marble introducedin Sect. 3.2 forms a superhydro (oleo) phobic structure on awater (oil) droplet surface, thereby eliminating contact withthe substrate. This allows the liquid marble to be regarded as asoft solid material (Fig. 8A) that can be prepared or trans-ported remotely by external stimuli. Changing the liquid typeto a reagent, indicator, or cell suspension is expected to enablelaboratory-in-a-marble applications, which allow chemicalreactions, sensing, and cell-culture experiments at dropletsizes. Readers should refer to our open-access review for moreinformation [15]. Most liquid marbles are prepared as dropletsmeasuring 2–3mm; however, we have successfully preparedliquid marbles of micrometer size (mist marbles) [16]. Agroup of micro liquid marbles behaves similarly to drypowder, despite having a moisture content of approximately99% (Fig. 8B). We successfully trapped individual cellsinside liquid marbles by forming a cell suspension in theinternal liquid of the liquid marbles and designing the size ofthe liquid marble to be equivalent to that of the cells (i.e.,approximately 10 µm) (Fig. 8C). The liquid marbles encap-sulating the cell forms a superhydrophobic structure on theirsurface; therefore, they do not adhere to each other andmaintain a dry powder-like state. Therefore, we name this a“drycell” and attempt to apply it as a cell-isolation tool forsingle-cell analysis techniques.5. SummaryIn this paper, we explained the basic theory of wettingphenomena and superhydrophobic/superoleophobic surfacedesign methods. Owing to space constraints, we have limitedour explanation to systems of droplets in air. Nonetheless, thetheory can be extended to systems that include bubbles inwater and immiscible liquid–liquid interfaces such as waterand oil. Recently, researchers have actively investigatedsurfaces that reduce the friction of liquids by achieving aflat surface on a molecular scale, thereby allowing themto “slide” [17,18]. The combinations of target liquids andsurface structures are numerous, and many challenges remainfor their practical application, such as durability, manufactur-ing process, and costs. Furthermore, their application scope isextremely broad; therefore, new demand may arise as otherfields continue to develop. Readers who are interested in thisfield should begin studying common wetting phenomena.References[1] P. G. Gennes, F. Brochard-Wyart, and D. Quéré, Capillarity and WettingPhenomena (Springer, 2004).[2] H.-J. Butt, K. Graf, and M. Kappl, Physics and Chemistry of Interfaces(Wiley-VCH, 2013).[3] T. Onda, S. Shibuichi, N. Satoh, and K. Tsujii, Langmuir 12, 2125 (1996).[4] R. Wang, K. Hashimoto, and A. Fujishima, Nature 388, 431 (1997).[5] M. Tenjimbayashi and K. Manabe, Sci. Technol. Adv. Mater. 23, 473(2022).[6] T. Young, Philos. Trans. R. Soc. London 95, 65 (1805).[7] R. N. Wenzel, Ind. Eng. Chem. 28, 988 (1936).[8] A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944).[9] A. Lafuma and D. Quéré, Nat. Mater. 2, 457 (2003).[10] P. Aussillous and D. Quéré, Nature 411, 924 (2001).[11] T. Verho, C. Bower, P. Andrew, S. Franssila, O. Ikkala, and R. H. A. Ras,Adv. Mater. 23, 673 (2011).[12] T. Mouterde, G. Lehoucq, S. Xavier, A. Checco, C. T. Black, A. Rahman,T. Midavaine, C. Clanet, and D. Quéré, Nat. Mater. 16, 658 (2017).[13] A. Tuteja, W. Choi, J. M. Mabry, G. H. McKinley, and R. E. Cohen, Proc.Natl. Acad. Sci. U.S.A. 105, 18200 (2008).[14] M. Tenjimbayashi, G. Hayase, T. Hiroi, and T. Ueki, Adv. Mater.Interfaces 9, 2200497 (2022).[15] M. Tenjimbayashi, T. Mouterde, P. K. Roy, and K. Uto, Nanoscale 15,18980 (2023).[16] M. Tenjimbayashi, S. Yamamoto, and K. Uto, Adv. Mater. 35, 2300486(2023).[17] L. Chen, S. Huang, R. H. A. Ras, and X. Tian, Nat. Rev. Chem. 7, 123(2023).[18] T.-S. Wong, S. H. Kang, S. K. Y. Tang, E. J. Smythe, B. D. Hatton, A.Grinthal, and J. Aizenberg, Nature 477, 443 (2011).ProfileMizuki Tenjimbayashi is an independent re-searcher at the Research Center for MaterialsNanoarchitectonics (MANA), National Institute forMaterials Science (NIMS). He received his Ph.D.from Keio University in 2017 and joined NIMS in2018 as a postdoctoral fellow. In 2019, he started hisindependent research career. He is concurrentlyworking as a visiting professor at Chuo University.Single cell isolationHandling dropletMist marbleNIH-3T3 cellABCFig. 8. (A) Liquid marble technology enabled the droplet handling.(B) Powder-like fluidity of micro-liquid marbles. (C) Cell isolation usingmicro-liquid Marbles. (A–C) Reproduced with permission from Ref. 16.© 2023 Wiley-VCH GmbH.JSAP ReviewJSAP Rev. 2024, 240212M. Tenjimbayashi240212-6 © 2024 The Author(s)https://doi.org/10.1021/la950418ohttps://doi.org/10.1038/41233https://doi.org/10.1080/14686996.2022.2116293https://doi.org/10.1080/14686996.2022.2116293https://doi.org/10.1098/rstl.1805.0005https://doi.org/10.1021/ie50320a024https://doi.org/10.1039/tf9444000546https://doi.org/10.1038/nmat924https://doi.org/10.1038/35082026https://doi.org/10.1002/adma.201003129https://doi.org/10.1038/nmat4868https://doi.org/10.1073/pnas.0804872105https://doi.org/10.1073/pnas.0804872105https://doi.org/10.1002/admi.202200497https://doi.org/10.1002/admi.202200497https://doi.org/10.1039/D3NR04966Chttps://doi.org/10.1039/D3NR04966Chttps://doi.org/10.1002/adma.202300486https://doi.org/10.1002/adma.202300486https://doi.org/10.1038/s41570-022-00455-whttps://doi.org/10.1038/s41570-022-00455-whttps://doi.org/10.1038/nature10447