{"title":"Actuating droplets with electrowetting: Force and dynamics","authors":"Robert Hennig, Vito Cacucciolo, Herbert Shea","doi":"10.1002/dro2.108","DOIUrl":"10.1002/dro2.108","url":null,"abstract":"<p>Electrowetting on dielectric (EWOD) allows rapid movement of liquid droplets on a smooth surface, with applications ranging from lab-on-chip devices to micro-actuators. The in-plane force on a droplet is a key indicator of EWOD performance. This force has been extensively modeled but few direct experimental measurements are reported. We study the EWOD force on a droplet using two setups that allow, for the first time, the simultaneous measurement of force and contact angle, while imaging the droplet shape at 6000 frames/s. For several liquids and surfaces, we observe that the force saturates at a voltage of approximately 150 V. Application of voltages of up 2 kV, that is, 10 times higher than is typical, does not significantly increase forces beyond the saturation point. However, we observe that the transient dynamics, localized at the front contact line, do not show saturation with voltage. At the higher voltages, the initial front contact line speed continues to increase, the front contact angle temporarily becomes near zero, creating a thin liquid film, and capillary waves form at the liquid–air interface. When the localized EWOD forces at the contact line exceed the capillary forces, projectile droplets form. Increasing surface tension allows for higher droplet forces, which we demonstrate with mercury.</p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-03-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.108","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140255390","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yuyang Wang, Zecong Fang, Sen Li, Kexin Lin, Zhifeng Zhang, Junyi Chen, Tingrui Pan
{"title":"Droplet Laplace valve-enabled glaucoma implant for intraocular pressure management","authors":"Yuyang Wang, Zecong Fang, Sen Li, Kexin Lin, Zhifeng Zhang, Junyi Chen, Tingrui Pan","doi":"10.1002/dro2.109","DOIUrl":"10.1002/dro2.109","url":null,"abstract":"<p>Glaucoma, the leading cause of irreversible blindness worldwide, is closely linked to aqueous overaccumulation and elevated intraocular pressure (IOP). For refractory glaucoma, aqueous shunts with valves are commonly implanted for effective aqueous drainage control and IOP stabilization. However, existing valved glaucoma implants have the disadvantages of inconsistent valve opening/closing pressures, poor long-term repeatability due to their reliance on moving parts, and complex architectures and fabrication processes. Here, we propose a novel valving concept, the droplet Laplace valve (DLV), a three-dimensional printable moving-parts-free microvalve with customizable and consistent threshold valving pressures. The DLV uses a flow discretization unit governed by capillarity, comprising a droplet-forming nozzle, and a separated reservoir to digitize continuous flow into quantifiable droplets. Unlike the classic one-time-use Laplace valves, the DLV's unique design allows for its reusability. The opening pressure is adjustable by varying the nozzle size, like the classic Laplace valves (following the Young–Laplace equation), while the closing pressure can be modified by tuning the separation distance and the reservoir size. Various DLVs with customizable opening pressures from 5 to 11 mmHg have been demonstrated, with opening/closing pressure differences suppressed down to <0.5 mmHg (<0.15 mmHg under the best conditions). Thanks to its moving-parts-free nature and digitized flow properties, the DLV shows a highly repeatable valving performance (<1.7%, 1000 cycles) and a predictable linear flow rate–pressure correlation (<i>R</i><sup>2</sup> > 0.99). Preliminary ex vivo validation in an enucleated porcine eye confirms the DLV's efficiency in aqueous shunting and prompt IOP stabilization. The DLV technology holds great promise in glaucoma implants for IOP management and various microsystems for flow control.</p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.109","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140429159","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Xue Qi Koh, Calvin Thenarianto, Ville Jokinen, Dan Daniel
{"title":"Quantifying droplet–solid friction using an atomic force microscope","authors":"Xue Qi Koh, Calvin Thenarianto, Ville Jokinen, Dan Daniel","doi":"10.1002/dro2.107","DOIUrl":"10.1002/dro2.107","url":null,"abstract":"<p>Controlling the wetting and spreading of microdroplets is key to technologies such as microfluidics, ink-jet printing, and surface coating. Contact angle goniometry is commonly used to characterize surface wetting by droplets, but the technique is ill-suited for high contact angles close to <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 \u0000 <mrow>\u0000 <mn>180</mn>\u0000 \u0000 <mo>°</mo>\u0000 </mrow>\u0000 </mrow>\u0000 <annotation> $180^circ $</annotation>\u0000 </semantics></math>. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly quantify droplet–solid friction on different surfaces (superhydrophobic and underwater superoleophobic) with sub-nanonewton force resolutions. We demonstrate the versatility of our approach by performing friction measurements using different liquids (water and oil droplets) and under different ambient environments (in air and underwater). Finally, we show that underwater superoleophobic surfaces can be qualitatively different from superhydrophobic surfaces: droplet–solid friction is highly sensitive to droplet speeds for the former but not for the latter surface.</p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.107","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"140448175","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jyotirmoy Sarma, Deepak Monga, Zongqi Guo, Fangying Chen, Xianming Dai
{"title":"Coarsening droplets for frosting delay on hydrophilic slippery liquid-infused porous surfaces","authors":"Jyotirmoy Sarma, Deepak Monga, Zongqi Guo, Fangying Chen, Xianming Dai","doi":"10.1002/dro2.106","DOIUrl":"10.1002/dro2.106","url":null,"abstract":"<p>Frosting occurs due to the freezing of condensed water droplets on a supercooled surface. The nucleated frost propagates through interdroplet bridges and covers the entire surface, resulting from the deposition of highly supersaturated vapor surrounding tiny droplets. While inhibition of the formation of frost bridges is not possible, the propagation of frost can be delayed by effectively removing tiny droplets. Passive technologies, such as superhydrophobic surfaces (SHS) and hydrophobic slippery liquid-infused porous surfaces (SLIPS), rely on static growth and direct contact with densely distributed droplets. However, use of these approaches in delaying frost propagation involves challenges, as the interdroplet distance remains small. Here, we report a new approach of spontaneous droplet movement on hydrophilic SLIPS to delay the formation of interdroplet frost bridges. Surface tension forces generated by the hydrophilic oil meniscus of a large water droplet efficiently pull neighboring droplets with a diameter of less than 20 μm from all directions. This causes a dynamic separation between water droplets and an adjacent frozen droplet. Such a process delays the formation and propagation of interdroplet frost bridges. Consequently, there is significant delay in frosting on hydrophilic SLIPS compared to those on SHS and hydrophobic SLIPS.</p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.106","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139960427","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Back Cover, Volume 3, Number 1, January 2024","authors":"Xi Zhao, Mengwen Qiao, Yingxin Zhou, Jing Liu","doi":"10.1002/dro2.114","DOIUrl":"https://doi.org/10.1002/dro2.114","url":null,"abstract":"<p><b>Back Cover</b>: The cover image is based on the Review Article <i>Liquid metal droplet dynamics</i> by Zhao et al.</p><p>Gallium-based liquid metal droplets are newly intriguing research targets in diverse areas due to their unique properties which combine the dual merits of both liquids and metals. This review summarizes the latest progress and presents an overview on basic findings related to liquid metal macro-droplet dynamics, including droplet types, fabrication methods, different dynamic behaviors, main challenges, and future perspectives. (DOI: 10.1002/dro2.104)\u0000\u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.114","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139550565","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Yanchen Wu, Joaquin E. Urrutia Gomez, Hongmin Zhang, Fei Wang, Pavel A. Levkin, Anna A. Popova, Britta Nestler
{"title":"Front Cover, Volume 3, Number 1, January 2024","authors":"Yanchen Wu, Joaquin E. Urrutia Gomez, Hongmin Zhang, Fei Wang, Pavel A. Levkin, Anna A. Popova, Britta Nestler","doi":"10.1002/dro2.115","DOIUrl":"https://doi.org/10.1002/dro2.115","url":null,"abstract":"<p><b>Front Cover</b>: The cover image is based on the Research Article <i>Digital twin of a droplet microarray platform: Evaporation behavior for multiple droplets on patterned chips for cell culture</i> by Wu et al.</p><p>This cover highlights evaporation phenomenon of multiple droplets on patterned surfaces, affected by humidity, temperature, and droplet distribution. We establish a digital twin system and propose a theoretical method to detect volumes and pH variation on evaporating droplets onto patterned substrates. The proposed strategy allows us to achieve an active maneuver of the collective evaporation of droplets on patterned surfaces. (DOI: 10.1002/dro2.94)\u0000\u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.115","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139550564","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Diego Sánchez-Saldaña, Maria Fernandino, Carlos A. Dorao
{"title":"Inside Front Cover, Volume 3, Number 1, January 2024","authors":"Diego Sánchez-Saldaña, Maria Fernandino, Carlos A. Dorao","doi":"10.1002/dro2.113","DOIUrl":"https://doi.org/10.1002/dro2.113","url":null,"abstract":"<p><b>Inside Front Cover</b>: The cover image is based on the Research Article <i>Acoustic micro-beam vortex generator for flow actuation inside droplets</i> by Sánchez-Saldaña et al.</p><p>A micro-size spiral IDT placed at the base of the droplet leads to an acoustic vortex that induces a poloidal flow inside the droplet with an ascendent flow in the centre and descendent flow in the periphery. This paper shows that the micro-size spiral IDT can control the scale of the flow actuation inside the droplet which in turn can be used for facilitating the operations on particles. (DOI: 10.1002/dro2.96)\u0000\u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.113","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139550566","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Inside Back Cover, Volume 3, Number 1, January 2024","authors":"Jingyu Shi, Yu Zhang, Yadi Fan, Yi Liu, Mo Yang","doi":"10.1002/dro2.112","DOIUrl":"https://doi.org/10.1002/dro2.112","url":null,"abstract":"<p><b>Inside Back Cover</b>: The cover image is based on the Review Article <i>Recent advances in droplet-based microfluidics in liquid biopsy for cancer diagnosis</i> by Shi et al.</p><p>Droplet-based microfluidics with high throughput, low contamination, high sensitivity, and single-cell/single-molecule/single-exosome analysis capabilities have shown great potential in the field of liquid biopsy. This review aims to summarize the recent development in droplet-based microfluidics in liquid biopsy for cancer diagnosis. (DOI: 10.1002/dro2.92)\u0000\u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.112","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139550567","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Frontispiece, Volume 3, Number 1, January 2024","authors":"Kenta Goto, Kyoka Nakanishi, Fumito Tani, Satoru Tokuda","doi":"10.1002/dro2.111","DOIUrl":"https://doi.org/10.1002/dro2.111","url":null,"abstract":"<p><b>Frontispiece</b>: The cover image is based on the Research Article <i>Chemical reaction in a liquid–liquid phase-separated multiple droplet: Synchronization of color change dynamics with droplet movement</i> by Goto et al. (This cover image is licensed under CC BY-NC-ND 4.0.)</p><p>Exemplifying self-similarity, a large Matryoshka doll holds a smaller one within. The shape is repeated at different reduced-scales to generate a nested structure. The smallest doll hosts a micro-channel, which formed nested droplets through liquid-liquid phase separation when oil-rich and aqueous alcohol liquids were mixed. (DOI: 10.1002/dro2.93)\u0000\u0000 <figure>\u0000 <div><picture>\u0000 <source></source></picture><p></p>\u0000 </div>\u0000 </figure></p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.111","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139550568","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"New insights into suspended drops: When soft matter meets acoustic levitation","authors":"Hongyue Chen, Zhenyu Hong, Duyang Zang","doi":"10.1002/dro2.95","DOIUrl":"https://doi.org/10.1002/dro2.95","url":null,"abstract":"<p>Acoustic levitation has developed into a popular but elegant tool for the study of drops as well as soft matter due to its exceptional levitation capabilities to a variety of liquid samples. The acoustically levitated drops offer opportunities for the investigation of a wide range of fundamental issues related to liquid drops. In this review, the unique physics/chemical processes involved in acoustically levitated drops are dealt with. We first introduce the dynamics of the acoustically levitated drops, including drop oscillation, coalescence, and the associated capillary phenomena. The bubble formation and stability are also discussed. Depending on the inhibition of solid surfaces and the nonlinear effects of ultrasound, the self-assembly of colloidal particles at the air–liquid interface as well as granular matter in air is reviewed. In particular, the exploration of biological drops by using acoustic levitation is also highlighted. In the end, the concept of acoustic-levitation-fluidics and possible potential topics are proposed.</p>","PeriodicalId":100381,"journal":{"name":"Droplet","volume":"3 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/dro2.95","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139550181","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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