Vincenzo Ferraro1, Massimiliano M Villone1, Volodymyr Tkachenko2, Lisa Miccio3, Lorenzo Lombardi1, Daniele Tammaro1, Ernesto Di Maio1, Gaetano D'Avino1, Pier Luca Maffettone4. 1. Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy; NEAPoLIS, Numerical and Experimental Advanced Program on Liquids and Interface Systems, Joint Research Center CNR - Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy. 2. CNR-ISASI, Institute of Applied Sciences and Intelligent Systems E. Caianiello, Via Campi Flegrei 34, 80078 Pozzuoli, Napoli, Italy; NEAPoLIS, Numerical and Experimental Advanced Program on Liquids and Interface Systems, Joint Research Center CNR - Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy. 3. NEAPoLIS, Numerical and Experimental Advanced Program on Liquids and Interface Systems, Joint Research Center CNR - Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy; CNR-ISASI, Institute of Applied Sciences and Intelligent Systems E. Caianiello, Via Campi Flegrei 34, 80078 Pozzuoli, Napoli, Italy. 4. Dipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy; NEAPoLIS, Numerical and Experimental Advanced Program on Liquids and Interface Systems, Joint Research Center CNR - Università degli Studi di Napoli Federico II, Piazzale Tecchio 80, 80125 Napoli, Italy. Electronic address: p.maffettone@unina.it.
Abstract
HYPOTHESIS: Thin liquid films are important in many scientific fields. In particular, films with both the surface layers exposed to a different fluid phase, known as freestanding films, are relevant in the ambit of foams and emulsions. Hence, there is a great interest in developing novel techniques allowing to form large and stable freestanding liquid films and to follow their dynamics. EXPERIMENTS: We develop a novel opto-mechanical tool allowing to perform and study the preparation and the capillary leveling flow of axisymmetric bare freestanding liquid films. The tool is composed by a customized motorized iris diaphragm and by an innovative joint imaging setup combining digital holography and white light color interferometry that enables real-time measurement of film thickness over a large field of view. The dynamics of films made of a model Newtonian fluid, i.e., high-viscosity silicone oil, is studied. Direct numerical simulations and a hydrodynamic model based on the lubrication theory are used to support the experimental results. FINDINGS: Iris opening induces the formation of large circular freestanding films with a stepped profile. Once iris opening is stopped, the films undergo a capillary leveling flow tending to flatten their profile. The leveling flow follows the theoretical scaling given by Ilton et al. [1]. We prove through numerical simulations that an equi-biaxial extensional flow occurs at the film center. Furthermore, we observe the formation and dynamics of dimples in bare freestanding films for the first time.
HYPOTHESIS: Thin liquid films are important in many scientific fields. In particular, films with both the surface layers exposed to a different fluid phase, known as freestanding films, are relevant in the ambit of foams and emulsions. Hence, there is a great interest in developing novel techniques allowing to form large and stable freestanding liquid films and to follow their dynamics. EXPERIMENTS: We develop a novel opto-mechanical tool allowing to perform and study the preparation and the capillary leveling flow of axisymmetric bare freestanding liquid films. The tool is composed by a customized motorized iris diaphragm and by an innovative joint imaging setup combining digital holography and white light color interferometry that enables real-time measurement of film thickness over a large field of view. The dynamics of films made of a model Newtonian fluid, i.e., high-viscosity silicone oil, is studied. Direct numerical simulations and a hydrodynamic model based on the lubrication theory are used to support the experimental results. FINDINGS:Iris opening induces the formation of large circular freestanding films with a stepped profile. Once iris opening is stopped, the films undergo a capillary leveling flow tending to flatten their profile. The leveling flow follows the theoretical scaling given by Ilton et al. [1]. We prove through numerical simulations that an equi-biaxial extensional flow occurs at the film center. Furthermore, we observe the formation and dynamics of dimples in bare freestanding films for the first time.