Joffrey Decourselle

Fabien Duchateau

Nicolas Lumineau

A Survey of FRBRization Techniques

The Functional Requirements for Bibliographic Records (FR-BR), an emerging model in the bibliographic domain, provide interesting possibilities in terms of cataloguing, representation and semantic enrichment of bibliographic data. However, the automated transformation of existing catalogs to fit this model is a requirement towards a wide adoption of FRBR in libraries. The cultural heritage community proposed a notable amount of FRBRization tools and projects, thus making it difficult for practitioners to compare and evaluate them. In this paper, we propose a synthetic and relevant classification of the FRBRization techniques according to specific criteria of comparison such as model expressiveness or specific enhancements.

Introduction

A large majority of current IT systems used by librarians to manage their bibliographic records are based on old standards like the well-known MAchine Readable Cataloguing (MARC) format, designed in 1965 and used around the world in different versions like MARC21 or UNIMARC. Although these formats are references in libraries, they suffer from many fields with ambiguous semantic (e.g. 7xx fields in MARC21) and from a lack of mechanisms to represent complex relationships between records, i.e., bibliographic families composed by different executions and many related works. Furthermore, librarians nowadays have to deal with a large amount of storage formats (e.g., ebook, streams) and new data sources. This may result in bad cataloguing practices, numerous typing errors and finally increasing cataloguing costs [START_REF] Denton | FRBR and the History of Cataloging. Understanding FRBR: What It Is and How It Will Affect Our Retrieval Tools[END_REF]. However, best practices from Semantic Web encourage the reuse of data which implies, in the context of cultural heritage data, to improve interoperability of bibliographic catalogs [START_REF] Alemu | Linked Data for libraries: Benefits of a conceptual shift from library-specific record structures to RDF-based data models[END_REF].

In response to the discussions around major librarian's topics such as reducing cataloguing costs and enhancing the representation of bibliographic families, the IFLA Study Group published, in 1998, new recommendations for cataloguing and a set of concepts to represent bibliographic data in the context of the Functional Requirements for Bibliographic Records (FRBR) [START_REF] Madison | The origins of the IFLA study on functional requirements for bibliographic records[END_REF]. FRBR offers a flexible framework to represent any cultural content in library [START_REF] Boeuf | FRBR: Hype or cure-all? Introduction[END_REF], new benefits for improving searching and visualization [START_REF] Dickey | FRBRization of a Library Catalog: Better Collocation of Records, Leading to Enhanced Search, Retrieval, and Display[END_REF] and new possibilities for semantic enrichment of cultural entities [START_REF] Buchanan | FRBR: enriching and integrating digital libraries[END_REF]. The adoption of FRBR in cultural institutions requires different issues to be solved like the transformation of legacy data into the new entity-relationship model, a process mainly called FRBRization [START_REF] Yee | FRBRization: a method for turning online public finding lists into online public catalogs[END_REF][START_REF] Aalberg | A Process and Tool for the Conversion of MARC Records to a Normalized FRBR Implementation[END_REF]. The last decade has seen the emergence of a significant amount of FR-BRization tools and projects, thus making it difficult for practitioners to gather relevant information [START_REF] Zhang | What Is Next for Functional Requirements for Bibliographic Records? A Delphi Study1[END_REF].

This paper presents a survey of FRBRization techniques, both research projects and full commercial tools. To the best of our knowledge, our work is the first classification of these techniques according to criteria such as the type of transformation, the model expressiveness and its enhancements (see Section 4). This classification is useful for practitioners who need the keys to quickly understand and identify suitable techniques w.r.t. their requirements. The rest of the paper is organized as follows: Section 2 introduces preliminaries about FRBR and FR-BRization, then we present the related work in Section 3. Our classification is detailed in Section 4. Section 5 concludes the article.

Preliminaries about FRBR and FRBRization

FRBR provides a set of concepts organized in an entity-relationship structure to represent bibliographic families and subject authority data of any cultural material with no ambiguity [START_REF] Committee | Functional requirements for bibliographic records: final report[END_REF]. The main contribution of FRBR is that it deeply redefines the notion of what an intellectual work is and its representation [START_REF] Bennett | The concept of a work in WorldCat: an application of FRBR[END_REF]. Entities are divided into three groups: the first group focuses on the record's representation, the second one is about related persons or corporate body and the third group represents what a work is about. Contrary to legacy models, several levels of metadata can be used to describe each bibliographic record [START_REF] O'neill | FRBR: Functional requirements for bibliographic records[END_REF].

As illustrated in Figure 1, the first group is composed of entities Work (the most abstract object which represents an intellectual or artistic idea), Expression (the realization of a Work in various intellectual forms, such as a translation), Manifestation (the physical representation of an Expression in terms of material attributes) and Item (a single example of a Manifestation, for example a single book on a library's shelf). The components of the second group represent humans, groups or organizations and have been later gathered under the Agent name. In Figure 1, each entity from the first group has a link with an Agent. The third and last group of the FRBR model concerns the subject relation of a work. It allows the model to answer the question of what a Work is about and to align several Works according to specific criteria (e.g., same topic, review of).

The Functional Requirements for Authority Data (FRAD) [START_REF] Patton | An Introduction to Functional Requirements for Authority Data (FRAD). Understanding FRBR: What It Is and How It Will Affect Our Retrieval Tools[END_REF] and the Functional Requirements for Subject Authority Data (FRSAD) [START_REF] Zeng | Functional requirements for subject authority data (FRSAD): a conceptual model[END_REF] have later been added to FRBR to cover authority data and complex relationships between enti- ties respectively. For instance, Figure 1 depicts examples of these improvements like the new entity Family from FRAD or the new concepts of Thema and Nomen from FRSAD. These three functional requirements, FRBR, FRAD and FRSAD are called the FRBR Family and must be seen as a single architecture for the new cataloguing concepts. Because the FRBR model have been created regardless of technological constraints a large amount of provided concepts covers a too large scope for being used in practical implementations. In other words, FRBR must not be used directly as a final object model. Thus, there is a branch of the FRBR researches which is focused on modelling practical ontologies based on FRBR but adapted to the user needs [START_REF] Riva | FRBRoo: enabling a common view of information from memory institutions[END_REF][START_REF] Coyle | FRBR, Twenty Years On[END_REF].

The adoption of a FRBR-based model in existing systems is known as the FRBRization process. Such a transformation relates to the well known challenges of data integration and data conversion [START_REF] Aalberg | The value of MARC data, or, challenges of frbrisation[END_REF]. This study focuses on a specific part of the process which aims at identifying, creating and connecting FRBR entities from existing records. In this context, FRBRization is widely regarded as a complete metadata migration framework rather than a simple algorithm [START_REF] Hegna | Data mining MARC to find: FRBR? International cataloguing and bibliographic control[END_REF]. The initial phases of a FRBRization process have been formalized as follows: pre-processing to harvest and normalize the input catalog, extraction to generate the FRBR entities and relationships and post-processing with deduplication and adaptation of data for visualization [START_REF] Aalberg | A Process and Tool for the Conversion of MARC Records to a Normalized FRBR Implementation[END_REF]. The next section presents the different attempts of surveys and classifications for FRBRization techniques.

Related Work

One of the earliest survey about FRBRization was conducted at the Online Computer Library Center (OCLC) [START_REF] Dickey | FRBRization of a Library Catalog: Better Collocation of Records, Leading to Enhanced Search, Retrieval, and Display[END_REF]. Although focused on the benefits of FRBR cataloguing, it also reviews the major solutions which implemented this model. However, the projects are simply listed along with their properties and available technical details to mainly illustrate the contrast between a growing interest in FRBRization and the lack of successful tools.

Another notable work is the observation of the whole landscape of FRBR implementations in libraries by Zhang & Salaba [START_REF] Zhang | Implementing FRBR in libraries: key issues and future directions[END_REF]. The existing implementations have been classified according to the following categories: the full-scale systems (i.e., that are mainly FRBRized systems), the prototypes or experimental systems and the algorithms and software that are essentially tools to FRBRize catalogs. The scope of the study is large since all kinds of FRBR-related projects are covered. Although exhaustive (at that time), such a broad classification is not sufficient to understand technical features between two different FRBRization approaches. Researchers from the TelPlus 1 project have also worked on a survey to introduce and list FRBR experiments [START_REF] Teixeira | Report on FRBR experiments[END_REF]. They have presented, with a rich description, all the relevant projects related to FRBR ordered in high level sections. The conversion tools are introduced under a Research section and presented as a list. More recently, Aalberg et al. presented a list of FRBRization projects [START_REF] Aalberg | The value of MARC data, or, challenges of frbrisation[END_REF]. For each project, they studied the possible issues raised during the transformation of records, and more specifically in terms of common structures. The authors were able to propose solutions for interpreting these structures and to FRBRize them correctly. However, the list of projects is not exhaustive since it mainly aims at illustrating structural problems.

To summarize, existing surveys presented the state of the art of FRBRization techniques, but mainly as an unsorted list. In addition, the techniques can be described with a specific goal in mind (e.g., illustrating problems of conversion). The properties and features of each technique are provided textually, which does not facilitate a quick comparison. In the next section, we propose the first classification dedicated to FRBRization techniques.

A Novel Classification of FRBRization Techniques

This survey aims at exploring the various issues faced by existing FRBRization techniques, bringing a more precise view of this migration process and facilitating the comparison of existing solutions through a multi-criteria classification. Because many projects gravitate around FRBR, we do not consider the Integrated Library Systems (ILS) which do not include a transformation tool. This choice is justified with regards to current and future challenges on FRBRization such as scalability and semantic enrichment. Besides, our classification focuses on semi-automated techniques, i.e., any FRBRization solution in which most of the algorithm proceeds without human intervention. This excludes manual projects which have not detailed any automated process to transform their catalog, for instance the Austlit project [START_REF] Ayres | Report on the successful austlit: Australian literature gateway. Implementation of the FRBR and INDECS event models, and implications for other FRBR implementations[END_REF] or Data.BNF [START_REF] Boeuf | Customized OPACs on the Semantic Web: the OpenCat prototype[END_REF].

Our classification makes use of three criteria of comparison:

1 http://www.theeuropeanlibrary.org/confluence/display/wiki/TELplus+project -Type of FRBRization which relates to the methods for identifying FRBR entities from the legacy model. Two strategies are found in the literature. The former consists of grouping physical form of records by comparing descriptive keys (e.g., a concatenation of author names and title) to deduce more abstract levels (see Section 4.1). The latter aims at interpreting the original fields of the records (e.g., with mapping rules) to build the FRBR entities and relationships (see Section 4.2).

-Model Expressiveness that deals with the models designed to receive FR-BRized data. Three variations are presented: a limited model means that the main entities from the first group of FRBR are not completely implemented, a standard model indicates that the FRBRization technique takes into account the entities and relations from the three groups of the basic FRBR model and an enhanced model corresponds to specific implementations where significant changes have been made to the initial FRBR model.

-Specific Enhancements used to improve the quality or the performance of the FRBRization process. It may include additional steps, improvements of algorithms or interoperability enhancements (see Section 4.3).

Figure 2 depicts the FRBRization techniques classified according to our criteria. For instance, the FRBRization project TelPlus is classified under rule-based interpretation of fields, standard model and it includes clustered deduplication and evaluation metrics as specific enhancements. The rest of this section provides details for each technique according to these criteria. When FRBRizing, it appears that a large part of the MARC records corresponds to the FRBR Manifestation entity. In such a case, the most complex task is to produce Work and Expression entities from the MARC records. Thus, the initial intuition is to group the derived Manifestations into relevant clusters which represent a more abstract level. A common solution deals with the detection of the Work entities based on generated description keys from Manifestation attributes, and to deduce the primal Expression that links the Works to the Manifestations.

Among the first projects about FRBRization, the studies from OCLC describe how to automatically group records at a more abstract level [START_REF] Bennett | The concept of a work in WorldCat: an application of FRBR[END_REF]. The wellknown Work-Set algorithm, applied in FictionFinder and WorldCat's catalog [START_REF] Hickey | FRBRizing OCLC's WorldCat[END_REF], was designed to produce "sets of Works" for MARC records based on the generation and the comparison of normalized keys [START_REF] Hickey | FRBR Work-Set Algorithm[END_REF]. This process is based on three main steps to build these keys: constructing an author portion, constructing a title portion and grouping these portions according to four specific patterns. Each portion is built according to mappings with MARC fields and matching with authority files. These studies reveal two major issues from this technique: the precision about the semantic level of the deduced Works (e.g., a too large clustering may lead to groups at more higher level than the original FRBR Work) and the difficulty to identify the variations of a same Work at Expression level.

Another experience in the grouping records category is PRIMO from Ex Libris [START_REF] Putz | FRBR The MAB2 Perspective[END_REF]. This commercial discovery tool provides FRBRization features by proposing transformation options like automatic grouping of records under a single Work. In addition, the FRBRization includes a pre-processing step which consists in harvesting catalog data into an intermediate model so that records can be cleaned. We cannot provide more details about PRIMO since technical information are not available, for example about deduplication and model expressiveness.

As a summary, the techniques based on grouping Manifestations under Works can be seen as a reorganization of the catalog rather than a real transformation. Furthermore, even if the patterns (based mainly on title and authors fields) are useful for clustering records, they are not sufficient to detect all the complex relationships that can exist between entities and must be completed by additional processes using the original fields to fully exploit the capabilities of FRBR.

Interpreting each fields of the original records

The second type of FRBRization aims at building a FRBR catalog by mainly applying mapping rules between the initial catalog metadata and the FRBR attributes. In the context of MARC catalogs, such a process is basically performed by reading each record's field and identifying whether it matches any rule. Depending on the quality and completeness of the input catalog data, the rules have to be more or less complex to create not only the FRBR entities but also the different relationships that link them. It is worth noting that a single MARC record usually generates more than one FRBR entity. Indeed, a MARC record contains information for different FRBR entities. Conversely, information about the same FRBR entity may appear in several MARC records, in particular when the second and the third FRBR groups are considered. In the rest of this section, we describe the major projects based on this kind of extraction technique.

The LC Display Tool is one of the first prototype for FRBRization provided by the researchers from the Library of Congress in 2003 [START_REF] Schneider | FRBRizing MARC records with the FRBR Display Tool[END_REF]. This solution takes as input MARC21 records and it can produce FRBR XML and HTML. The process uses MARCXML as an intermediate format to benefit from the XML format and from the MARC leader fields. Rules stored in XSL files are applied to the MARCXML catalog to clean the data and to generate FRBR entities. The process returns a list of Works, each of them hierarchically linked to one Expression and one Manifestation and containing title and author attributes. This representation focuses on the main entities from the first group of the FRBR model which is a very limited expression of the capabilities of FRBR. Such choices for displaying results have also raised early issues in terms of visualization of FRBR entities. Indeed the hierarchical organization of XML (e.g., if using HTML) combined with the large amount of relationships available in FRBR may produce very large files which makes it more difficult to read. The TelPlus project [START_REF] Manguinhas | FRBRization of MARC records in multiple catalogs[END_REF] allowed the realization of a semi-automatic transformation process for MARC records. The FRBRization is based on a preprocessing stage for correcting and enriching records, a rule-based extraction step and a deduplication phase. Several issues are raised by this study such as the minimum information needed in each record to guarantee a good quality of the process, the large amount of duplicate FRBR entities generated and the high complexity of some mapping rules. They face the first issue by providing a list of requirements to filter the records in input. The second one (i.e., duplicates entities) is managed with a clustered deduplication process (see Section 4.3). They deal with the last issue (i.e., complex rules) by using an extension of the extraction tool from Aalberg [START_REF] Aalberg | A Process and Tool for the Conversion of MARC Records to a Normalized FRBR Implementation[END_REF] to enable the implementation of more complex rules. The experiments have been performed on a large number of heterogeneous records (from TelPlus project) and the output model used is a quite standard version of initial FRBR model designed in RDF 2LibFRBR is another rule-based implementation built in the context of FR-BRizing the Chinese Koha system [START_REF] Chang | Experimenting with implementing FRBR in a Chinese Koha system[END_REF]. This project includes a FRBRization tool written in Perl which can harvest data in MARC21 or CMARC, extract FRBR entities from the three groups and store them in FRBRXML or in the Koha database. There are only few information about the model used by the tool which seems to be close of the standard initial FRBR model. A cataloguing interface for performing corrections on the transformed data and for clustering equivalent FRBR entities has been implemented and evaluated. Compared to previous tools, the advantages of LibFRFR reside both in the edition of the mapping rules (which are not hard-coded) and in the cataloguing interface which involves librarians in the process.

Virtua VTLS is also a solution which provide a rule-based interpretation of input fields to extract FRBR concepts [START_REF] Espley | The VTLS implementation of FRBR[END_REF]. A specificity of this commercial tool is that MARC records can coexist with FRBR entities, i.e., FRBR is implemented as an optional layer. Although only few technical informations are given about the FRBRization, many options and user interfaces have been presented to analyse MARC records and FRBRize the catalog. Furthermore, the FRBR model has been extended to allow complex relationships between different works of a same bibliographic family (e.g., Super Works).

The Variations or V/FRBR project, dedicated to musical content, brings a new FRBRization tool with an extended FRBR implementation [START_REF] Riley | Enhancing Interoperability of FRBR-Based Metadata[END_REF]. A major proposal of this project deals with the model because three XML-based schemas have been released: a strict version of FRBR and FRAD entities, an extended model which globally allows to add more meta-information to the previous strict model and the V/FRBR model which is based on the extended model in which attributes have been adapted to the musical context. The other contribution is a Java-based tool that has been designed to FRBRize musical data to build a FRBRized OPAC called Scherzo [START_REF] Notess | Scherzo: A FRBR-Based Music Discovery System[END_REF]. The results of the project show that FRBRization may also succeed in real-world contexts by using sophisticated extraction rules. However, the mapping rules were apparently hardcoded even if a complete documentation have been freely provided with the project.

In eXtensible Catalog (XC) [START_REF] Bowen | Moving Library Metadata Toward Linked Data: Opportunities Provided by the eXtensible Catalog[END_REF], FRBRization is handled by the Metadata Service Toolkit with the goal of improving the quality of the migration. This open-source tool is composed of a pre-processing phase with mechanisms to harvest and normalize records (from OAI-PMH repositories 3 ), a transformation phase to migrate the normalized data (FRBRization) and an aggregation phase to detect and merge duplicate entities [START_REF] Bowen | Supporting the eXtensible Catalog through Metadata Design and Services[END_REF]. In this project, the Metadata Service Toolkit is a component inside a full ILS solution, and the implemented model (the XC schema) has been extended, mainly with the Holding entity, i.e., a specific implementation of the FRBR Item entity to handle MARC21 Holding records. Several interesting challenges of FRBRization are faced by XC in terms of input model management and normalization of extracted data (see Section 4.3). [START_REF] Takhirov | FRBR-ML: A FRBR-based framework for semantic interoperability[END_REF] is an extended version of Aalberg's FRBRization tool [START_REF] Aalberg | A Process and Tool for the Conversion of MARC Records to a Normalized FRBR Implementation[END_REF]. In addition to the transformation aspects, it aims at promoting interoperability between different models such as MARC and FRBR. Thus, this new system is built on an XML based intermediate model which was designed to ease exporting data in various semantic formats. The FRBRized records are represented with an hybrid semantic format which may use external attributes from the Linked Open Data.

FRBR-ML

To summarize, all these rule-based solutions cover various model needs. FR-BRization techniques based on enhanced model may be more complex to design but offer a better completeness in terms of transformed data. Furthermore, a major part of the solutions has managed to improve the quality of their transformation at different steps. At the beginning of the process, some have chosen to involve the user feedback with an intuitive interface to create or refine the rules and to ease building the output model. At the end of the process, several solutions have provided an aggregation phase to find and merge duplicate records. It is a critical step especially when rules are applied on fields that may contain duplicate values.

Specific Enhancements

The last criterion of our classification is about specific enhancements, mainly designed for tackling quality or performance issues. These enhancements have been isolated from the standard features presented previously because they don't represent a fundamental step of the initial FRBRization process but are needed to face the metadata migration challenges (see Section 2). The FRBR model and its extensions are suitable for the representation of many information in the cultural heritage domain. However, the original model only provides a limited set of attributes and relationships labels which are mainly concepts. Thus, to cover the practical requirements of librarians and to enhance related information, it may be useful to include additional attributes while modelling with FRBR. Several tools are able to integrate vocabularies from other models. For instance, Virtua VTLS uses RDA vocabulary and rules 4 to provide a more structured and interoperable catalog. Similarly, eXtensible Catalog extends its models with elements from RDA and Dublin Core [START_REF] Bowen | Moving Library Metadata Toward Linked Data: Opportunities Provided by the eXtensible Catalog[END_REF]. The exploitation of added entries from the initial data has been widely studied to find solutions for extracting data stored in the MARC21 7xx fields 5 . Such fields may contain additional authors, geographic names, information about Agents at the Expression level (e.g., cover drawer, translator) and the challenge is to store this information in the correct FRBR entity. Variations [START_REF] Riley | Enhancing Interoperability of FRBR-Based Metadata[END_REF] and eXtensible Catalog [START_REF] Bowen | Moving Library Metadata Toward Linked Data: Opportunities Provided by the eXtensible Catalog[END_REF] roughly use the same method for exploiting these added entries: creation of new FRBR entities from specific fields (e.g., in MARC21 700, 710 or 740 with a second indicator equal to 2 ). Variations may create several levels of Works6 while eXtensible Catalog can directly create a new Expression and its parent Work 7 . In FRBR-ML, the idea for disambiguating information contained in the MARC21 7xx fields is to analyse similar records that may not contain the ambiguity [START_REF] Takhirov | FRBR-ML: A FRBR-based framework for semantic interoperability[END_REF]. This search is performed either in the same collection, in external collections (z39.50 or SRU/SRW) or in knowledge bases such as the Linked Open Data cloud8 (LOD). An interesting enhancement for FRBR catalogs is the semantic enrichment, i.e., the addition of extra information such as new attributes or relationships. Using the potential of external data can both enhance the completeness of the process and also improve the validation of the results. However, this task involves many new issues because of the heterogeneity of the databases that can be used. For instance, using both information from structured sources (e.g., LOD) and non-structured sources (e.g., websites) involve effective and efficient matching tools at schema and instance levels. FRBR-ML performs matching to external knowledge bases (LOD cloud) for discovering the semantic type of an ambiguous field (e.g., person, location) and detecting missing relationships between FRBR entities [START_REF] Takhirov | FRBR-ML: A FRBR-based framework for semantic interoperability[END_REF]. As for eXtensible Catalog, it can perform a specific data fusion process to merge the attributes from equivalent entities [START_REF] Bowen | Supporting the eXtensible Catalog through Metadata Design and Services[END_REF]. Descriptive keys are used for comparing records and for detecting duplicates, thus improving quality [START_REF] Bowen | Supporting the eXtensible Catalog through Metadata Design and Services[END_REF][START_REF] Manguinhas | FRBRization of MARC records in multiple catalogs[END_REF][START_REF] Chang | Experimenting with implementing FRBR in a Chinese Koha system[END_REF][START_REF] Takhirov | FRBR-ML: A FRBR-based framework for semantic interoperability[END_REF]. The issue is to find the most relevant patterns for descriptive keys. In the Work-Set project, the OCLC has proposed four patterns to fulfil this goal [START_REF] Hickey | FRBRizing OCLC's WorldCat[END_REF]. For instance, one of the patterns is the combination of author name and title. These four OCLC patterns are still a reference for the generation of descriptive keys for bibliographic content, and they have been applied in the most recent FRBRization techniques. The deduplication, which aims at detecting duplicate records, is one of the most time consuming step because of the Cartesian product applied between all records. The clustered deduplication enhancement impacts efficiency by reducing the execution time of the deduplication. In the TelPlus project, each FRBR entity produces a set of keys (according to the patterns for descriptive keys from OCLC [START_REF] Hickey | FRBRizing OCLC's WorldCat[END_REF]). The intuition is to group in the same cluster all entities that share at least one identical value for one of their keys. The last step identifies duplicates inside clusters by comparing their keys and computing similarity values with thresholds. Evaluation of the FRBRization process is crucial, but it requires the definition of evaluation metrics. TelPlus proposes a metric for the evaluation of the aggregation level (i.e., the percentage of duplicate entities extracted). The FRBR-ML project [START_REF] Takhirov | FRBR-ML: A FRBR-based framework for semantic interoperability[END_REF] defines three metrics to measure the degree of completeness (i.e., the amount of information lost during FRBRization), the minimality rate (i.e., the amount of redundant information, at the property, record and collection levels) and the extension rate (i.e., the amount of enriched information). Finally, our classification enables the identification of relevant techniques accord-ing to the type of FRBRization and the model expressiveness. Compared to the initial FRBRization process, most tools have proposed specific enhancements either for effectiveness (quality aspects) or efficiency (performance aspects).

Conclusion

This paper introduces a survey about FRBRization tools and projects to provide a more comprehensive view of this process. To the best of our knowledge, this is the first classification according to criteria such as the type of FRBRization and the model expressiveness. Our contribution is useful both for librarians who need to choose a FRBRization technique and for IT company working on cultural heritage data who plan to design mature FRBRization tools. According to the results of our study, we advocate that the FRBRization process must be refined to cover the most recent challenges of metadata migration. For instance, involving user feedback at the initialization phase (e.g., rule selection) can reduce human mistakes. Furthermore, the quality of the process can be enhanced by providing additional steps at the post-processing phase such as an automatic evaluation step or a semantic enrichment step based on external sources.

Fig. 2 .

 2 Fig. 2. Classification of the FRBRization Techniques, with the Type of FRBRization in light-grey circled boxes, the Model Expressiveness in dark-grey circled boxes, the Solutions in white squared boxes and the Specific Enhancements in light-grey squared boxes

FRBR in RDF, http://vocab.org/frbr/core.html

Virtua does not provide any public specification about its use of RDA.

MARC21 7xx fields, http://www.loc.gov/marc/bibliographic/bd70x75x.html

Converting MARC to FRBR, http://www.dlib.indiana.edu/projects/vfrbr/projectDoc

XC, https://code.google.com/p/xcmetadataservicestoolkit/wiki/TranserviceIntro

Linked Open Data cloud, http://linkeddata.org/

This work has been partially supported by the French Agency ANRT (www.anrt.asso.fr), the company PROGILONE (http://www.progilone.fr/), a PHC Aurora funding (#34047VH) and a CNRS PICS funding.


Yousra Timounay

Elise Lorenceau

Florence Rouyer

Opening and retraction of particulate soap films

Keywords: 68.15.+e -liquid films 47.57.Gc -Granular flow, complex fluids 47.55.nb -Capillary effects, interfacial flows

We study for the first time the bursting dynamics of thin liquid films laden with hydrophobic micronic particles either with free or constrained edges. We highlight that the particles can arrange in bilayer or monolayer configurations and explore a range of particles coverage from zero to random close packing. When the particles bridge the two interfaces (monolayer configuration) of free edge films, the hole opens intermittently. For the other cases, we observe constant retraction velocities, modeled by balancing liquid and particles inertia against surface tension as in Taylor-Culick theory. But, this approach is only valid up to a critical value of particles coverage due to the interplay between the interfaces and the friction between particles.

Solid hydrophobic particles attached at an air/liquid interface create hybrid interfaces, with a mechanical behavior simultaneously driven by contacts between grains at the 2D granular medium and capillarity [1,2]. Previous studies mainly focus on particle rafts [3][4][5][6] and coated droplets [2,[7][8][9][10][11][12], where the ratio between the depth of the liquid reservoir and the particles diameter is far larger than 1. However, some recent works on foams stabilized by particles consider particulate films for which the depth of the liquid reservoir is smaller than a particle diameter in an aerated medium [13,14]. Yet, particulate films still raise many fundamental questions and practical issues. What is the mechanics of particulate films? Do they deform like liquid films or solid membranes? Do the particles concentration and arrangement at the liquid/air interface affect their mechanics? At the scale of particle laden aqueous foams, a recent work has shown that nanoparticles can stop the coarsening [15]. This behavior has been rationalized using continuum mechanics's framework and deriving a surface elasticity modulus for the particle laden interfaces. Yet, is this analysis also accurate for microparticles or is the framework developed for granular materials more adapted? Moreover, do the particles also modify the other aging processes of foams, namely drainage and film coalescence? These are crucial issues, in particular from a practical point of view, since particle laden foams are encountered in different industries such as cosmetics, food or civil engineering industries. In these applications, aqueous foams are often used as templates to obtain solid aerated materials. The stability of the liquid aqueous foam (fresh material) but also the mechanical and acoustical properties of the solid foam after drying, which depend especially on the fraction of open cells and the apertures sizes of the membranes [16,17], are affected by the presence of the particles. Understanding how the particles affect this aspects is challenging.

In the present work, we study the dynamics of particulate films during their retraction either when they are simply punctured at their center or when the retracting edge is attached to a rigid but mobile boundary. The first mode of retraction, called "free edge" because the retracting edge is the hole boundary, corresponds to the coalescence mechanism at the scale of the foam. The second mode of retraction, called "constrained edge", is similar to film shrinkage during the topological rearrangements T1 and T2 at the scale of the foam, due to coarsening or shear.

Thin bare soap films are known to open at constant velocity [18] until total disappearance as predicted by the well-know Taylor-Culick law where a balance of the rim inertia and the capillary forces is assumed. But, in the cases of viscous films [19,20] and elastic surfaces [21], deviations from this law are observed. For particulate films, the high packing of grains at the interface might lead to jamming, thus to a freezing of the retraction dynamics. We thus explore a range of 2D solid fractions from zero to close packing. Different retraction dynamics (intermittent or inertial) are observed depending on the particle configuration and the constraints at the edge.

The liquid used to create the particulate films is an aqueous solution of Sodium Dodecyl Sulfate (SDS) at the critical micelle concentration [START_REF] Mukerjee | National Bureau of Standards of NSRDS-NBS 36[END_REF]. This concentration is chosen because any increase of the SDS concentration results in an increase of the film's stability but also leads up to a decrease of the hydrophobicity of the particles in contact with the solution. The CMC therefore happens to be a good compromise between this two aspects. The liquid's volumetric mass density is ρ l = 1000 kg/m 3 and its surface tension is γ =36 mN/m. The particles, in turn, are monosized polystyrene beads (Dynoseeds from Microbeads) with a volumetric mass density ρ P = 1050 kg/m 3 . Particles with four different diameters D p are used: 20, 40, 140 and 250 µm. To modify their wetting properties, the particles have been immersed into a mixture made up of octane solution and dissolved silane. After the silanization, the contact angle θ between the particles and the SDS solution at the CMC is equal to 95 • ± 10 • making them slightly hydrophobic.

To create particulate films, we immerse and withdraw a frame made of chrome wires of 60 µm diameter from a container filled with the SDS solution with a dense raft of particles on its interface. The surface area of the frame ( ∼ 1 cm 2 ) is in the order of the surface of the liquid films of a dry foam constituted of centimetric bubbles. Moreover, such liquid films are stable during the typical time of one experiment(up to one minute). Using a microscope, we observe two particle configurations on the particulate films. In the monolayer configuration (ML), the particles cross the film and their median plan is within the liquid phase (see FIG 1-a). In the bilayer configuration (BL), the particles are attached to each one of the liquid-air interfaces (see FIG 1-b) and we observe that one interface is generally much more covered in particles than the other one. For one given particle diameter, the particles configuration in the liquid film is perfectly reproducible. We are not able yet to predict this configuration but it seems that it mainly depends on the weight of the particles.

To characterize the particles coverage of the particulate films, we define φ, the surface fraction of particles per interface, which takes into account the projected surface of the particles on the interfaces:

φ = α.N particles .S particle 2.S f ilm (1)
with N particles the number of particles present in the film, S particle the projected surface of a single particle, S f ilm the surface area of the film and α=1 for BL and 2 for ML films, since each particle is attached to both liquid/air interfaces in this configuration. By exploiting the images prior to the film retraction, we compute the number of particles present in the film (see FIG 1-c). For monolayer films, an image analysis with thresholding of gray levels and circularity as criteria counts the exact number of particles yielding to an incertitude over φ less than 2%. For bilayer films, we use the same method as above in the areas where the particles are present only on one single interface. But for the areas where the particles are present on both interfaces, we consider that the random close packing for hard disks (φ RCP =0.82±0.1 [START_REF] Berryman | [END_REF]) is achieved relying on experimental observations (see FIG 1-d). Adding these two contributions, we obtain the total number of particles with an incertitude over φ typically less than 5%.

The thickness of the liquid film e is measured by light interference spectroscopy (Ocean Optics spectrometer used in transmission). The thickness is proportional to the wavelengths for which intensity peaks are observed. A light absorbance method is also used on films colored (see FIG 1-e) with the Acid blue dye (from Fluotechnik). The typical liquid film thickness of bare liquid films, e thick varies between 40 and 50 µm for all the frames we used. On particulate films, e is very heterogeneous and ranges from e thick ≈ 45 µm in the vicinity of the particles to e thin ≈5 µm where there is no particle (see FIG 1-e, 1-g and 1-h). This heterogeneous film thickness is presumably due to the fact that the particles need a liquid's reservoir to fulfill the contact angle condition. Assuming that the area of the thick film is proportional to φ/φ RCP , we approximate an average thickness (e φ ) as a function of φ, e thick and e thin , that meets the good limit for films covered with closely packed particles:

e φ = e thick φ φ RCP + e thin (1 - φ φ RCP ) (2)
To study the hole opening in particulate films, we use a hexagonal frame with a side length equal to 5 mm. The film is gently punctured in its center by a thin needle wrapped in absorbing paper that locally thins the film up to its rupture and thus creates a hole that expands radially (see FIG 2-a). When a heterogeneity in particles coverage is noticed, the hexagonal films are divided into eight parts, where different surface fractions of particles and radial velocities of the hole are measured in each part.

To study the retraction of particulate films when one of the edges is constrained, we use rectangular frames; the length is kept constant equal to 1.5 cm while the width a can either be equal to 2 mm, 5 mm or 9 mm. Once the rectangular film is formed, we gather all of the particles on the right side of the frame using a stick that can slide on the wires of the frame. This stick is is a hollow cylinder whose length, diameter and mass are respectively 1.6 cm, 6 mm and 0.01 g and will constitute the constrained edge of the particulate film. To trigger the particulate film's retraction, we puncture gently the bare film in the vicinity of the stick in order that no liquid is collected on this side of the stick. We follow the retraction's velocity of the film perpendicular to the mobile stick (see FIG 2-b). Only experiments with a quite uniform particles coverage are considered. Indeed, if the particles coverage is heterogeneous, the stick will not remain perpendicular to the frame's wires during its motion and it can not be described by a unique velocity. Each rectangular film is thus characterized by one average surface fraction of particles.

All the experiments are followed with a Photron high speed camera placed above the frame and recording at 1000 fps for rectangular films and 2000 fps for hexagonal ones.

Depending on the particles configuration, the holes in the particulate films open differently as can be observed in FIG 3, where the outlines correspond to the hole boundary taken at a regular time interval for ML and BL films, respectively on the left and right sides. Holes created in the center of ML films expand intermittently and can have an anisotropic shape. On the contrary, holes created in the center of BL films expand uniformly in time and space, they are circular. We describe in the following these two antagonist behaviors and explain them in regards to the particle position at the interfaces.

The anisotropic and intermittent expansion of the hole in ML particulate films, triggered by the successive breakages of capillary bridges between the particles advances with a typical velocity ≈ 0.1 m/s. This opening pattern is qualitatively similar to the fracture of particle rafts induced by the localized addition of surfactant [24] and to the capillary fingering regime in immersed granular media [25]. Moreover, the hole does not necessarily reach the frame (see the right side of FIG 3). These observations can be explained as follows. In this configuration, the particles bridge the two liquid/air interfaces, which can inhibit the growth of the rim in the vertical direction. Indeed, a rim would not form unless the particles are expelled from the film during the hole opening. To be expelled, a particle needs an energy input greater than its attachment energy to the liquid film ∆E a . ∆E a can be approximated by comparing the two energy states: the first one being the case where the particle and the liquid film are totally separated and the second one being the case where the median plane of the particle is within the liquid phase. Without taking into account the meniscus around the particle, we estimate ∆E a to be about 10 -9 J. Obviously for microparticles, thermal activation is not sufficient to remove particle, but as the hole expands, the particles at the free moving edge acquire a kinetic energy E c . For velocity ∼ 0.1m/s: ∆Ea Ec ≈ 25. The expulsion of the particles from the liquid phase is thus not possible which is consistent with our experimental observations. Besides, the capillary bridges between grains can stop the coalescence of ML films: their ruptures would require to open small holes between the particles, which is energetically unfavorable. Indeed, assuming hexagonal close packing of particles, the maximal hole radius is r = ( 1

√ 3 -1
2 )D p ≈ 20 µm which gives a ratio r e ≈ 0.4 close to 0.3, the critical value below which, based on energy considerations, a hole in a soap film would not expand [26]. Moreover, during the opening of the hole, the liquid might be redistributed thus thickening the film and making the ratio r e even smaller. On the other hand, for all BL experiments, the film opens uniformly at constant velocity ranging from 0.4 m/s to 1.4m/s depending on the surface fraction of particles, as long as the edge of the frame is not reached, as can be seen in the left side of FIG 3. Some particles are ejected from the particulate film during the experiments. It can be explained by comparing the same energies than above.

In this case, ∆Ea

Ec ranges from 0.3 to 4 depending on the size of the particles. This ratio is much smaller than the one computed in the BL configuration because, in this latter configuration, the particles are attached to only one interface and that their kinetic energy can be significantly larger than in the ML case. However, a darkening of the outer rim of the hole is noticeable, due to the accumulation of particles in this portion of the film (see FIG 1-f). All of these observations coincide qualitatively with what has been observed for bare liquid films despite the presence of solid particles on the interfaces. Indeed, the bursting of a bare liquid film also exhibits a constant velocity that results from the balance between surface tension and inertia of the rim formed at the edge of the hole, V =

√ 2γ eρ ≈ 1.4 m/s [27-29].
From these bursting experiments, we observed that BL films burst in a quite similar way than bare films, and that the velocity of the rim is lower when particles are present in the film. For ML films, the rim can not form because the particles bridge the two interfaces of the film and the only way to increase the hole size is to break the small capillary bridges between the grains, leading to an intermittent opening that can even be stopped.

We now move to the second mode of retraction that is the "constrained edge" geometry where the retracting edge is attached to a mobile stick and thus is forced to be of a constant width. ML and BL films retract in a similar way: the stick velocity reaches a constant value ranging from 0.1 m/s to 0.5m/s depending on the surface fraction of particles within few milliseconds. Instead of a rim, folds where liquid and particles are collected appear near the mobile stick. Moreover, the stick can stop before reaching the frame.

More quantitatively, we report V /V 0 as a function of φ on FIG 4, where V is either the hole opening velocity for BL films or the mobile stick velocity and V 0 the velocity of bare films (φ = 0) in the same geometry. V /V 0 decreases with φ, a feature qualitatively expected from a simple balance of inertia and capillary as in Taylor-Culick model [START_REF] Taylor | [END_REF]29] since the mass collected per unit time increases with φ. In the following, we present a model to predict V . Assuming that both the liquid and the particles are collected at the edge of the retracting film (in the outer rim of the opening hole for BL films or within the folds for constrained edge rectangular films), the variation of the mass over time per unit length is :

dm dt = Aρ l e φ V + Bρ P φD p V (3)
where (A; B) = (1; 4 3 ) for BL films and (A; B) = (1 -φ; 2 3 ) for ML films. The value of the parameter B in the monolayer configuration is half its value in the bilayer configuration because the particles are attached to the two liquid/air interfaces of the film, so for each interface, we count half the particle mass .

Then, we write the momentum balance per unit of length:

βγ -F = V dm dt + m dV dt ( 4
)
where β is a dimensionless coefficient equal to 2 for hexagonal films and smaller than 2 for rectangular films because of the meniscus on the upper air/liquid interface around the stick. As for F , it is the global linear resistant force exerted on the rim or on the mobile stick that takes into account the friction of the stick on the wires as well as the friction of the liquid menisci on the wires in the rectangular frame. Determining F is a difficult task as observed in slightly different geometries [30][31][32], which is far beyond the scope of this paper. Neglecting the contribution of the particles, F in the case of particulate films is assumed to be identical to F in the case of bare liquid films. Moreover, observing that retraction velocities of bare and particulate films are constant over time, it comes:

V = V 0 √
ρ l e thick ρ l e φ A + Bρ P φD p (5) For ML constrained edge films, the experimental normalized velocities are in good agreement with Eq.5 as long as φ < 0.75 (see . This confirms our hypothesis that the particles do not play any role in the resisting force F at low φ. However, for φ > 0.75, the normalized velocities are lower than expected. At such φ, approaching φ RCP = 0.82, all particles are in contact with at least three neighbors and the inter-particle friction starts to be significant. Experimental data for BL films covered with small particles, either with constrained or free edges (see FIG 4-b), agree well with Eq.5 for φ < 0.5 ± 0.05. Because of gravity, the free retracting edge is out of plane for the largest particles, we thus do not represent the experimental points relative to diameter larger than 40 µm for the free edge experiments. For φ > 0.5±0.05 all experimental points are below the theoretical curves. The deviation for BL films can be explained by the inter-particle friction as well. This friction is presumably more intense due to higher number of contacts. Indeed, at given φ and particles diameter, the particles are twice as many in the BL configuration than in the ML configuration. This friction can also be more intense due to the absence of lubricated contacts as reported for 2D granular medium at high packing fractions [33]. We emphasize that this second critical value of φ = 0.5 is around the connectivity threshold for particle rafts [34]. Below this threshold, the packing is very loose and particles have few collective effects but once this threshold is reached, the particle assembly may have a granular behavior which results in the deviation from Taylor-Culick law. On another hand, one can argue that this critical φ is around half the 2D maximal close packing value, thus the two nearby interfaces interact and considering a projection over the surface area of the film rather than the interfaces to define φ might be relevant. Moreover, the deviation from the theoretical curve might be due an increase of the overall viscosity of the films because of the presence of the particles. Indeed, during the opening of viscous films, a transient regime where the rim velocity is smaller than Taylor-Culick velocity is observed [20]. Finally, we remind that we used the surface tension of the solution in Eq.5 rather than an effective surface tension despite the presence of particles on the interfaces. It has been shown that, when adsorbed at an interface, nanoparticles as well as microparticles can change the surface tension of the particle laden interface they are attached to [35][36][37]. This change becomes significant at high surface coverage (∼ 0.8) and might thus be one of the causes of deviation from Taylor-Culick law that we observe. To conclude, we have shown that the dynamics of retraction of particulate films depends on the particle position at the interfaces as well as on the constraints on the film edges, the results are summed up in the table on FIG 5. Particles that bridge the two film's interfaces in free edge experiments inhibit the expansion of the hole because the rim formation and the opening of the capillary bridges between the grains are unfavorable. This behavior is quite different from what we have observed for free edge experiments in the bilayer configuration where the opening leads to the total disappearance of the films. In this latter case as well as in the case of constrained edges, the velocities are constant and can be predicted by a modified Taylor-Culick model considering the particles' inertia for low φ. At large φ, experimental normalized velocities are lower than expected. Inter-particle friction resists the retraction. The critical φ is lower for BL films, suggesting an interaction of the two particulate interfaces through the thin film. The modeling of this granular dissipation would require a full characterization of the rheology of the quasi-2D granular medium expected to depend in a non trivial way on both solid fraction and inertia [38,39] which can not be controlled independently in our experiments. To go further, PIV measurements as well as a study on the final state of our experiments are needed.

Fig. 1 :

 1 Fig. 1: Pictures of particulate films. (a) Side view of a laden film of 250 µm's particles in the monolayer configuration. (b) Side view of a laden film of 140 µm's particles in the bilayer configuration. The black horizontal line corresponds to the frame wire. (c) Top view of a laden film of 140 µm's particles in the bilayer configuration. (d) A close up on the area where particles of 140 µm are attached to both interfaces. (e) A coloured film with 250 µm's particles showing the variation in liquid's thickness. (f ) Hexagonal film with 140 µm's particles with a dark rim along the expanding hole. (g) and (h) are schematic side views presenting the film and the trapped particles respectively in the monolayer and the bilayer configuration.

Fig. 2 :Fig. 3 :

 23 Fig. 2: The two experimental setups. (a) Retraction on an hexagonal frame of a free edge film that has been perforated in the center. The dotted lines delimit the eight slices where the surface fraction of particles is calculated. (b) Retraction on a rectangular frame of a constrained edge film via the motion of the mobile stick triggered by bursting the right side of the film. The red arrows illustrate the rim dynamics (a) and the motion of the stick (b)

Fig. 4 :

 4 Fig. 4: Retraction velocities normalized by the velocity of bare liquid films as a function of particles surface fraction. Each point of the graph corresponds to an average over an interval of φ equal to 0.1 with at least 3 experimental measurements. Data relative to the monolayer configuration are presented in (a) and data relative to the bilayer configuration are presented in (b). Lines are plotted from Equ. 5 with Dp=250 µm for (a) and Dp=140 µm for (b).

Fig. 5 :

 5 Fig. 5: Summary of the dynamics of retraction of particulate films depending on the particle position at the interfaces (raw) and the constraints at the edge (column).

The authors would like to thank O. Pitois and X. Chateau for fruitful discussion and C. Méziere for technical help and frames support. We acknowledge CNES for financial support and IFSTTAR for the PhD grant of Y.T.