STUDIES IN INTERFACE SCIENCE Drops and Bubbles in Interfacial Research STUDIES IN INTERFACE SCIENCE SERIES EDITORS D. M6bius and R. Miller Vol. I Dynamics of Adsorption at Liquid Interfaces Theory, Experiment, Application by S.S. Dukhin, G. Kretzschmar and R. Miller Vol. 2 An Introduction to Dynamics of Colloids by J.K.G. Dhont Vol. 3 Interracial Tensiometry by A.I. Rusanov and V.A. Prokhorov Vol. 4 New Developments in Construction and Functions of Organic Thin Films edited by T. Kajiyama and M. Aizawa Vol. 5 Foam and Foam Films by D. Exerowa and P.M. Kruglyakov Vol. 6 Drops and Bubbles in Interfacial Research edited by D. M6bius and R. Miller Drops and Bubbles in lnterfacial Research Edited by D. MOBIUS Max-Planck-lnstitut fur Biophysikalische Chemie P.O. Box 2841 G6ttingen Germany R. MILLER Max-Planck-lnstitut fur Kolloid- und Grenzfl~ichenforschung Rudower Chaussee 5 Berlin-Adlershof Germany I998 ELSEVIER Amsterdam - Lausanne - New York- Oxford- Shannon - Singapore - Tokyo ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 2ii, IOOO AE Amsterdam, The Netherlands ISBN: o 444 82894 X (cid:14)9 I998 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 52i, I OOO AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA, 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science BV, unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Foreword The shape of drops and bubbles and their specific properties make them a centre of interest of many interfacial scientists. There are classical methods for measuring the surface and interfacial tension, such as the maximum bubble pressure technique or the drop volume method. Milestones in the development of the theoretical basis were set by Thomas Young in 1805 and P.S. de Laplace in1806. On the basis of an analysis of the forces acting at an interface, they derived a description of both the capillary elevation and the contact angle. While Young only described the phenomena qualitatively, Laplace developed a very clear and well developed physical and mathematical theory without any analytical solution, given in 1830 by Gauss. This theory, named Gauss-Laplace-Equation is the basis for all coming methods on surface and interfacial tension. In the early years of this century many methods had been established as standard methods. However, a number of physically principle questions were under discussion at that time. Remember the discussion of Lohnstein and Tate in 1905 through 1913 about the correction factors for the drop volume methods. In recent time this method was further developed to make it applicable at drop times less than one second, which brings along additional problems connected with the hydrodynamics of the drop formation and detachment. Also, in presence of surfactants, surface rheological parameters come into play and have to be considered. Another method, the maximum bubble pressure technique, used in literature for many decades, has had a renaissance also very recently due to the availability of high precision pressure sensors which made this method faster and more accurate and allow nowadays measurements of dynamic surface tensions down to adsorption times of some hundred microseconds. The shape of a pendent drop as another example gives access to the interfacial tension and various developed sol, ware packages such as ADSA or ASTRA supply simultaneously the area vi and volume of the studied drops. This makes the pendent drop method very variable and allows even interfacial relaxation experiments with a single liquid drop. The spinning drop has been especially developed for ultra-low interfacial tensions. Experiments have shown that such ultra-low tensions are important for the stability of emulsions and lead under certain conditions even to a spontaneous emulsification. However this technique has a number of limitations and needs attention when used at higher interfacial or surface tensions. A completely new methodology for studying the dilational rheology of adsorption layers has been developed only during the last 25 years, the oscillating bubble technique. This method allows to determine the dilational rheology of adsorbed layers and simultaneously the exchange of matter of these adsorbed molecules or other relaxation mechanisms. This technique has been applied for so interesting scientific problems like the compression/expansion of layers formed by lung surfactants. It is also a method of choice for fast matter exchange or interfacial relaxation processes for example of mixed protein/surfactant systems the intrinsic mechanisms of which is not understood so far. The properties of rising bubbles in surfactants solutions is another highly interesting system in which bubbles play a key role. The rising characteristics of a bubble in a liquid are significantly influences by adsorbed layers of surfactants or polymers. This makes the phenomenon interesting for example for the purity test of drinking water or for the progress of purification procedures in water treatment plants. The same scenario is the basis for a technology of large economic and ecological importance - the flotation. The efficiency of this technology is controlled by the interfacial properties of the floating bubbles and their interaction with the particles to be flotated. Several of the described techniques are available as commercial instruments. The book will serve as an up to date guide to understand the methods, to show their advantages and disadvantages, to specify the range of application and to explain their experimental limits. For vii other methods just laboratory set-ups exist and the book will give insight into how to design them and what are the key questions to be solved in order to get accurate results. Especially these prototype instruments give access to completely new phenomena or allow studies of particular properties, such as it is true for the oscillation mode analysis of drops, the force measurements between two liquid phases or the shape analysis of growing drops using modern light fibre techniques. The are more fields of application of bubbles and drops, for example in medicine and biology. The lungs are constructed by bubbles, by so-called alveols. The functionality of alveols and their mechanical properties are still not fully understood, although live is based on the perfect functioning of the lungs since millions of years. Another very practical problem closely linked to drops is the metallurgy. Metals or alloys during their processing are usually liquids. This fact is used to study their properties - to determine the surface tension of metals from the shape of a molten metal drop, to understand reactions on the drop surface etc. The book contains 15 chapters dedicated to either of the above mentioned topics. In Volume 1 of this series (Dynamics of Adsorption at Liquid Interfaces) few of the classical methods have been described more from the point of view of interfacial dynamics so than only a small overlap with this books exists. Moreover, there is a basic introduction of the surface and interfacial tension methods in Volume 3 (Surface Tensiometry). However, chapters on these methods given here do not introduce into the physical basics but describe in detail the most recent developments and how to use them correctly. Also experimental examples are given to demonstrate the data interpretation properly. After a general introduction into the topic the chapter about the mechanics of axisymmetric liquid menisci gives an overview of the theoretical background for all drop and bubble experiments. Subsequent chapters are dedicated to drop and bubble methods. The chapters viii about the three main drop experiments provide the theoretical basis, a description of experimental set-ups, specific advantages and disadvantages, correction and calibration problems, experimental examples, and their interpretation: pendent and sessile drop, drop volume, and spinning drop technique. The chapter about capillary pressure methods summarises different techniques and gives examples of applications, for instance measurements under microgravity conditions in space experiments. This type of methods comprises bubbles and drops. The maximum bubble pressure technique as a particular capillary pressure method will be described with emphasis on the decisive developments in the last five years, during which this method was developed as the most reliable one for extremely short adsorption times, down to the range of milliseconds and less. Problems connected with aerodynamics and hydrodynamics are discussed and used to show the limits of this widely used standard method. The oscillating bubble technique has been first developed about 25 years ago and does exist still in form of individual set-ups only. However it provides a number of information not available by other techniques, for example about the dilational rheology of adsorption layers and relaxation processes at the interface. Especially the frequency interval spans a range which is of large importance for many surfactants. The theory as well experimental details and results will be presented and compared with other techniques. The description of rising bubbles in surfactant solutions will contain the hydrodynamic basis as well as the theoretical description of the effect of interfacial layers on the movement of bubbles. Besides the theoretical basis also experimental data and the relevance for practical applications will be presented, such as water purification, flotation processes etc. The chapter about lung alveols demonstrates how important bubbles built by biological membranes are in everyday life. The relevance for medicine and biology as well as model studies will be discussed. ix An important example for the application of drops is the metallurgy, where the surface tension of metals and alloys is an important parameter for many applications. Also the chapters on drop shape analysis by using fibre technique and on force measurements between emulsion droplets are of much practical relevance. These chapters however will close the present book although a number of further interesting fields could be added, such as on formation and stabilisation of emulsions, or drops in shear fields, thin films between drops or bubbles, are extensive enough to fill separate books and thus will be excluded here. A book especially dedicated to "Foams and Foam Films" has been published within this series. This book will be interesting for all users of commercial instruments based on principles such as drop volume, spinning drop and maximum bubble pressure techniques. Also for those scientists and engineers using unique or home-made set-ups for example of the pendent or sessile drop technique will find the state of the art of the respective methods and comparison with other techniques. The different examples of application are interesting for a number of groups, in material sciences as well as life sciences. For those scientists starting up research in the field of characterisation of liquid interfaces this book will serve as guide to find the right methods and to realise the certain peculiarities inherent in specific liquid systems. For the very valuable support during the whole preparation of the book manuscript we want to express our gratefulness to Dr. Martina Bree and Sabine Siegmund. September 1997 D. M6bius R. Miller